diff --git a/core-java/functional-programming/functional-interfaces/.gitignore b/core-java/functional-programming/functional-interfaces/.gitignore
new file mode 100644
index 000000000..3aff99610
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/.gitignore
@@ -0,0 +1,29 @@
+HELP.md
+target/*
+!.mvn/wrapper/maven-wrapper.jar
+!**/src/main/**/target/
+!**/src/test/**/target/
+
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+
+### IntelliJ IDEA ###
+.idea
+*.iws
+*.iml
+*.ipr
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+build/
+!**/src/main/**/build/
+!**/src/test/**/build/
+
+### VS Code ###
+.vscode/
\ No newline at end of file
diff --git a/core-java/functional-programming/functional-interfaces/.mvn/wrapper/MavenWrapperDownloader.java b/core-java/functional-programming/functional-interfaces/.mvn/wrapper/MavenWrapperDownloader.java
new file mode 100644
index 000000000..b901097f2
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/.mvn/wrapper/MavenWrapperDownloader.java
@@ -0,0 +1,117 @@
+/*
+ * Copyright 2007-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+import java.net.*;
+import java.io.*;
+import java.nio.channels.*;
+import java.util.Properties;
+
+public class MavenWrapperDownloader {
+
+    private static final String WRAPPER_VERSION = "0.5.6";
+    /**
+     * Default URL to download the maven-wrapper.jar from, if no 'downloadUrl' is provided.
+     */
+    private static final String DEFAULT_DOWNLOAD_URL = "https://repo.maven.apache.org/maven2/io/takari/maven-wrapper/"
+        + WRAPPER_VERSION + "/maven-wrapper-" + WRAPPER_VERSION + ".jar";
+
+    /**
+     * Path to the maven-wrapper.properties file, which might contain a downloadUrl property to
+     * use instead of the default one.
+     */
+    private static final String MAVEN_WRAPPER_PROPERTIES_PATH =
+            ".mvn/wrapper/maven-wrapper.properties";
+
+    /**
+     * Path where the maven-wrapper.jar will be saved to.
+     */
+    private static final String MAVEN_WRAPPER_JAR_PATH =
+            ".mvn/wrapper/maven-wrapper.jar";
+
+    /**
+     * Name of the property which should be used to override the default download url for the wrapper.
+     */
+    private static final String PROPERTY_NAME_WRAPPER_URL = "wrapperUrl";
+
+    public static void main(String args[]) {
+        System.out.println("- Downloader started");
+        File baseDirectory = new File(args[0]);
+        System.out.println("- Using base directory: " + baseDirectory.getAbsolutePath());
+
+        // If the maven-wrapper.properties exists, read it and check if it contains a custom
+        // wrapperUrl parameter.
+        File mavenWrapperPropertyFile = new File(baseDirectory, MAVEN_WRAPPER_PROPERTIES_PATH);
+        String url = DEFAULT_DOWNLOAD_URL;
+        if(mavenWrapperPropertyFile.exists()) {
+            FileInputStream mavenWrapperPropertyFileInputStream = null;
+            try {
+                mavenWrapperPropertyFileInputStream = new FileInputStream(mavenWrapperPropertyFile);
+                Properties mavenWrapperProperties = new Properties();
+                mavenWrapperProperties.load(mavenWrapperPropertyFileInputStream);
+                url = mavenWrapperProperties.getProperty(PROPERTY_NAME_WRAPPER_URL, url);
+            } catch (IOException e) {
+                System.out.println("- ERROR loading '" + MAVEN_WRAPPER_PROPERTIES_PATH + "'");
+            } finally {
+                try {
+                    if(mavenWrapperPropertyFileInputStream != null) {
+                        mavenWrapperPropertyFileInputStream.close();
+                    }
+                } catch (IOException e) {
+                    // Ignore ...
+                }
+            }
+        }
+        System.out.println("- Downloading from: " + url);
+
+        File outputFile = new File(baseDirectory.getAbsolutePath(), MAVEN_WRAPPER_JAR_PATH);
+        if(!outputFile.getParentFile().exists()) {
+            if(!outputFile.getParentFile().mkdirs()) {
+                System.out.println(
+                        "- ERROR creating output directory '" + outputFile.getParentFile().getAbsolutePath() + "'");
+            }
+        }
+        System.out.println("- Downloading to: " + outputFile.getAbsolutePath());
+        try {
+            downloadFileFromURL(url, outputFile);
+            System.out.println("Done");
+            System.exit(0);
+        } catch (Throwable e) {
+            System.out.println("- Error downloading");
+            e.printStackTrace();
+            System.exit(1);
+        }
+    }
+
+    private static void downloadFileFromURL(String urlString, File destination) throws Exception {
+        if (System.getenv("MVNW_USERNAME") != null && System.getenv("MVNW_PASSWORD") != null) {
+            String username = System.getenv("MVNW_USERNAME");
+            char[] password = System.getenv("MVNW_PASSWORD").toCharArray();
+            Authenticator.setDefault(new Authenticator() {
+                @Override
+                protected PasswordAuthentication getPasswordAuthentication() {
+                    return new PasswordAuthentication(username, password);
+                }
+            });
+        }
+        URL website = new URL(urlString);
+        ReadableByteChannel rbc;
+        rbc = Channels.newChannel(website.openStream());
+        FileOutputStream fos = new FileOutputStream(destination);
+        fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
+        fos.close();
+        rbc.close();
+    }
+
+}
diff --git a/core-java/functional-programming/functional-interfaces/.mvn/wrapper/maven-wrapper.jar b/core-java/functional-programming/functional-interfaces/.mvn/wrapper/maven-wrapper.jar
new file mode 100644
index 000000000..2cc7d4a55
Binary files /dev/null and b/core-java/functional-programming/functional-interfaces/.mvn/wrapper/maven-wrapper.jar differ
diff --git a/core-java/functional-programming/functional-interfaces/.mvn/wrapper/maven-wrapper.properties b/core-java/functional-programming/functional-interfaces/.mvn/wrapper/maven-wrapper.properties
new file mode 100644
index 000000000..642d572ce
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/.mvn/wrapper/maven-wrapper.properties
@@ -0,0 +1,2 @@
+distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.6.3/apache-maven-3.6.3-bin.zip
+wrapperUrl=https://repo.maven.apache.org/maven2/io/takari/maven-wrapper/0.5.6/maven-wrapper-0.5.6.jar
diff --git a/core-java/functional-programming/functional-interfaces/README.md b/core-java/functional-programming/functional-interfaces/README.md
new file mode 100644
index 000000000..ae82b2723
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/README.md
@@ -0,0 +1,3 @@
+# Related Blog Posts
+
+* [Use Cases for Java Records](https://reflectoring.io/beginner-friendly-guide-to-java-records/)
diff --git a/core-java/functional-programming/functional-interfaces/mvnw b/core-java/functional-programming/functional-interfaces/mvnw
new file mode 100644
index 000000000..41c0f0c23
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/mvnw
@@ -0,0 +1,310 @@
+#!/bin/sh
+# ----------------------------------------------------------------------------
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+# ----------------------------------------------------------------------------
+
+# ----------------------------------------------------------------------------
+# Maven Start Up Batch script
+#
+# Required ENV vars:
+# ------------------
+#   JAVA_HOME - location of a JDK home dir
+#
+# Optional ENV vars
+# -----------------
+#   M2_HOME - location of maven2's installed home dir
+#   MAVEN_OPTS - parameters passed to the Java VM when running Maven
+#     e.g. to debug Maven itself, use
+#       set MAVEN_OPTS=-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000
+#   MAVEN_SKIP_RC - flag to disable loading of mavenrc files
+# ----------------------------------------------------------------------------
+
+if [ -z "$MAVEN_SKIP_RC" ] ; then
+
+  if [ -f /etc/mavenrc ] ; then
+    . /etc/mavenrc
+  fi
+
+  if [ -f "$HOME/.mavenrc" ] ; then
+    . "$HOME/.mavenrc"
+  fi
+
+fi
+
+# OS specific support.  $var _must_ be set to either true or false.
+cygwin=false;
+darwin=false;
+mingw=false
+case "`uname`" in
+  CYGWIN*) cygwin=true ;;
+  MINGW*) mingw=true;;
+  Darwin*) darwin=true
+    # Use /usr/libexec/java_home if available, otherwise fall back to /Library/Java/Home
+    # See https://developer.apple.com/library/mac/qa/qa1170/_index.html
+    if [ -z "$JAVA_HOME" ]; then
+      if [ -x "/usr/libexec/java_home" ]; then
+        export JAVA_HOME="`/usr/libexec/java_home`"
+      else
+        export JAVA_HOME="/Library/Java/Home"
+      fi
+    fi
+    ;;
+esac
+
+if [ -z "$JAVA_HOME" ] ; then
+  if [ -r /etc/gentoo-release ] ; then
+    JAVA_HOME=`java-config --jre-home`
+  fi
+fi
+
+if [ -z "$M2_HOME" ] ; then
+  ## resolve links - $0 may be a link to maven's home
+  PRG="$0"
+
+  # need this for relative symlinks
+  while [ -h "$PRG" ] ; do
+    ls=`ls -ld "$PRG"`
+    link=`expr "$ls" : '.*-> \(.*\)$'`
+    if expr "$link" : '/.*' > /dev/null; then
+      PRG="$link"
+    else
+      PRG="`dirname "$PRG"`/$link"
+    fi
+  done
+
+  saveddir=`pwd`
+
+  M2_HOME=`dirname "$PRG"`/..
+
+  # make it fully qualified
+  M2_HOME=`cd "$M2_HOME" && pwd`
+
+  cd "$saveddir"
+  # echo Using m2 at $M2_HOME
+fi
+
+# For Cygwin, ensure paths are in UNIX format before anything is touched
+if $cygwin ; then
+  [ -n "$M2_HOME" ] &&
+    M2_HOME=`cygpath --unix "$M2_HOME"`
+  [ -n "$JAVA_HOME" ] &&
+    JAVA_HOME=`cygpath --unix "$JAVA_HOME"`
+  [ -n "$CLASSPATH" ] &&
+    CLASSPATH=`cygpath --path --unix "$CLASSPATH"`
+fi
+
+# For Mingw, ensure paths are in UNIX format before anything is touched
+if $mingw ; then
+  [ -n "$M2_HOME" ] &&
+    M2_HOME="`(cd "$M2_HOME"; pwd)`"
+  [ -n "$JAVA_HOME" ] &&
+    JAVA_HOME="`(cd "$JAVA_HOME"; pwd)`"
+fi
+
+if [ -z "$JAVA_HOME" ]; then
+  javaExecutable="`which javac`"
+  if [ -n "$javaExecutable" ] && ! [ "`expr \"$javaExecutable\" : '\([^ ]*\)'`" = "no" ]; then
+    # readlink(1) is not available as standard on Solaris 10.
+    readLink=`which readlink`
+    if [ ! `expr "$readLink" : '\([^ ]*\)'` = "no" ]; then
+      if $darwin ; then
+        javaHome="`dirname \"$javaExecutable\"`"
+        javaExecutable="`cd \"$javaHome\" && pwd -P`/javac"
+      else
+        javaExecutable="`readlink -f \"$javaExecutable\"`"
+      fi
+      javaHome="`dirname \"$javaExecutable\"`"
+      javaHome=`expr "$javaHome" : '\(.*\)/bin'`
+      JAVA_HOME="$javaHome"
+      export JAVA_HOME
+    fi
+  fi
+fi
+
+if [ -z "$JAVACMD" ] ; then
+  if [ -n "$JAVA_HOME"  ] ; then
+    if [ -x "$JAVA_HOME/jre/sh/java" ] ; then
+      # IBM's JDK on AIX uses strange locations for the executables
+      JAVACMD="$JAVA_HOME/jre/sh/java"
+    else
+      JAVACMD="$JAVA_HOME/bin/java"
+    fi
+  else
+    JAVACMD="`which java`"
+  fi
+fi
+
+if [ ! -x "$JAVACMD" ] ; then
+  echo "Error: JAVA_HOME is not defined correctly." >&2
+  echo "  We cannot execute $JAVACMD" >&2
+  exit 1
+fi
+
+if [ -z "$JAVA_HOME" ] ; then
+  echo "Warning: JAVA_HOME environment variable is not set."
+fi
+
+CLASSWORLDS_LAUNCHER=org.codehaus.plexus.classworlds.launcher.Launcher
+
+# traverses directory structure from process work directory to filesystem root
+# first directory with .mvn subdirectory is considered project base directory
+find_maven_basedir() {
+
+  if [ -z "$1" ]
+  then
+    echo "Path not specified to find_maven_basedir"
+    return 1
+  fi
+
+  basedir="$1"
+  wdir="$1"
+  while [ "$wdir" != '/' ] ; do
+    if [ -d "$wdir"/.mvn ] ; then
+      basedir=$wdir
+      break
+    fi
+    # workaround for JBEAP-8937 (on Solaris 10/Sparc)
+    if [ -d "${wdir}" ]; then
+      wdir=`cd "$wdir/.."; pwd`
+    fi
+    # end of workaround
+  done
+  echo "${basedir}"
+}
+
+# concatenates all lines of a file
+concat_lines() {
+  if [ -f "$1" ]; then
+    echo "$(tr -s '\n' ' ' < "$1")"
+  fi
+}
+
+BASE_DIR=`find_maven_basedir "$(pwd)"`
+if [ -z "$BASE_DIR" ]; then
+  exit 1;
+fi
+
+##########################################################################################
+# Extension to allow automatically downloading the maven-wrapper.jar from Maven-central
+# This allows using the maven wrapper in projects that prohibit checking in binary data.
+##########################################################################################
+if [ -r "$BASE_DIR/.mvn/wrapper/maven-wrapper.jar" ]; then
+    if [ "$MVNW_VERBOSE" = true ]; then
+      echo "Found .mvn/wrapper/maven-wrapper.jar"
+    fi
+else
+    if [ "$MVNW_VERBOSE" = true ]; then
+      echo "Couldn't find .mvn/wrapper/maven-wrapper.jar, downloading it ..."
+    fi
+    if [ -n "$MVNW_REPOURL" ]; then
+      jarUrl="$MVNW_REPOURL/io/takari/maven-wrapper/0.5.6/maven-wrapper-0.5.6.jar"
+    else
+      jarUrl="https://repo.maven.apache.org/maven2/io/takari/maven-wrapper/0.5.6/maven-wrapper-0.5.6.jar"
+    fi
+    while IFS="=" read key value; do
+      case "$key" in (wrapperUrl) jarUrl="$value"; break ;;
+      esac
+    done < "$BASE_DIR/.mvn/wrapper/maven-wrapper.properties"
+    if [ "$MVNW_VERBOSE" = true ]; then
+      echo "Downloading from: $jarUrl"
+    fi
+    wrapperJarPath="$BASE_DIR/.mvn/wrapper/maven-wrapper.jar"
+    if $cygwin; then
+      wrapperJarPath=`cygpath --path --windows "$wrapperJarPath"`
+    fi
+
+    if command -v wget > /dev/null; then
+        if [ "$MVNW_VERBOSE" = true ]; then
+          echo "Found wget ... using wget"
+        fi
+        if [ -z "$MVNW_USERNAME" ] || [ -z "$MVNW_PASSWORD" ]; then
+            wget "$jarUrl" -O "$wrapperJarPath"
+        else
+            wget --http-user=$MVNW_USERNAME --http-password=$MVNW_PASSWORD "$jarUrl" -O "$wrapperJarPath"
+        fi
+    elif command -v curl > /dev/null; then
+        if [ "$MVNW_VERBOSE" = true ]; then
+          echo "Found curl ... using curl"
+        fi
+        if [ -z "$MVNW_USERNAME" ] || [ -z "$MVNW_PASSWORD" ]; then
+            curl -o "$wrapperJarPath" "$jarUrl" -f
+        else
+            curl --user $MVNW_USERNAME:$MVNW_PASSWORD -o "$wrapperJarPath" "$jarUrl" -f
+        fi
+
+    else
+        if [ "$MVNW_VERBOSE" = true ]; then
+          echo "Falling back to using Java to download"
+        fi
+        javaClass="$BASE_DIR/.mvn/wrapper/MavenWrapperDownloader.java"
+        # For Cygwin, switch paths to Windows format before running javac
+        if $cygwin; then
+          javaClass=`cygpath --path --windows "$javaClass"`
+        fi
+        if [ -e "$javaClass" ]; then
+            if [ ! -e "$BASE_DIR/.mvn/wrapper/MavenWrapperDownloader.class" ]; then
+                if [ "$MVNW_VERBOSE" = true ]; then
+                  echo " - Compiling MavenWrapperDownloader.java ..."
+                fi
+                # Compiling the Java class
+                ("$JAVA_HOME/bin/javac" "$javaClass")
+            fi
+            if [ -e "$BASE_DIR/.mvn/wrapper/MavenWrapperDownloader.class" ]; then
+                # Running the downloader
+                if [ "$MVNW_VERBOSE" = true ]; then
+                  echo " - Running MavenWrapperDownloader.java ..."
+                fi
+                ("$JAVA_HOME/bin/java" -cp .mvn/wrapper MavenWrapperDownloader "$MAVEN_PROJECTBASEDIR")
+            fi
+        fi
+    fi
+fi
+##########################################################################################
+# End of extension
+##########################################################################################
+
+export MAVEN_PROJECTBASEDIR=${MAVEN_BASEDIR:-"$BASE_DIR"}
+if [ "$MVNW_VERBOSE" = true ]; then
+  echo $MAVEN_PROJECTBASEDIR
+fi
+MAVEN_OPTS="$(concat_lines "$MAVEN_PROJECTBASEDIR/.mvn/jvm.config") $MAVEN_OPTS"
+
+# For Cygwin, switch paths to Windows format before running java
+if $cygwin; then
+  [ -n "$M2_HOME" ] &&
+    M2_HOME=`cygpath --path --windows "$M2_HOME"`
+  [ -n "$JAVA_HOME" ] &&
+    JAVA_HOME=`cygpath --path --windows "$JAVA_HOME"`
+  [ -n "$CLASSPATH" ] &&
+    CLASSPATH=`cygpath --path --windows "$CLASSPATH"`
+  [ -n "$MAVEN_PROJECTBASEDIR" ] &&
+    MAVEN_PROJECTBASEDIR=`cygpath --path --windows "$MAVEN_PROJECTBASEDIR"`
+fi
+
+# Provide a "standardized" way to retrieve the CLI args that will
+# work with both Windows and non-Windows executions.
+MAVEN_CMD_LINE_ARGS="$MAVEN_CONFIG $@"
+export MAVEN_CMD_LINE_ARGS
+
+WRAPPER_LAUNCHER=org.apache.maven.wrapper.MavenWrapperMain
+
+exec "$JAVACMD" \
+  $MAVEN_OPTS \
+  -classpath "$MAVEN_PROJECTBASEDIR/.mvn/wrapper/maven-wrapper.jar" \
+  "-Dmaven.home=${M2_HOME}" "-Dmaven.multiModuleProjectDirectory=${MAVEN_PROJECTBASEDIR}" \
+  ${WRAPPER_LAUNCHER} $MAVEN_CONFIG "$@"
diff --git a/core-java/functional-programming/functional-interfaces/mvnw.cmd b/core-java/functional-programming/functional-interfaces/mvnw.cmd
new file mode 100644
index 000000000..86115719e
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/mvnw.cmd
@@ -0,0 +1,182 @@
+@REM ----------------------------------------------------------------------------
+@REM Licensed to the Apache Software Foundation (ASF) under one
+@REM or more contributor license agreements.  See the NOTICE file
+@REM distributed with this work for additional information
+@REM regarding copyright ownership.  The ASF licenses this file
+@REM to you under the Apache License, Version 2.0 (the
+@REM "License"); you may not use this file except in compliance
+@REM with the License.  You may obtain a copy of the License at
+@REM
+@REM    http://www.apache.org/licenses/LICENSE-2.0
+@REM
+@REM Unless required by applicable law or agreed to in writing,
+@REM software distributed under the License is distributed on an
+@REM "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+@REM KIND, either express or implied.  See the License for the
+@REM specific language governing permissions and limitations
+@REM under the License.
+@REM ----------------------------------------------------------------------------
+
+@REM ----------------------------------------------------------------------------
+@REM Maven Start Up Batch script
+@REM
+@REM Required ENV vars:
+@REM JAVA_HOME - location of a JDK home dir
+@REM
+@REM Optional ENV vars
+@REM M2_HOME - location of maven2's installed home dir
+@REM MAVEN_BATCH_ECHO - set to 'on' to enable the echoing of the batch commands
+@REM MAVEN_BATCH_PAUSE - set to 'on' to wait for a keystroke before ending
+@REM MAVEN_OPTS - parameters passed to the Java VM when running Maven
+@REM     e.g. to debug Maven itself, use
+@REM set MAVEN_OPTS=-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000
+@REM MAVEN_SKIP_RC - flag to disable loading of mavenrc files
+@REM ----------------------------------------------------------------------------
+
+@REM Begin all REM lines with '@' in case MAVEN_BATCH_ECHO is 'on'
+@echo off
+@REM set title of command window
+title %0
+@REM enable echoing by setting MAVEN_BATCH_ECHO to 'on'
+@if "%MAVEN_BATCH_ECHO%" == "on"  echo %MAVEN_BATCH_ECHO%
+
+@REM set %HOME% to equivalent of $HOME
+if "%HOME%" == "" (set "HOME=%HOMEDRIVE%%HOMEPATH%")
+
+@REM Execute a user defined script before this one
+if not "%MAVEN_SKIP_RC%" == "" goto skipRcPre
+@REM check for pre script, once with legacy .bat ending and once with .cmd ending
+if exist "%HOME%\mavenrc_pre.bat" call "%HOME%\mavenrc_pre.bat"
+if exist "%HOME%\mavenrc_pre.cmd" call "%HOME%\mavenrc_pre.cmd"
+:skipRcPre
+
+@setlocal
+
+set ERROR_CODE=0
+
+@REM To isolate internal variables from possible post scripts, we use another setlocal
+@setlocal
+
+@REM ==== START VALIDATION ====
+if not "%JAVA_HOME%" == "" goto OkJHome
+
+echo.
+echo Error: JAVA_HOME not found in your environment. >&2
+echo Please set the JAVA_HOME variable in your environment to match the >&2
+echo location of your Java installation. >&2
+echo.
+goto error
+
+:OkJHome
+if exist "%JAVA_HOME%\bin\java.exe" goto init
+
+echo.
+echo Error: JAVA_HOME is set to an invalid directory. >&2
+echo JAVA_HOME = "%JAVA_HOME%" >&2
+echo Please set the JAVA_HOME variable in your environment to match the >&2
+echo location of your Java installation. >&2
+echo.
+goto error
+
+@REM ==== END VALIDATION ====
+
+:init
+
+@REM Find the project base dir, i.e. the directory that contains the folder ".mvn".
+@REM Fallback to current working directory if not found.
+
+set MAVEN_PROJECTBASEDIR=%MAVEN_BASEDIR%
+IF NOT "%MAVEN_PROJECTBASEDIR%"=="" goto endDetectBaseDir
+
+set EXEC_DIR=%CD%
+set WDIR=%EXEC_DIR%
+:findBaseDir
+IF EXIST "%WDIR%"\.mvn goto baseDirFound
+cd ..
+IF "%WDIR%"=="%CD%" goto baseDirNotFound
+set WDIR=%CD%
+goto findBaseDir
+
+:baseDirFound
+set MAVEN_PROJECTBASEDIR=%WDIR%
+cd "%EXEC_DIR%"
+goto endDetectBaseDir
+
+:baseDirNotFound
+set MAVEN_PROJECTBASEDIR=%EXEC_DIR%
+cd "%EXEC_DIR%"
+
+:endDetectBaseDir
+
+IF NOT EXIST "%MAVEN_PROJECTBASEDIR%\.mvn\jvm.config" goto endReadAdditionalConfig
+
+@setlocal EnableExtensions EnableDelayedExpansion
+for /F "usebackq delims=" %%a in ("%MAVEN_PROJECTBASEDIR%\.mvn\jvm.config") do set JVM_CONFIG_MAVEN_PROPS=!JVM_CONFIG_MAVEN_PROPS! %%a
+@endlocal & set JVM_CONFIG_MAVEN_PROPS=%JVM_CONFIG_MAVEN_PROPS%
+
+:endReadAdditionalConfig
+
+SET MAVEN_JAVA_EXE="%JAVA_HOME%\bin\java.exe"
+set WRAPPER_JAR="%MAVEN_PROJECTBASEDIR%\.mvn\wrapper\maven-wrapper.jar"
+set WRAPPER_LAUNCHER=org.apache.maven.wrapper.MavenWrapperMain
+
+set DOWNLOAD_URL="https://repo.maven.apache.org/maven2/io/takari/maven-wrapper/0.5.6/maven-wrapper-0.5.6.jar"
+
+FOR /F "tokens=1,2 delims==" %%A IN ("%MAVEN_PROJECTBASEDIR%\.mvn\wrapper\maven-wrapper.properties") DO (
+    IF "%%A"=="wrapperUrl" SET DOWNLOAD_URL=%%B
+)
+
+@REM Extension to allow automatically downloading the maven-wrapper.jar from Maven-central
+@REM This allows using the maven wrapper in projects that prohibit checking in binary data.
+if exist %WRAPPER_JAR% (
+    if "%MVNW_VERBOSE%" == "true" (
+        echo Found %WRAPPER_JAR%
+    )
+) else (
+    if not "%MVNW_REPOURL%" == "" (
+        SET DOWNLOAD_URL="%MVNW_REPOURL%/io/takari/maven-wrapper/0.5.6/maven-wrapper-0.5.6.jar"
+    )
+    if "%MVNW_VERBOSE%" == "true" (
+        echo Couldn't find %WRAPPER_JAR%, downloading it ...
+        echo Downloading from: %DOWNLOAD_URL%
+    )
+
+    powershell -Command "&{"^
+		"$webclient = new-object System.Net.WebClient;"^
+		"if (-not ([string]::IsNullOrEmpty('%MVNW_USERNAME%') -and [string]::IsNullOrEmpty('%MVNW_PASSWORD%'))) {"^
+		"$webclient.Credentials = new-object System.Net.NetworkCredential('%MVNW_USERNAME%', '%MVNW_PASSWORD%');"^
+		"}"^
+		"[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12; $webclient.DownloadFile('%DOWNLOAD_URL%', '%WRAPPER_JAR%')"^
+		"}"
+    if "%MVNW_VERBOSE%" == "true" (
+        echo Finished downloading %WRAPPER_JAR%
+    )
+)
+@REM End of extension
+
+@REM Provide a "standardized" way to retrieve the CLI args that will
+@REM work with both Windows and non-Windows executions.
+set MAVEN_CMD_LINE_ARGS=%*
+
+%MAVEN_JAVA_EXE% %JVM_CONFIG_MAVEN_PROPS% %MAVEN_OPTS% %MAVEN_DEBUG_OPTS% -classpath %WRAPPER_JAR% "-Dmaven.multiModuleProjectDirectory=%MAVEN_PROJECTBASEDIR%" %WRAPPER_LAUNCHER% %MAVEN_CONFIG% %*
+if ERRORLEVEL 1 goto error
+goto end
+
+:error
+set ERROR_CODE=1
+
+:end
+@endlocal & set ERROR_CODE=%ERROR_CODE%
+
+if not "%MAVEN_SKIP_RC%" == "" goto skipRcPost
+@REM check for post script, once with legacy .bat ending and once with .cmd ending
+if exist "%HOME%\mavenrc_post.bat" call "%HOME%\mavenrc_post.bat"
+if exist "%HOME%\mavenrc_post.cmd" call "%HOME%\mavenrc_post.cmd"
+:skipRcPost
+
+@REM pause the script if MAVEN_BATCH_PAUSE is set to 'on'
+if "%MAVEN_BATCH_PAUSE%" == "on" pause
+
+if "%MAVEN_TERMINATE_CMD%" == "on" exit %ERROR_CODE%
+
+exit /B %ERROR_CODE%
diff --git a/core-java/functional-programming/functional-interfaces/pom.xml b/core-java/functional-programming/functional-interfaces/pom.xml
new file mode 100644
index 000000000..a871d179b
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/pom.xml
@@ -0,0 +1,85 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>io.reflectoring</groupId>
+    <artifactId>functional-interfaces</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <name>Java Functional Interfaces</name>
+    <url>https://reflectoring.io</url>
+
+    <properties>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        <maven.compiler.source>17</maven.compiler.source>
+        <maven.compiler.target>17</maven.compiler.target>
+    </properties>
+
+    <dependencies>
+        <!-- https://mvnrepository.com/artifact/org.junit.jupiter/junit-jupiter-api -->
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-api</artifactId>
+            <version>5.10.0</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-params</artifactId>
+            <version>5.10.0</version>
+            <scope>test</scope>
+        </dependency>        <!-- https://mvnrepository.com/artifact/org.assertj/assertj-core -->
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>3.24.2</version>
+            <scope>test</scope>
+        </dependency>
+
+        <!-- https://mvnrepository.com/artifact/org.slf4j/slf4j-simple -->
+        <dependency>
+            <groupId>org.slf4j</groupId>
+            <artifactId>slf4j-simple</artifactId>
+            <version>2.0.9</version>
+            <scope>test</scope>
+        </dependency>
+
+        <dependency>
+            <groupId>org.slf4j</groupId>
+            <artifactId>slf4j-api</artifactId>
+            <version>2.0.9</version>
+        </dependency>
+
+        <dependency>
+            <groupId>org.projectlombok</groupId>
+            <artifactId>lombok</artifactId>
+            <version>1.18.28</version>
+        </dependency>
+
+        <dependency>
+            <groupId>jakarta.validation</groupId>
+            <artifactId>jakarta.validation-api</artifactId>
+            <version>3.0.2</version>
+        </dependency>
+
+        <dependency>
+            <groupId>org.hibernate.validator</groupId>
+            <artifactId>hibernate-validator</artifactId>
+            <version>8.0.1.Final</version>
+        </dependency>
+    </dependencies>
+
+    <dependencyManagement>
+        <dependencies>
+            <dependency>
+                <groupId>org.junit</groupId>
+                <artifactId>junit-bom</artifactId>
+                <version>5.10.0</version>
+                <type>pom</type>
+                <scope>import</scope>
+            </dependency>
+        </dependencies>
+    </dependencyManagement>
+</project>
diff --git a/core-java/functional-programming/functional-interfaces/src/main/java/io/reflectoring/function/custom/ArithmeticOperation.java b/core-java/functional-programming/functional-interfaces/src/main/java/io/reflectoring/function/custom/ArithmeticOperation.java
new file mode 100644
index 000000000..5ca8c9164
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/src/main/java/io/reflectoring/function/custom/ArithmeticOperation.java
@@ -0,0 +1,13 @@
+package io.reflectoring.function.custom;
+
+/** The arithmetic operation functional interface. */
+public interface ArithmeticOperation {
+  /**
+   * Operates on two integer inputs to calculate a result.
+   *
+   * @param a the first number
+   * @param b the second number
+   * @return the result
+   */
+  int operate(int a, int b);
+}
diff --git a/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/ConsumerTest.java b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/ConsumerTest.java
new file mode 100644
index 000000000..74737658a
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/ConsumerTest.java
@@ -0,0 +1,183 @@
+package io.reflectoring.function;
+
+import java.text.MessageFormat;
+import java.time.Instant;
+import java.time.LocalDateTime;
+import java.util.*;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.*;
+import java.util.stream.DoubleStream;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+public class ConsumerTest {
+  @Test
+  void consumer() {
+    Consumer<List<String>> trim =
+        strings -> {
+          if (strings != null) {
+            strings.replaceAll(s -> s == null ? null : s.trim());
+          }
+        };
+    Consumer<List<String>> upperCase =
+        strings -> {
+          if (strings != null) {
+            strings.replaceAll(s -> s == null ? null : s.toUpperCase());
+          }
+        };
+
+    List<String> input = null;
+    input = Arrays.asList(null, "", " Joy", " Joy ", "Joy ", "Joy");
+    trim.accept(input);
+    Assertions.assertEquals(Arrays.asList(null, "", "Joy", "Joy", "Joy", "Joy"), input);
+
+    input = Arrays.asList(null, "", " Joy", " Joy ", "Joy ", "Joy");
+    trim.andThen(upperCase).accept(input);
+    Assertions.assertEquals(Arrays.asList(null, "", "JOY", "JOY", "JOY", "JOY"), input);
+  }
+
+  @Test
+  void biConsumer() {
+    BiConsumer<List<Double>, Double> discountRule =
+        (prices, discount) -> {
+          if (prices != null && discount != null) {
+            prices.replaceAll(price -> price * discount);
+          }
+        };
+    BiConsumer<List<Double>, Double> bulkDiscountRule =
+        (prices, discount) -> {
+          if (prices != null && discount != null && prices.size() > 2) {
+            // 20% discount cart has 2 items or more
+            prices.replaceAll(price -> price * 0.80);
+          }
+        };
+
+    double discount = 0.90; // 10% discount
+    List<Double> prices = null;
+    prices = Arrays.asList(20.0, 30.0, 100.0);
+    discountRule.accept(prices, discount);
+    Assertions.assertEquals(Arrays.asList(18.0, 27.0, 90.0), prices);
+
+    prices = Arrays.asList(20.0, 30.0, 100.0);
+    discountRule.andThen(bulkDiscountRule).accept(prices, discount);
+    Assertions.assertEquals(Arrays.asList(14.4, 21.6, 72.0), prices);
+  }
+
+  @ParameterizedTest
+  @CsvSource({
+    "15,Turning off AC.",
+    "22,---",
+    "25,Turning on AC.",
+    "52,Alert! Temperature not safe for humans."
+  })
+  void intConsumer(int temperature, String expected) {
+    AtomicReference<String> message = new AtomicReference<>();
+    IntConsumer temperatureSensor =
+        t -> {
+          message.set("---");
+          if (t <= 20) {
+            message.set("Turning off AC.");
+          } else if (t >= 24 && t <= 50) {
+            message.set("Turning on AC.");
+          } else if (t > 50) {
+            message.set("Alert! Temperature not safe for humans.");
+          }
+        };
+
+    temperatureSensor.accept(temperature);
+    Assertions.assertEquals(expected, message.toString());
+  }
+
+  @Test
+  void longConsumer() {
+    long duration = TimeUnit.MINUTES.toMillis(20);
+    long stopTime = Instant.now().toEpochMilli() + duration;
+    AtomicReference<String> message = new AtomicReference<>();
+
+    LongConsumer timeCheck =
+        millis -> {
+          message.set("---");
+          if (millis >= stopTime) {
+            message.set("STOP");
+          } else {
+            message.set("CONTINUE");
+          }
+        };
+
+    // Current time in milliseconds
+    long currentTimeMillis = Instant.now().toEpochMilli();
+    timeCheck.accept(currentTimeMillis);
+    Assertions.assertEquals("CONTINUE", message.toString());
+
+    long pastStopTime = currentTimeMillis + duration + 10000L;
+    timeCheck.accept(pastStopTime);
+    Assertions.assertEquals("STOP", message.toString());
+  }
+
+  @Test
+  void doubleConsumer() {
+    AtomicReference<Double> temperature = new AtomicReference<>(0.0);
+    DoubleConsumer celsiusToFahrenheit = celsius -> temperature.set(celsius * 9 / 5 + 32);
+    celsiusToFahrenheit.accept(100);
+    Assertions.assertEquals(212.0, temperature.get());
+
+    // radius of circles
+    List<Integer> input = Arrays.asList(1, 2, 3, 4, 5);
+    // calculate area of circle
+    BiConsumer<Integer, DoubleConsumer> biConsumer =
+        (radius, consumer) -> {
+          consumer.accept(Math.PI * radius * radius);
+        };
+    DoubleStream result = input.stream().mapMultiToDouble(biConsumer);
+    Assertions.assertArrayEquals(
+        new double[] {3.14, 12.56, 28.27, 50.26, 78.53}, result.toArray(), 0.01);
+  }
+
+  @Test
+  void objIntConsumer() {
+    AtomicReference<String> result = new AtomicReference<>();
+    ObjIntConsumer<String> trim =
+        (input, len) -> {
+          if (input != null && input.length() > len) {
+            result.set(input.substring(0, len));
+          }
+        };
+
+    trim.accept("123456789", 3);
+    Assertions.assertEquals("123", result.get());
+  }
+
+  @Test
+  void objLongConsumer() {
+    AtomicReference<LocalDateTime> result = new AtomicReference<>();
+    ObjLongConsumer<LocalDateTime> trim =
+        (input, delta) -> {
+          if (input != null) {
+            result.set(input.plusSeconds(delta));
+          }
+        };
+
+    LocalDateTime input = LocalDateTime.now().toLocalDate().atStartOfDay();
+    trim.accept(input, TimeUnit.DAYS.toMillis(1));
+    Assertions.assertEquals(0, result.get().getMinute());
+  }
+
+  @ParameterizedTest
+  @CsvSource(
+      value = {"{0};12,345.678", "{0,number,#.##};12345.68", "{0,number,currency};$12,345.68"},
+      delimiter = ';')
+  void objDoubleConsumer(String formatString, String expected) {
+    AtomicReference<String> result = new AtomicReference<>();
+    ObjDoubleConsumer<String> format =
+        (formatStr, input) -> {
+          result.set(MessageFormat.format(formatStr, input));
+        };
+
+    double number = 12345.678;
+    format.accept(formatString, number);
+    Assertions.assertEquals(expected, result.get());
+  }
+}
diff --git a/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/FunctionTest.java b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/FunctionTest.java
new file mode 100644
index 000000000..abdb2eb79
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/FunctionTest.java
@@ -0,0 +1,194 @@
+package io.reflectoring.function;
+
+import java.time.Instant;
+import java.time.LocalDateTime;
+import java.time.ZoneOffset;
+import java.util.Date;
+import java.util.function.*;
+import java.util.stream.DoubleStream;
+import java.util.stream.IntStream;
+import java.util.stream.LongStream;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class FunctionTest {
+
+  @Test
+  void simpleFunction() {
+    Function<String, String> toUpper = s -> s == null ? null : s.toUpperCase();
+    Assertions.assertEquals("JOY", toUpper.apply("joy"));
+    Assertions.assertNull(toUpper.apply(null));
+  }
+
+  @Test
+  void functionComposition() {
+    Function<String, String> toUpper = s -> s == null ? null : s.toUpperCase();
+    Function<String, String> replaceVowels =
+        s ->
+            s == null
+                ? null
+                : s.replace("A", "")
+                    .replace("E", "")
+                    .replace("I", "")
+                    .replace("O", "")
+                    .replace("U", "");
+    Assertions.assertEquals("APPLE", toUpper.compose(replaceVowels).apply("apple"));
+    Assertions.assertEquals("PPL", toUpper.andThen(replaceVowels).apply("apple"));
+  }
+
+  @Test
+  void biFunction() {
+    BiFunction<Integer, Integer, Integer> bigger =
+        (first, second) -> first > second ? first : second;
+    Function<Integer, Integer> square = number -> number * number;
+
+    Assertions.assertEquals(10, bigger.apply(4, 10));
+    Assertions.assertEquals(100, bigger.andThen(square).apply(4, 10));
+  }
+
+  @Test
+  void intFunction() {
+    IntFunction<Integer> square = number -> number * number;
+    Assertions.assertEquals(100, square.apply(10));
+  }
+
+  @Test
+  void intToDoubleFunction() {
+    int principalAmount = 1000; // Initial investment amount
+    double interestRate = 0.05; // Annual accruedInterest rate (5%)
+
+    IntToDoubleFunction accruedInterest = principal -> principal * interestRate;
+    Assertions.assertEquals(50.0, accruedInterest.applyAsDouble(principalAmount));
+  }
+
+  @Test
+  void intToLongFunction() {
+    IntToLongFunction factorial =
+        n -> {
+          long result = 1L;
+          for (int i = 1; i <= n; i++) {
+            result *= i;
+          }
+          return result;
+        };
+    IntStream input = IntStream.range(1, 6);
+    final long[] result = input.mapToLong(factorial).toArray();
+    Assertions.assertArrayEquals(new long[] {1L, 2L, 6L, 24L, 120L}, result);
+  }
+
+  @Test
+  void longFunction() {
+    LongFunction<Double> squareArea = side -> (double) (side * side);
+    Assertions.assertEquals(400d, squareArea.apply(20L));
+  }
+
+  @Test
+  void longToDoubleFunction() {
+    LongToDoubleFunction squareArea = side -> (double) (side * side);
+    Assertions.assertEquals(400d, squareArea.applyAsDouble(20L));
+
+    LongStream input = LongStream.range(1L, 6L);
+    final double[] result = input.mapToDouble(squareArea).toArray();
+    Assertions.assertArrayEquals(new double[] {1.0, 4.0, 9.0, 16.0, 25.0}, result);
+  }
+
+  @Test
+  void longToIntFunction() {
+    LongToIntFunction digitCount = number -> String.valueOf(number).length();
+    LongStream input = LongStream.of(1L, 120, 15L, 12345L);
+    final int[] result = input.mapToInt(digitCount).toArray();
+    Assertions.assertArrayEquals(new int[] {1, 3, 2, 5}, result);
+  }
+
+  @Test
+  void doubleFunction() {
+    // grouping separator like a comma for thousands
+    //  exactly two digits after the decimal point
+    DoubleFunction<String> numberFormatter = number -> String.format("%1$,.2f", number);
+    Assertions.assertEquals("999,999.12", numberFormatter.apply(999999.123));
+  }
+
+  @Test
+  void doubleToIntFunction() {
+    DoubleToIntFunction wholeNumber = number -> Double.valueOf(number).intValue();
+    DoubleStream input = DoubleStream.of(1.0, 12.34, 99.0, 101.444);
+    int[] result = input.mapToInt(wholeNumber).toArray();
+    Assertions.assertArrayEquals(new int[] {1, 12, 99, 101}, result);
+  }
+
+  @Test
+  void doubleToLongFunction() {
+    DoubleToLongFunction celsiusToFahrenheit = celsius -> Math.round(celsius * 9 / 5 + 32);
+    DoubleStream input = DoubleStream.of(0.0, 25.0, 100.0);
+    long[] result = input.mapToLong(celsiusToFahrenheit).toArray();
+    Assertions.assertArrayEquals(new long[] {32, 77, 212}, result);
+  }
+
+  @Test
+  void toDoubleFunction() {
+    ToDoubleFunction<Integer> fahrenheitToCelsius =
+        (fahrenheit) -> (double) ((fahrenheit - 32) * 5) / 9;
+    Assertions.assertEquals(0.0, fahrenheitToCelsius.applyAsDouble(32));
+    Assertions.assertEquals(25.0, fahrenheitToCelsius.applyAsDouble(77));
+    Assertions.assertEquals(100.0, fahrenheitToCelsius.applyAsDouble(212));
+  }
+
+  @Test
+  void toDoubleBiFunction() {
+    // 30% discount when it is SALE else 10% standard discount
+    ToDoubleBiFunction<String, Double> discountedPrice =
+        (code, price) -> "SALE".equals(code) ? price * 0.7 : price * 0.9;
+    Assertions.assertEquals(14.0, discountedPrice.applyAsDouble("SALE", 20.0));
+    Assertions.assertEquals(18.0, discountedPrice.applyAsDouble("OFF_SEASON", 20.0));
+  }
+
+  @Test
+  void toIntFunction() {
+    ToIntFunction<String> charCount = input -> input == null ? 0 : input.trim().length();
+
+    Assertions.assertEquals(0, charCount.applyAsInt(null));
+    Assertions.assertEquals(0, charCount.applyAsInt(""));
+    Assertions.assertEquals(3, charCount.applyAsInt("JOY"));
+  }
+
+  @Test
+  void toIntBiFunction() {
+    // discount on product
+    ToIntBiFunction<String, Integer> discount =
+        (season, quantity) -> "WINTER".equals(season) || quantity > 100 ? 40 : 10;
+
+    Assertions.assertEquals(40, discount.applyAsInt("WINTER", 50));
+    Assertions.assertEquals(40, discount.applyAsInt("SUMMER", 150));
+    Assertions.assertEquals(10, discount.applyAsInt("FALL", 50));
+  }
+
+  @Test
+  void toLongFunction() {
+    ToLongFunction<Date> elapsedTime =
+        input -> input == null ? 0 : input.toInstant().toEpochMilli();
+
+    Assertions.assertEquals(0L, elapsedTime.applyAsLong(null));
+    long now = System.currentTimeMillis();
+    Date nowDate = Date.from(Instant.ofEpochMilli(now));
+    Assertions.assertEquals(now, elapsedTime.applyAsLong(nowDate));
+  }
+
+  @Test
+  void toLongBiFunction() {
+    // discount on product
+    ToLongBiFunction<LocalDateTime, ZoneOffset> elapsed =
+        (localDateTime, zoneOffset) ->
+            zoneOffset == null
+                ? localDateTime.toEpochSecond(ZoneOffset.UTC)
+                : localDateTime.toEpochSecond(zoneOffset);
+
+    final long now = System.currentTimeMillis();
+    final LocalDateTime nowLocalDateTime = LocalDateTime.ofEpochSecond(now, 0, ZoneOffset.UTC);
+    Assertions.assertEquals(now, elapsed.applyAsLong(nowLocalDateTime, null));
+
+    final long later = now + 1000;
+    final ZoneOffset offset = ZoneOffset.ofHours(5);
+    final LocalDateTime laterLocalDateTime = LocalDateTime.ofEpochSecond(later, 0, offset);
+    Assertions.assertEquals(later, elapsed.applyAsLong(laterLocalDateTime, offset));
+  }
+}
diff --git a/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/OperatorTest.java b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/OperatorTest.java
new file mode 100644
index 000000000..8f0b76ec4
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/OperatorTest.java
@@ -0,0 +1,162 @@
+package io.reflectoring.function;
+
+import java.util.OptionalDouble;
+import java.util.OptionalInt;
+import java.util.OptionalLong;
+import java.util.function.*;
+import java.util.stream.DoubleStream;
+import java.util.stream.IntStream;
+import java.util.stream.LongStream;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import io.reflectoring.function.custom.ArithmeticOperation;
+import jakarta.validation.constraints.NotNull;
+
+public class OperatorTest {
+  @Test
+  void unaryOperator() {
+    ArithmeticOperation add = (var a, var b) -> a + b;
+    ArithmeticOperation addNullSafe = (@NotNull var a, @NotNull var b) -> a + b;
+    UnaryOperator<String> trim = value -> value == null ? null : value.trim();
+    UnaryOperator<String> upperCase = value -> value == null ? null : value.toUpperCase();
+    Function<String, String> transform = trim.andThen(upperCase);
+
+    Assertions.assertEquals("joy", trim.apply("  joy "));
+    Assertions.assertEquals("  JOY ", upperCase.apply("  joy "));
+    Assertions.assertEquals("JOY", transform.apply("  joy "));
+  }
+
+  @Test
+  void intUnaryOperator() {
+    // formula y = x^2 + 2x + 1
+    IntUnaryOperator formula = x -> (x * x) + (2 * x) + 1;
+    Assertions.assertEquals(36, formula.applyAsInt(5));
+
+    IntStream input = IntStream.of(2, 3, 4);
+    final int[] result = input.map(formula).toArray();
+    Assertions.assertArrayEquals(new int[] {9, 16, 25}, result);
+
+    // the population doubling every 3 years, one fifth migrate and 10% mortality
+    IntUnaryOperator growth = number -> number * 2;
+    IntUnaryOperator migration = number -> number * 4 / 5;
+    IntUnaryOperator mortality = number -> number * 9 / 10;
+    IntUnaryOperator population = growth.andThen(migration).andThen(mortality);
+    Assertions.assertEquals(1440000, population.applyAsInt(1000000));
+  }
+
+  @Test
+  void longUnaryOperator() {
+    // light travels 186282 miles per seconds
+    LongUnaryOperator distance = time -> time * 186282;
+    // denser medium slows light down
+    LongUnaryOperator slowDown = dist -> dist * 2 / 3;
+    LongUnaryOperator actualDistance = distance.andThen(slowDown);
+
+    Assertions.assertEquals(931410, distance.applyAsLong(5));
+    Assertions.assertEquals(620940, actualDistance.applyAsLong(5));
+
+    final LongStream input = LongStream.of(5, 10, 15);
+    final long[] result = input.map(distance).toArray();
+    Assertions.assertArrayEquals(new long[] {931410L, 1862820L, 2794230L}, result);
+  }
+
+  @Test
+  void doubleUnaryOperator() {
+    DoubleUnaryOperator circleArea = radius -> radius * radius * Math.PI;
+    DoubleUnaryOperator doubleIt = area -> area * 4;
+    DoubleUnaryOperator scaling = circleArea.andThen(doubleIt);
+
+    Assertions.assertEquals(153.93D, circleArea.applyAsDouble(7), 0.01);
+    Assertions.assertEquals(615.75D, scaling.applyAsDouble(7), 0.01);
+
+    final DoubleStream input = DoubleStream.of(7d, 14d, 21d);
+    final double[] result = input.map(circleArea).toArray();
+    Assertions.assertArrayEquals(new double[] {153.93D, 615.75D, 1385.44D}, result, 0.01);
+  }
+
+  @Test
+  void binaryOperator() {
+    LongUnaryOperator factorial =
+        n -> {
+          long result = 1L;
+          for (int i = 1; i <= n; i++) {
+            result *= i;
+          }
+          return result;
+        };
+    // Calculate permutations
+    BinaryOperator<Long> npr = (n, r) -> factorial.applyAsLong(n) / factorial.applyAsLong(n - r);
+    // Verify permutations
+    // 3P2 means the number of permutations of 2 that can be achieved from a choice of 3.
+    final Long result3P2 = npr.apply(3L, 2L);
+    Assertions.assertEquals(6L, result3P2);
+
+    // Add two prices
+    BinaryOperator<Double> addPrices = Double::sum;
+    // Apply discount
+    UnaryOperator<Double> applyDiscount = total -> total * 0.9; // 10% discount
+    // Apply tax
+    UnaryOperator<Double> applyTax = total -> total * 1.07; // 7% tax
+    // Composing the final operation
+    BiFunction<Double, Double, Double> finalCost =
+        addPrices.andThen(applyDiscount).andThen(applyTax);
+
+    // Prices of two items
+    double item1 = 50.0;
+    double item2 = 100.0;
+    // Calculate final cost
+    double cost = finalCost.apply(item1, item2);
+    // Verify the final calculated cost
+    Assertions.assertEquals(144.45D, cost, 0.01);
+  }
+
+  @Test
+  void intBinaryOperator() {
+    IntBinaryOperator add = Integer::sum;
+    Assertions.assertEquals(10, add.applyAsInt(4, 6));
+
+    IntStream input = IntStream.of(2, 3, 4);
+    OptionalInt result = input.reduce(add);
+    Assertions.assertEquals(OptionalInt.of(9), result);
+  }
+
+  @Test
+  void longBinaryOperator() {
+    // Greatest Common Divisor
+    LongBinaryOperator gcd =
+        (a, b) -> {
+          while (b != 0) {
+            long temp = b;
+            b = a % b;
+            a = temp;
+          }
+          return a;
+        };
+    Assertions.assertEquals(6L, gcd.applyAsLong(54L, 24L));
+
+    LongBinaryOperator add = Long::sum;
+    // Time car traveled
+    LongStream input = LongStream.of(1715785375164L, 1715785385771L);
+    final OptionalLong result = input.reduce(add);
+    Assertions.assertEquals(OptionalLong.of(3431570760935L), result);
+  }
+
+  @Test
+  void doubleBinaryOperator() {
+    DoubleBinaryOperator subtractAreas = (a, b) -> a - b;
+    // Area of a rectangle
+    double rectangleArea = 20.0 * 30.0;
+    // Area of a circle
+    double circleArea = Math.PI * 7.0 * 7.0;
+
+    // Subtract the two areas
+    double difference = subtractAreas.applyAsDouble(rectangleArea, circleArea);
+    Assertions.assertEquals(446.06, difference, 0.01);
+
+    DoubleBinaryOperator add = Double::sum;
+    DoubleStream input = DoubleStream.of(10.2, 5.6, 15.8, 20.12);
+    OptionalDouble result = input.reduce(add);
+    Assertions.assertEquals(OptionalDouble.of(51.72), result);
+  }
+}
diff --git a/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/PredicateTest.java b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/PredicateTest.java
new file mode 100644
index 000000000..19f92f42b
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/PredicateTest.java
@@ -0,0 +1,256 @@
+package io.reflectoring.function;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.function.*;
+import java.util.stream.DoubleStream;
+import java.util.stream.IntStream;
+import java.util.stream.LongStream;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class PredicateTest {
+  // C = Carpenter, W = Welder
+  private final Object[][] workers = {
+    {"C", 24},
+    {"W", 32},
+    {"C", 35},
+    {"W", 40},
+    {"C", 50},
+    {"W", 44},
+    {"C", 30}
+  };
+
+  @Test
+  void testFiltering() {
+    List<Integer> numbers = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
+    Predicate<Integer> isEven = num -> num % 2 == 0;
+
+    List<Integer> actual = numbers.stream().filter(isEven).toList();
+
+    List<Integer> expected = List.of(2, 4, 6, 8, 10);
+    Assertions.assertEquals(expected, actual);
+  }
+
+  @Test
+  void testPredicate() {
+    List<Integer> numbers = List.of(-5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5);
+    Predicate<Integer> isZero = num -> num == 0;
+    Predicate<Integer> isPositive = num -> num > 0;
+    Predicate<Integer> isNegative = num -> num < 0;
+    Predicate<Integer> isOdd = num -> num % 2 == 1;
+
+    Predicate<Integer> isPositiveOrZero = isPositive.or(isZero);
+    Predicate<Integer> isPositiveAndOdd = isPositive.and(isOdd);
+    Predicate<Integer> isNotPositive = Predicate.not(isPositive);
+    Predicate<Integer> isNotZero = isZero.negate();
+    Predicate<Integer> isAlsoZero = isPositive.negate().and(isNegative.negate());
+
+    // check zero or greater
+    Assertions.assertEquals(
+        List.of(0, 1, 2, 3, 4, 5), numbers.stream().filter(isPositiveOrZero).toList());
+
+    // check greater than zero and odd
+    Assertions.assertEquals(List.of(1, 3, 5), numbers.stream().filter(isPositiveAndOdd).toList());
+
+    // check less than zero and negative
+    Assertions.assertEquals(
+        List.of(-5, -4, -3, -2, -1, 0), numbers.stream().filter(isNotPositive).toList());
+
+    // check not zero
+    Assertions.assertEquals(
+        List.of(-5, -4, -3, -2, -1, 1, 2, 3, 4, 5), numbers.stream().filter(isNotZero).toList());
+
+    // check neither positive nor negative
+    Assertions.assertEquals(
+        numbers.stream().filter(isZero).toList(), numbers.stream().filter(isAlsoZero).toList());
+  }
+
+  @Test
+  void testBiPredicate() {
+
+    BiPredicate<String, Integer> juniorCarpenterCheck =
+        (worker, age) -> "C".equals(worker) && (age >= 18 && age <= 40);
+
+    BiPredicate<String, Integer> groomedCarpenterCheck =
+        (worker, age) -> "C".equals(worker) && (age >= 30 && age <= 40);
+
+    BiPredicate<String, Integer> allCarpenterCheck =
+        (worker, age) -> "C".equals(worker) && (age >= 18);
+
+    BiPredicate<String, Integer> juniorWelderCheck =
+        (worker, age) -> "W".equals(worker) && (age >= 18 && age <= 40);
+
+    BiPredicate<String, Integer> juniorWorkerCheck = juniorCarpenterCheck.or(juniorWelderCheck);
+
+    BiPredicate<String, Integer> juniorGroomedCarpenterCheck =
+        juniorCarpenterCheck.and(groomedCarpenterCheck);
+
+    BiPredicate<String, Integer> allWelderCheck = allCarpenterCheck.negate();
+
+    final long juniorCarpenterCount =
+        Arrays.stream(workers)
+            .filter(person -> juniorCarpenterCheck.test((String) person[0], (Integer) person[1]))
+            .count();
+    Assertions.assertEquals(3L, juniorCarpenterCount);
+
+    final long juniorWelderCount =
+        Arrays.stream(workers)
+            .filter(person -> juniorWelderCheck.test((String) person[0], (Integer) person[1]))
+            .count();
+    Assertions.assertEquals(2L, juniorWelderCount);
+
+    final long juniorWorkerCount =
+        Arrays.stream(workers)
+            .filter(person -> juniorWorkerCheck.test((String) person[0], (Integer) person[1]))
+            .count();
+    Assertions.assertEquals(5L, juniorWorkerCount);
+
+    final long juniorGroomedCarpenterCount =
+        Arrays.stream(workers)
+            .filter(
+                person -> juniorGroomedCarpenterCheck.test((String) person[0], (Integer) person[1]))
+            .count();
+    Assertions.assertEquals(2L, juniorGroomedCarpenterCount);
+
+    final long allWelderCount =
+        Arrays.stream(workers)
+            .filter(person -> allWelderCheck.test((String) person[0], (Integer) person[1]))
+            .count();
+    Assertions.assertEquals(3L, allWelderCount);
+  }
+
+  @Test
+  void testBiPredicateDefaultMethods() {
+
+    BiPredicate<String, Integer> juniorCarpenterCheck =
+        (worker, age) -> "C".equals(worker) && (age >= 18 && age <= 40);
+
+    BiPredicate<String, Integer> groomedCarpenterCheck =
+        (worker, age) -> "C".equals(worker) && (age >= 30 && age <= 40);
+
+    BiPredicate<String, Integer> allCarpenterCheck =
+        (worker, age) -> "C".equals(worker) && (age >= 18);
+
+    BiPredicate<String, Integer> juniorWelderCheck =
+        (worker, age) -> "W".equals(worker) && (age >= 18 && age <= 40);
+
+    BiPredicate<String, Integer> juniorWorkerCheck = juniorCarpenterCheck.or(juniorWelderCheck);
+
+    BiPredicate<String, Integer> juniorGroomedCarpenterCheck =
+        juniorCarpenterCheck.and(groomedCarpenterCheck);
+
+    BiPredicate<String, Integer> allWelderCheck = allCarpenterCheck.negate();
+
+    // test or()
+    final long juniorWorkerCount =
+        Arrays.stream(workers)
+            .filter(person -> juniorWorkerCheck.test((String) person[0], (Integer) person[1]))
+            .count();
+    Assertions.assertEquals(5L, juniorWorkerCount);
+
+    // test and()
+    final long juniorGroomedCarpenterCount =
+        Arrays.stream(workers)
+            .filter(
+                person -> juniorGroomedCarpenterCheck.test((String) person[0], (Integer) person[1]))
+            .count();
+    Assertions.assertEquals(2L, juniorGroomedCarpenterCount);
+
+    // test negate()
+    final long allWelderCount =
+        Arrays.stream(workers)
+            .filter(person -> allWelderCheck.test((String) person[0], (Integer) person[1]))
+            .count();
+    Assertions.assertEquals(3L, allWelderCount);
+  }
+
+  @Test
+  void testIntPredicate() {
+    IntPredicate isZero = num -> num == 0;
+    IntPredicate isPositive = num -> num > 0;
+    IntPredicate isNegative = num -> num < 0;
+    IntPredicate isOdd = num -> num % 2 == 1;
+
+    IntPredicate isPositiveOrZero = isPositive.or(isZero);
+    IntPredicate isPositiveAndOdd = isPositive.and(isOdd);
+    IntPredicate isNotZero = isZero.negate();
+    IntPredicate isAlsoZero = isPositive.negate().and(isNegative.negate());
+
+    // check zero or greater
+    Assertions.assertArrayEquals(
+        new int[] {0, 1, 2, 3, 4, 5}, IntStream.range(-5, 6).filter(isPositiveOrZero).toArray());
+
+    // check greater than zero and odd
+    Assertions.assertArrayEquals(
+        new int[] {1, 3, 5}, IntStream.range(-5, 6).filter(isPositiveAndOdd).toArray());
+
+    // check not zero
+    Assertions.assertArrayEquals(
+        new int[] {-5, -4, -3, -2, -1, 1, 2, 3, 4, 5},
+        IntStream.range(-5, 6).filter(isNotZero).toArray());
+
+    // check neither positive nor negative
+    Assertions.assertArrayEquals(
+        IntStream.range(-5, 6).filter(isZero).toArray(),
+        IntStream.range(-5, 6).filter(isAlsoZero).toArray());
+  }
+
+  @Test
+  void testLongPredicate() {
+    LongPredicate isStopped = num -> num == 0;
+    LongPredicate firstGear = num -> num > 0 && num <= 20;
+    LongPredicate secondGear = num -> num > 20 && num <= 35;
+    LongPredicate thirdGear = num -> num > 35 && num <= 50;
+    LongPredicate forthGear = num -> num > 50 && num <= 80;
+    LongPredicate fifthGear = num -> num > 80;
+    LongPredicate max = num -> num < 150;
+
+    LongPredicate cityDriveCheck = firstGear.or(secondGear).or(thirdGear);
+    LongPredicate drivingCheck = isStopped.negate();
+    LongPredicate highwayCheck = max.and(forthGear.or(fifthGear));
+
+    // check stopped
+    Assertions.assertArrayEquals(
+        new long[] {0L}, LongStream.of(0L, 40L, 60L, 100L).filter(isStopped).toArray());
+
+    // check city speed limit
+    Assertions.assertArrayEquals(
+        new long[] {20L, 50L}, LongStream.of(0L, 20L, 50L, 100L).filter(cityDriveCheck).toArray());
+
+    // check negate
+    Assertions.assertArrayEquals(
+        new long[] {70L, 100L}, LongStream.of(70L, 100L, 200L).filter(highwayCheck).toArray());
+  }
+
+  @Test
+  void testDoublePredicate() {
+    // weight categories (weight in lbs)
+    DoublePredicate underweight = weight -> weight <= 125;
+    DoublePredicate healthy = weight -> weight >= 126 && weight <= 168;
+    DoublePredicate overweight = weight -> weight >= 169 && weight <= 202;
+    DoublePredicate obese = weight -> weight >= 203;
+    DoublePredicate needToLose = weight -> weight >= 169;
+    DoublePredicate notHealthy = healthy.negate();
+    DoublePredicate alsoNotHealthy = underweight.or(overweight).or(obese);
+    DoublePredicate skipSugar = needToLose.and(overweight.or(obese));
+
+    // check need to lose weight
+    Assertions.assertArrayEquals(
+        new double[] {200D}, DoubleStream.of(100D, 140D, 160D, 200D).filter(needToLose).toArray());
+
+    // check need to lose weight
+    Assertions.assertArrayEquals(
+        new double[] {100D, 200D},
+        DoubleStream.of(100D, 140D, 160D, 200D).filter(notHealthy).toArray());
+
+    // check negate()
+    Assertions.assertArrayEquals(
+        DoubleStream.of(100D, 140D, 160D, 200D).filter(notHealthy).toArray(),
+        DoubleStream.of(100D, 140D, 160D, 200D).filter(alsoNotHealthy).toArray());
+
+    // check and()
+    Assertions.assertArrayEquals(
+        new double[] {200D}, DoubleStream.of(100D, 140D, 160D, 200D).filter(skipSugar).toArray());
+  }
+}
diff --git a/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/SupplierTest.java b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/SupplierTest.java
new file mode 100644
index 000000000..9f22d5f49
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/SupplierTest.java
@@ -0,0 +1,59 @@
+package io.reflectoring.function;
+
+import java.util.Random;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.*;
+import java.util.stream.DoubleStream;
+import java.util.stream.LongStream;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+public class SupplierTest {
+  @Test
+  void supplier() {
+    // Supply random numbers
+    Supplier<Integer> randomNumberSupplier = () -> new Random().nextInt(100);
+    int result = randomNumberSupplier.get();
+    Assertions.assertTrue(result >= 0 && result < 100);
+  }
+
+  @Test
+  void intSupplier() {
+    IntSupplier nextWinner = () -> new Random().nextInt(100, 200);
+    int result = nextWinner.getAsInt();
+    Assertions.assertTrue(result >= 100 && result < 200);
+  }
+
+  @Test
+  void longSupplier() {
+    LongSupplier nextWinner = () -> new Random().nextLong(100, 200);
+    LongStream winners = LongStream.generate(nextWinner).limit(10);
+    Assertions.assertEquals(10, winners.toArray().length);
+  }
+
+  @Test
+  void doubleSupplier() {
+    // Random data for plotting graph
+    DoubleSupplier weightSupplier = () -> new Random().nextDouble(100, 200);
+    DoubleStream dataSample = DoubleStream.generate(weightSupplier).limit(10);
+    Assertions.assertEquals(10, dataSample.toArray().length);
+  }
+
+  @ParameterizedTest
+  @CsvSource(value = {"ON,true", "OFF,false"})
+  void booleanSupplier(String statusCode, boolean expected) {
+    AtomicReference<String> status = new AtomicReference<>();
+    status.set(statusCode);
+    // Simulate a service health check
+    BooleanSupplier isServiceHealthy =
+        () -> {
+          // Here, we could check the actual health of a service.
+          // simplified for test purpose
+          return status.toString().equals("ON");
+        };
+    boolean result = isServiceHealthy.getAsBoolean();
+    Assertions.assertEquals(expected, result);
+  }
+}
diff --git a/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/custom/ArithmeticOperationTest.java b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/custom/ArithmeticOperationTest.java
new file mode 100644
index 000000000..5e1afbeb3
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/custom/ArithmeticOperationTest.java
@@ -0,0 +1,23 @@
+package io.reflectoring.function.custom;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+class ArithmeticOperationTest {
+
+  @Test
+  void operate() {
+    // Define operations
+    ArithmeticOperation add = (a, b) -> a + b;
+    ArithmeticOperation subtract = (a, b) -> a - b;
+    ArithmeticOperation multiply = (a, b) -> a * b;
+    ArithmeticOperation divide = (a, b) -> a / b;
+
+    // Verify results
+    assertEquals(15, add.operate(10, 5));
+    assertEquals(5, subtract.operate(10, 5));
+    assertEquals(50, multiply.operate(10, 5));
+    assertEquals(2, divide.operate(10, 5));
+  }
+}
diff --git a/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/custom/MethodReferenceTest.java b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/custom/MethodReferenceTest.java
new file mode 100644
index 000000000..96ab8c8ba
--- /dev/null
+++ b/core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/custom/MethodReferenceTest.java
@@ -0,0 +1,80 @@
+package io.reflectoring.function.custom;
+
+import java.math.BigInteger;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+/** Method reference test. */
+public class MethodReferenceTest {
+  /** Static method reference. */
+  @Test
+  void staticMethodReference() {
+    List<Integer> numbers = List.of(1, -2, 3, -4, 5);
+    List<Integer> positiveNumbers = numbers.stream().map(Math::abs).toList();
+    positiveNumbers.forEach(number -> Assertions.assertTrue(number > 0));
+  }
+
+  /** The String number comparator. */
+  static class StringNumberComparator implements Comparator<String> {
+    @Override
+    public int compare(String o1, String o2) {
+      if (o1 == null) {
+        return o2 == null ? 0 : 1;
+      } else if (o2 == null) {
+        return -1;
+      }
+      return o1.compareTo(o2);
+    }
+  }
+
+  /** Instance method reference. */
+  @Test
+  void containingClassInstanceMethodReference() {
+    List<String> numbers = List.of("One", "Two", "Three");
+    List<Integer> numberChars = numbers.stream().map(String::length).toList();
+    numberChars.forEach(length -> Assertions.assertTrue(length > 0));
+  }
+
+  /** Instance method reference. */
+  @Test
+  void containingObjectInstanceMethodReference() {
+    List<String> numbers = List.of("One", "Two", "Three");
+    StringNumberComparator comparator = new StringNumberComparator();
+    final List<String> sorted = numbers.stream().sorted(comparator::compare).toList();
+    final List<String> expected = List.of("One", "Three", "Two");
+    Assertions.assertEquals(expected, sorted);
+  }
+
+  /** Instance method arbitrary object particular type. */
+  @Test
+  void instanceMethodArbitraryObjectParticularType() {
+    List<Number> numbers = List.of(1, 2L, 3.0f, 4.0d);
+    List<Integer> numberIntValues = numbers.stream().map(Number::intValue).toList();
+    Assertions.assertEquals(List.of(1, 2, 3, 4), numberIntValues);
+  }
+
+  /** Constructor reference. */
+  @Test
+  void constructorReference() {
+    List<String> numbers = List.of("1", "2", "3");
+    Map<String, BigInteger> numberMapping =
+        numbers.stream()
+            .map(BigInteger::new)
+            .collect(Collectors.toMap(BigInteger::toString, Function.identity()));
+    Map<String, BigInteger> expected =
+        new HashMap<>() {
+          {
+            put("1", BigInteger.valueOf(1));
+            put("2", BigInteger.valueOf(2));
+            put("3", BigInteger.valueOf(3));
+          }
+        };
+    Assertions.assertEquals(expected, numberMapping);
+  }
+}