The lambda functionality in java 8 is very well documented - I found the following to be more than enough!

As of 0_11 CFR is able to decompile lambda expressions... as of 0_27, post beta103 lambdas.

please note - when desugaring lambdas, CFR relies on your JRE, i.e. if you're trying to desugar a call to Consumer::andThen, and you've got Java 7 - CFR will not be able to desugar it!

What's particularly interesting (to me!) is that before I dug into this, I was expecting lambdas to simply be yet more syntactic sugar around anonymous inner classes - indeed Idea 12 'hides' anonymous inners behind lambda syntax, which is quite funny...

CFR treatment

Original code

public class LambdaTest2 {

    Integer invoker(int arg, UnaryFunction<Integer, Integer> fn) {
        return fn.invoke(arg);
    }

    public int test(int y) {
        return invoker(3, x -> x + y + 1);
    }

}

CFR 0_11 decompilation, with --decodelambdas false

There are a couple of things of interest here, which mean that un-desugared, we can't treat this as real java:

• the lambdaMetafactory (as referenced above) takes MethodHandles and MethodTypes, which aren't nice to represent as literals, and the first three arguments are filled in by the JVM, so I treat them as null.
• ... and the curried arguments can't really be represented - I've shown this as (y) after the metaFactory call.

public class LambdaTest2
{

    public LambdaTest2(){
    }

    Integer invoker(int arg, UnaryFunction<Integer, Integer> fn){
        return fn.invoke(Integer.valueOf(arg));
    }

    public int test(int y){
        return this.invoker(3, (UnaryFunction)LambdaMetafactory.metaFactory(null, null, null, 
            invoke(X ), lambda$2(int Integer ), ConstantUTF8[(Ljava/lang/Integer;)Ljava/lang/Integer;])(y)).intValue();
    }

    private static /* synthetic */ Integer lambda$2(int v0, Integer v1){
        return Integer.valueOf((v1.intValue() + v0 + 1));
    }

}

CFR 0_11 decompilation with lambda desugaring

As of 0_11, I haven't figured out if it's possible to determine the names which were given to the lambda arguments, so I'm making up names for them...

public class LambdaTest2
{

    public LambdaTest2(){
    }

    Integer invoker(int arg, UnaryFunction<Integer, Integer> fn){
        return fn.invoke(Integer.valueOf(arg));
    }

    public int test(int y){
        return this.invoker(3, arg_0 -> Integer.valueOf((arg_0.intValue() + y + 1))).intValue();
    }

}

Appendix - Bytecode

public class org.benf.cfr.tests.LambdaTest2 {
  public org.benf.cfr.tests.LambdaTest2();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        

  java.lang.Integer invoker(int, org.benf.cfr.reader.util.functors.UnaryFunction);
    Code:
       0: aload_2       
       1: iload_1       
       2: invokestatic  #2                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
       5: invokeinterface #3,  2            // InterfaceMethod org/benf/cfr/reader/util/functors/UnaryFunction.invoke:(Ljava/lang/Object;)Ljava/lang/Object;
      10: checkcast     #4                  // class java/lang/Integer
      13: areturn       

  public int test(int);
    Code:
       0: aload_0       
       1: iconst_3      
       2: iload_1       
       3: invokedynamic #5,  0              // InvokeDynamic #0:lambda$:(I)Lorg/benf/cfr/reader/util/functors/UnaryFunction;
       8: invokevirtual #6                  // Method invoker:(ILorg/benf/cfr/reader/util/functors/UnaryFunction;)Ljava/lang/Integer;
      11: invokevirtual #7                  // Method java/lang/Integer.intValue:()I
      14: ireturn       

  private static java.lang.Integer lambda$2(int, java.lang.Integer);
    Code:
       0: aload_1       
       1: invokevirtual #7                  // Method java/lang/Integer.intValue:()I
       4: iload_0       
       5: iadd          
       6: iconst_1      
       7: iadd          
       8: invokestatic  #2                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      11: areturn       
}