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Lambda Expressions
Java 8 lambda expressions help eliminate boilerplate code that makes the syntax verbose and less clear. For instance, even basic click listeners syntax can be simplified:
myButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Log.d("debug", "Button clicked");
}
});
With event listeners that only have one method that need to be implemented, lambda expressions can be used to simplify the syntax:
myButton.setOnClickListener(v -> {
Log.d("debug", "Button clicked");
});
Lambda expressions are especially helpful in RxJava as well. Take a look at the code below for creating an Observable and subscribing an Observer to it.
Creating and subscribing to an observable without lambdas:
Observable.just("Hello RxAndroid")
.subscribe(new Action1<String>() {
@Override
public void call(String s) {
Log.d("Emitted", s);
}
});
Consider the same code with lambda expressions:
Observable.just("Hello RxAndroid")
.subscribe(s -> Log.d("Emitted", s));
Make sure you have JDK 8 installed or higher. Click here in case you need to download it. If you are using a continuous integration service, you should also make sure to be specifying a JDK 8 environment.
To use Java 8 lambda expressions in your Android code, you can either use the Gradle Retrolambda plugin developed by Evan Tatarka or use the new Android Jack toolchain.
- Open the root
build.gradle
file and add the following dependency:
buildscript {
dependencies {
classpath 'me.tatarka:gradle-retrolambda:3.2.5'
}
}
- Modify the app module
build.gradle
file to apply the me.tatarka.retrolambda plugin:
apply plugin: 'com.android.application'
apply plugin: 'me.tatarka.retrolambda'
- Add a new
compileOptions
block, thensourceCompatibility
andtargetCompatibility
Java version should be set as 1.8
compileOptions {
sourceCompatibility JavaVersion.VERSION_1_8
targetCompatibility JavaVersion.VERSION_1_8
}
- If you have multiple Java versions installed, set the Java 8 JDK path in
retrolambda
block:
retrolambda {
jdk '/path/to/java-8/jdk'
}
- Using the Android lint detector will trigger a
java.lang.UnsupportedOperationException: Unknown ASTNode child: LambdaExpression
. To get around this issue, you need to add this third-party package that replaces the parsing engine for Java to support lamda expressions:
buildscript {
dependencies {
classpath 'me.tatarka.retrolambda.projectlombok:lombok.ast:0.2.3.a2'
}
}
Android provided a way to use some Java 8 language features including lambda expressions
in your Android project by enabling the Jack toolchain. To do this, edit your module level build.gradle
file as follows:
android {
...
defaultConfig {
...
jackOptions {
enabled true
}
}
compileOptions {
sourceCompatibility JavaVersion.VERSION_1_8
targetCompatibility JavaVersion.VERSION_1_8
}
}
Sync your gradle file, if you encounter any build error, you may need to download the latest Android SDK Build-tools
from the SDK Manager
.
Known issues with using the Jack toolchain
-
Android Studio Instant Run does not currently work with Jack and will be disabled while using the new toolchain.
-
Because Jack does not generate intermediate class files when compiling an app, tools that depend on these files for example, lint detectors, do not currently work with Jack.
-
Tools like
android-apt
which is required for usingDagger 2
in your Android project do not currently work with Jack.
This guide was originally drafted by Adegeye Mayowa
Created by CodePath with much help from the community. Contributed content licensed under cc-wiki with attribution required. You are free to remix and reuse, as long as you attribute and use a similar license.
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