在Java 8中是否可以使用方法引用定义/实现方法？

Question

Is there any way to define methods via method references, as demonstrated by the following imaginary code (and imaginary syntax)?

class A
{
    public Sometype somefunction(Argtype arg)
    { ... return somevalue; ..}
}

class B
{
    public Sometype method = A::somefunction;
}

or

class C
{
    public Sometype method(Argtype arg) = A::somefunction;
}

A similar feature would seem quite useful to me. Methods are known at compile time, so any definition in class B or C might simply (after a type check) take over a reference to an already known method in A . The advantage: faster calls (one less required than even in a direct call from a new method) - where I am not sure whether Java compilers do not optimize anyway?

Is there any way to achieve that already now in Java (if so, it has evaded my search successfully)? Or are there good reasons not to introduce that to Java? Are there any workarounds which provide similar performance advantage?

PS: I get the message that similar question titles have been downvoted and/or closed. If this applies here, too, please help me to understand why. Thanks in advance!

Answer 1

This question can be split in 2 different ones.

1. How can I use class A methods from other classes
2. How can I specify the return type of a method at runtime

Regarding the first one, take a look at Java inheritance, polymorphism, abstract classes and interfaces.

I would have defined class A as abstract:

abstract class A{
  public Sometype somefunction(Argtype arg)
     { ... return somevalue; ..}
}

and B, C extends it:

class B extends A{
}

In this way B and C inherits all the methods implemented in A and you can perform something like this:

B b = new B();
b.somefunction(arg);

Moreover, referring to the second question if you don't know the return type of this method at compile time you can exploit java generics (if polymorphism isn't enough). In this way class A becomes:

abstract class A{
    public T somefunction(Class<T> type)
    { ... return somevalue; ..}
}

and the previous example code becomes:

B b = new B();
b.somefunction(String.class);

Answer 2

You could do something like this:

class A {
    public static Sometype someStaticFunction(Argtype aArg) {
      ...
    }

    public Sometype someNonStaticFunction(Argtype aArg) {
      ...
    }
}

class B {
    private Function<Argtype, Sometype> mStaticFunction = A::someStaticFunction;

    private Function<Argtype, Sometype> mNonStaticFunction = new A()::someNonStaticFunction;

    public Sometype applyStatic(Argtype aArg) {
      return mStaticFunction.apply(aArg);
    }

    public Sometype applyNonStatic(Argtype aArg) {
      return mNonStaticFunction.apply(aArg);
    }
}

However I don't see any point in this, regrading design, performance or any other aspects.