Using Java 8 Lambda Expressions in Java 7 or Older

I think nobody declines the usefulness of Lambda expressions, introduced by Java 8. However, many projects are stuck with Java 7 or even older versions. Upgrading can be time consuming and costly. If third party components are incompatible with Java 8 upgrading might not be possible at all.
Besides that, the whole Android platform is stuck on Java 6 and 7. 

Nevertheless, there is still hope for Lambda expressions!

Retrolambda provides a backport of Lambda expressions for Java 5, 6 and 7.

From the Retrolambda documentation:

Retrolambda lets you run Java 8 code with lambda expressions and method references on Java 7 or lower. It does this by transforming your Java 8 compiled bytecode so that it can run on a Java 7 runtime. After the transformation they are just a bunch of normal .class files, without any additional runtime dependencies.

To get Retrolambda running, you can use the Maven or Gradle plugin.

If you want to use Lambda expressions on Android, you only have to add the following lines to your gradle build files:

<project>/build.gradle:

buildscript {
  dependencies {
    classpath 'me.tatarka:gradle-retrolambda:2.4.0'    
  }
}

<project>/app/build.gradle:

apply plugin: 'com.android.application'
 
// Apply retro lambda plugin after the Android plugin
apply plugin: 'retrolambda' 
 
android {
  compileOptions {
    // change compatibility to Java 8 to get Java 8 IDE support  
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
  }
}