How Lambdas and Anonymous Inner Classes Work

1. Key Points

Lambdas implement a functional interface.
Anonymous Inner Classes can extend a class or implement an interface with any number of methods.
Variables – Lambdas can only access final or effectively final.
State – Anonymous inner classes can use instance variables and thus can have state, lambdas cannot.
Scope – Lambdas can’t define a variable with the same name as a variable in enclosing scope.
Compilation – Anonymous compiles to a class, while lambda is an invokedynamic instruction.

2. How They Work

2.1 Anonymous Inner Classes (AICs)

The compiler generates a class file for each anonymous inner class.
- For example – AnonymousInnerClass$1.class
Like all classes, it needs to be loaded and verified at startup.

2.2 Lambdas

The key to lambda implementation is the InvokeDynamic instruction, introduced in Java 7. This allows dynamic languages to bind to symbols at runtime.

A lambda works like this:

Generates invokedynamic call site and uses a lambdafactory to return the functional implementation.
Lambda converted to a method to be invoked by invokedynamic.
The method is stored in a class as a private static method.
There are two lambda types. Non-capturing lambdas only use fields inside their bodies, whereas capturing lambdas access fields outside their bodies.

2.2.1 Non-Capturing Lambda

Doesn’t access fields outside its body:

public class NonCapturingLambda {
  public static void main(String[] args) {
    Runnable nonCapturingLambda = () -> System.out.println("NonCapturingLambda");
    nonCapturingLambda.run();
  }
}

If we decode the class file using the CFR decompiler, we see the:

LambdaMetafactory
Lambda is a static void method in our class

java -jar cfr_0_119.jar NonCapturingLambda --decodelambdas false
/*  * Decompiled with CFR 0_119.  */
import java.io.PrintStream;
import java.lang.invoke.LambdaMetafactory;
public class NonCapturingLambda {
    public static void main(String[] args) {
        Runnable nonCapturingLambda = (Runnable)LambdaMetafactory.metafactory(null,
           null, null, ()V, lambda$0(), ()V)();
        nonCapturingLambda.run();
    }
    private static /* synthetic */ void lambda$0() {
        System.out.println("NonCapturingLambda");
    }
}

2.2.2 Capturing Lambdas

Accesses final or effectively final fields outside their bodies:

public class CapturingLambda {
    public static void main(String[] args) {
        String effectivelyFinal = "effectivelyFinal";
        Runnable capturingLambda = () -> System.out.println("capturingLambda " + effectivelyFinal);
        capturingLambda.run();
    }
}

This decompiles to:

java -jar cfr_0_119.jar CapturingLambda --decodelambdas false
/*  * Decompiled with CFR 0_119.  */
import java.io.PrintStream;
import java.lang.invoke.LambdaMetafactory;
public class CapturingLambda {
    public static void main(String[] args) {
        String effectivelyFinal = "effectivelyFinal";
        Runnable capturingLambda =
          (Runnable)LambdaMetafactory.metafactory(null, null, null, ()V,
                                                  lambda$0(java.lang.String ),
                                                  ()V)((String)effectivelyFinal);
          capturingLambda.run();
    }
    private static /* synthetic */ void lambda$0(String string) {
        System.out.println("capturingLambda " + string);
    }
}

The interesting part is that the lambda$0 method signature has gone from empty to taking a parameter String.

Understanding Lambdas and Anonymous Inner Classes