Lambda behaving differently than anonymous inner class

Question

While doing some basic lambda exercises, the output from an apparently identical anonymous inner class was giving me a different output than the lambda.

interface Supplier<T> {      T get(T t); } 

Scenario #1

Supplier<Integer> s1 = new Supplier<Integer>() {     @Override     public Integer get(Integer t) {         return t;     } }; Supplier<Integer> s2 = t -> t; System.out.println(s1.get(2)); System.out.println(s2.get(2)); 

Outputs 2 and 2. Nothing new here.

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But when I do this:

Scenario #2

Supplier<Integer> s1 = new Supplier<Integer>() {     @Override     public Integer get(Integer t) {         return t++;     } }; Supplier<Integer> s2 = t -> t++; System.out.println(s1.get(2)); System.out.println(s2.get(2)); 

Outputs 2 and 3

QUESTION: Shouldn't both outputs be identical? Am I missing something?

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For the sake of completeness: Scenario #3

Supplier<Integer> s1 = new Supplier<Integer>() {     @Override     public Integer get(Integer t) {         return ++t;     } }; Supplier<Integer> s2 = t -> ++t; System.out.println(s1.get(2)); System.out.println(s2.get(2)); 

Outputs 3 and 3. Nothing new here as well.

UPDATE: Still getting same output from 1.8.0-b132

UPDATE #2: Bug report: https://bugs.openjdk.java.net/browse/JDK-8038420

UPDATE #3: The bug has been fixed in javac, you should be able to obtain the same result now.

Answer 1

According to generated bytecode:

Java(TM) SE Runtime Environment (build 1.8.0-b132)

Lambda:

 private static java.lang.Integer lambda$main$0(java.lang.Integer);    descriptor: (Ljava/lang/Integer;)Ljava/lang/Integer;    flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC    Code:      stack=2, locals=2, args_size=1         0: aload_0         1: invokevirtual #9                  // Method java/lang/Integer.intValue:()I         4: iconst_1         5: iadd         6: invokestatic  #6                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;         9: dup        10: astore_0        11: astore_1        12: aload_0        13: areturn      LineNumberTable:        line 20: 0      LocalVariableTable:        Start  Length  Slot  Name   Signature            0      14     0     t   Ljava/lang/Integer; 

Anonymous class:

  public java.lang.Integer get(java.lang.Integer);     descriptor: (Ljava/lang/Integer;)Ljava/lang/Integer;     flags: ACC_PUBLIC     Code:       stack=2, locals=4, args_size=2          0: aload_1          1: astore_2          2: aload_1          3: invokevirtual #2                  // Method java/lang/Integer.intValue:()I          6: iconst_1          7: iadd          8: invokestatic  #3                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;         11: dup         12: astore_1         13: astore_3         14: aload_2         15: areturn       LineNumberTable:         line 16: 0       LocalVariableTable:         Start  Length  Slot  Name   Signature             0      16     0  this   LTest$1;             0      16     1     t   Ljava/lang/Integer; 

As you can see, in anonymous class after loading variable from local variable table (method parameter t) runtime store copy of parameter in another variable (astore_2) and then use this copy of parameter as returning value.

Lambda method doesn't make copy of parameter (load -> unbox -> add 1 -> box -> store -> load -> return).

UPDATE

It's definitely a javac bug.

I got source from http://hg.openjdk.java.net/jdk8u/jdk8u

Anonymous class and lambda converts to following intermediate representations:

@Override() public Integer get(Integer t) {     return (let /*synthetic*/ final Integer $112619572 = t in         (let /*synthetic*/ final Integer $1295226194 = t = Integer.valueOf((int)(t.intValue() + 1)) in $112619572)); }  /*synthetic*/ private static Integer lambda$main$0(final Integer t) {     return (let /*synthetic*/ final Integer $1146147158 = t = Integer.valueOf((int)(t.intValue() + 1)) in t); } 

In lambda generated method parameter marked as final, because LambdaToMethod translator marks all parameters as FINAL (according source code LambdaTranslationContext.translate(…):1899).

Then let expression builder checks variable flags and when if it’s final omits temporary variable generation (according source code Lower.abstractRval(…):2277), because modification considered to be prohibited.

Possible solutions:

1. Forbid parameter modification inside lambda or

2. Remove FINAL flag from local variable (LambdaTranslationContext.translate(…):1894) and parameter (LambdaTranslationContext.translate(…):1899) in lamda generated method:

    case LOCAL_VAR:    ret = new VarSymbol(FINAL, name, types.erasure(sym.type), translatedSym);  ...   case PARAM:    ret = new VarSymbol(FINAL | PARAMETER, name, types.erasure(sym.type), translatedSym);  ... 

I removed FINAL flag and got expected results on tests from: https://bugs.openjdk.java.net/browse/JDK-8038420