How to enable Java 8 language features in Android studio

Clean answer-

Just add following to app level build.gradle and Sync

android {
  ...
  compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
  }
}

Enable Java 8 Support:

To start using supported Java 8 language features, update the Android plugin to 2.4.0-alpha4 (or higher) and add the following to your module’s build.gradle file:

android {
  ...
  compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
  }
}

Disable jackOptions:

We can disable Jack and switch to the default toolchain, by removing the jackOptions block from module’s build.gradle file:

android {
    ...
    defaultConfig {
        ...
        // Remove this block.
        jackOptions {
            enabled true
        }
    }

}

Note: If your project is using Jack, Retrolambda, or DexGuard, then Android studio default uses Java 8 support provided by those tool.

Disable Java 8 Support:

We can also disable Java 8 features in your project in case you are facing any issue related Java 8. We can update gradle.properties file by adding below line to disable Java 8 features:

android.enableDesugar=false

Check Use Java 8 language features for more details about Java 8 features.

I know this has been already answered, but after the new Gradle and android studio update, jackOptions is deprecated.

 android {
      .....

        defaultConfig {
        ..........
            //remove jackOptions and add
            android.compileOptions.sourceCompatibility 1.8
            android.compileOptions.targetCompatibility 1.8

        }
        // Keep the following configuration in order to target Java 8.
         compileOptions {

           sourceCompatibility JavaVersion.VERSION_1_8
           targetCompatibility JavaVersion.VERSION_1_8
       }
  }

Simple process -

Right click on Project > Open Module Setting (F4) > Modules (app) >

Select -

Source Compatibility - 1.8
Target Compatibility - 1.8