Lambdas and JVM bytecode - Java 8 in Action: Lambdas, Streams, and Functional-Style Programming - page 464

Let's try to implement an instance of the Function interface using the old Java 7 syntax, as an
anonymous inner class, as shown in the following listing.

Listing D.1. A Function implemented as an anonymous inner class

import java.util.function.Function;
public class InnerClass {
Function<Object, String> f = new Function<Object, String>() {
@Override
public String apply(Object obj) {
return obj.toString();

}

};

}

Doing this, the corresponding generated bytecode for the Function created as an anonymous
inner class will be something along the lines of this:

0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V

4: aload_0
5: new #2 // class InnerClass$1

8: dup
9: aload_0
10: invokespecial #3

// Method InnerClass$1."<init>":(LInnerClass;)V

13: putfield #4 // Field f:Ljava/util/function/Function;

16: return

This code shows the following:

 An object of type InnerClass$1 is instantiated using the bytecode operation new . A reference to the
newly created object is pushed on the stack at the same time.
 The operation dup duplicates that reference on the stack.
 This value then gets consumed by the instruction invokespecial , which initializes the object.
 The top of the stack now still contains a reference to the object, which is stored in the f1 field of the
LambdaBytecode class using the putfield instruction.

InnerClass$1 is the name generated by the compiler for the anonymous class. If you want to
reassure yourself, you can inspect the InnerClass$1 class file as well, and you'll find the code for
the implementation of the Function interface: