How is the Java ByteCode generated for Lambda Expressions in Java?

How is the Java ByteCode generated for Lambda Expressions in Java?

Here is an article that explains exactly how lambda expressions are compiled: Java 8 Lambdas - A Peek Under the Hood

And here another (older, from 2012) article from the horse's mouth (Brian Goetz, who led the lambda project at Oracle): Translation of Lambda Expressions

Note: This is not really simple to understand; but the most important thing to know is that it's not as simple as replacing the lambda by an anonymous inner class. Other things are done, involving the invokedynamic bytecode instruction, which make lambdas more efficient than anonymous inner classes. So, that is in itself a good reason to use lambda expressions instead of anonymous inner classes (besides the fact that lambda expressions enable you to write shorter and more readable code compared to anonymous inner classes).

Why don't we have a look?
import java.util.*; import java.util.function.*; final class LambdaTest {    private static final Optional<String> OPTIONAL_GREETING = Optional.of("Hello World!");  static void useLambda() {    OPTIONAL_GREETING.ifPresent(greeting -> System.out.println(greeting));  }    static void useAnonymousClass() {    OPTIONAL_GREETING.ifPresent(new Consumer<String>() {      @Override      public void accept(String greeting) {        System.out.println();      }    });  } }final class LambdaTest$1 implements java.util.function.Consumer<java.lang.String> {  LambdaTest$1();    Code:       0: aload_0       1: invokespecial #1                  // Method java/lang/Object."<init>":()V       4: return  public void accept(java.lang.String);    Code:       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;       3: invokevirtual #3                  // Method java/io/PrintStream.println:()V       6: return  public void accept(java.lang.Object);    Code:       0: aload_0       1: aload_1       2: checkcast     #4                  // class java/lang/String       5: invokevirtual #5                  // Method accept:(Ljava/lang/String;)V       8: return }final class LambdaTest {  private static final java.util.Optional<java.lang.String> OPTIONAL_GREETING;  LambdaTest();    Code:       0: aload_0       1: invokespecial #1                  // Method java/lang/Object."<init>":()V       4: return  static void useLambda();    Code:       0: getstatic     #2                  // Field OPTIONAL_GREETING:Ljava/util/Optional;       3: invokedynamic #3,  0              // InvokeDynamic #0:accept:()Ljava/util/function/Consumer;       8: invokevirtual #4                  // Method java/util/Optional.ifPresent:(Ljava/util/function/Consumer;)V      11: return  static void useAnonymousClass();    Code:       0: getstatic     #2                  // Field OPTIONAL_GREETING:Ljava/util/Optional;       3: new           #5                  // class LambdaTest$1       6: dup       7: invokespecial #6                  // Method LambdaTest$1."<init>":()V      10: invokevirtual #4                  // Method java/util/Optional.ifPresent:(Ljava/util/function/Consumer;)V      13: return  private static void lambda$useLambda$0(java.lang.String);    Code:       0: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;       3: aload_0       4: invokevirtual #8                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V       7: return  static {};    Code:       0: ldc           #9                  // String Hello World!       2: invokestatic  #10                 // Method java/util/Optional.of:(Ljava/lang/Object;)Ljava/util/Optional;       5: putstatic     #2                  // Field OPTIONAL_GREETING:Ljava/util/Optional;       8: return }
As you can see, the compiler generates an entire class with a constructor and two methods for the anonymous consumer, while it generates one private static method for the lambda expression: lambda$useLambda$0(java.lang.String).

useAnonymousClass() performs new, dup and invokespecial to create an instance of LambdaTest$1 and put it on the operand stack, while useLambda() performs invokedynamic. Both methods then use invokevirtual to call Optional.ifPresent() on whatever is on the operand stack. This means that whatever invokedynamic does with lambda$useLambda$0(java.lang.String), the result must be something that's compatible with Consumer<String>.