CODEWITHSUNDEEP
javalambdajava-8
Alexandr
Alexandr

Reputation: 3969

Java - Is functional interface "lever" for lambda expressions?

Is it true my statement?

There is my code:

public void start() {
    Consumer<Integer> someFunc = (someInt) -> {
        System.out.println("Hello lambda!");
    };
}

And there is bytecode of my code:

~ Start method

  // access flags 0x1
  public start()V
   L0
    LINENUMBER 9 L0
    INVOKEDYNAMIC accept()Ljava/util/function/Consumer; [
      // handle kind 0x6 : INVOKESTATIC
      java/lang/invoke/LambdaMetafactory.metafactory(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
      // arguments:
      (Ljava/lang/Object;)V, 
      // handle kind 0x6 : INVOKESTATIC
      me/alexandr/SomeMainClass.lambda$start$0(Ljava/lang/Integer;)V, 
      (Ljava/lang/Integer;)V
    ]
    ASTORE 1
   L1
    LINENUMBER 12 L1
    ALOAD 1
    ICONST_1
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    INVOKEINTERFACE java/util/function/Consumer.accept (Ljava/lang/Object;)V
   L2
    LINENUMBER 13 L2
    RETURN
   L3
    LOCALVARIABLE this Lme/alexandr/SomeMainClass; L0 L3 0
    LOCALVARIABLE someFunc Ljava/util/function/Consumer; L1 L3 1
    // signature Ljava/util/function/Consumer<Ljava/lang/Integer;>;
    // declaration: java.util.function.Consumer<java.lang.Integer>
    MAXSTACK = 2
    MAXLOCALS = 2

~ Translated lambda expression

  // access flags 0x100A
  private static synthetic lambda$start$0(Ljava/lang/Integer;)V
   L0
    LINENUMBER 10 L0
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    LDC "Hello lambda!"
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L1
    LINENUMBER 11 L1
    RETURN
   L2
    LOCALVARIABLE someInt Ljava/lang/Integer; L0 L2 0
    MAXSTACK = 2
    MAXLOCALS = 1

As far as I can see - a lambda expression translated into a static method. So, can I say that functional interface is "lever" for the static method that allow me to call it?

Upvotes: 1

Views: 272

Answers (1)

Peter Lawrey
Peter Lawrey

Reputation: 533530

If you use the -> notation, the actual method implementing the code is placed in a `static method. The lambda created at runtime will call this method.

If you use the :: notation, there is no additional method required and the lambda will be generated at runtime to call the ::method

So, can I say that functional interface is "lever" for the static method that allow me to call it?

You can call it a "lever" if you like. It might be called a bridging functions. As @Dici points out the term javap uses is "bootstrap method" Note: these bootstrap methods are stored as Strings in the meta-data and don't appear as Method in reflection or in the byte code via ASM (the library the JVM uses internally) ASM sees them as meta data.

Upvotes: 4

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