可以避免使用Class :: method语法对Lambda进行装箱和拆箱吗？ （JAVA）

Question

I've recently stumbled upon the Class::method syntax which allows you to insert lambda code which from a static method like this:

public class Main {

  public static void foo(int foo) {
    System.out.println(foo);
  }

  public static void bar(Consumer<Integer> bar) {
    bar.accept(1);
  }

  public static void main(String[] args) {
    bar(Main::foo);
  }

}

The question is, does boxing and unboxing of 1 still occur? After all, bar 's parameter is a Consumer<Integer> which should normally box primitives, yet foo(int) is a method which accepts a primitive, thus boxing is not neccessary.

So, what happens? Does 1 get turned into an Integer or does it remain primitive?

On a side note, I am aware that IntConsumer delivers a solution to get rid of boxing and unboxing, but not every single functional interface has an alternative for every single primitive type, hence the question.

Answer 1

Yes it is boxed and then unboxed.

Compiling the code and then decompiling the class file reveals the following:

import java.util.function.Consumer;

public class Main {
    public Main() {
    }

    public static void foo(int foo) {
        System.out.println(foo);
    }

    public static void bar(Consumer<Integer> bar) {
        bar.accept(Integer.valueOf(1));
    }

    public static void main(String[] args) {
        bar(Main::foo);
    }
}

The compiler automatically boxes 1, meaning it will be unboxed when foo() is called.

Answer 2

If the answer you accepted surprises you, than because you don't know enough about Java yet. So don't call it a disappointment, but a disillusion .

In contrast to C++ templates Java generics work with for reference types only .

Thus there is absolutely no way any generic-using construct can directly work with primitive types! So whenever you use a primitive type value in such a context, it must be boxed.

Maybe, one day far in the future, when the parents of Java decide to redo generics, that might change, but we are not there yet.