对特定类型的所有对象的Java调用方法

Question

Okay, the title is a little bit cryptic, an example will show better what I mean, suppose the following structure:

interface I { 
    methodCall();
}

class A implements I {
}

class B implements I {
}

class C implements I {
}

class Main {
    private A a;
    private B b;
    private C c;

    //other interesting stuff

    void doSomeMainMethod() {
        a.methodCall();
        b.methodCall();
        c.methodCall();
    }
}

This code has been heavily simplified, the classes A , B and C implement methodCall() obviously, but there is no need to explicitely show that in the code.

What I want to ask is the following:
Is there a way to tell Java to generate my doSomeMainMethod method? I want to tell Java to call methodCall() on all objects of type I in class Main .

Preferably without the use of reflection, because with reflection I think it is possible or if reflection is needed, is there a way to wrap it up such that it at least looks non-hackish? Ofcourse it needs to be safe (as safe as possible) aswell.

Answer 1

class Main {
private A a;
private B b;
private C c;
private I[] instances = new I[]{a, b, c};

//other interesting stuff

void doSomeMainMethod() {
    for(I instance : instances) {
        instance.methodCall();
    }
}

Answer 2

If you want the method to be generated then you can use ASM generation or (probably simpler option) implement your own AnnotationProcessor. With the second option you should annotate your classes and then scan for this annotation in compile time and generate decired method.

Answer 3

interface I {
  void methodCall();
  List<I> INSTANCES = new ArrayList<I>();
}

Now, in every constructor of classes that implements interface I, do this:

I.INSTANCES.add(this);

In main:

for (I i : I.INSTANCES) {
  // do something with every instance of I
}

EDIT: This will add all instances of I. To limit it to the objects created in Main, let´s add a parameter, createdBy. It is not quite elegant, but it is just Java, no Reflection ...

public A(Class createdBy) {
  if (createdBy.equals(Main.class) {
    I.INSTANCES.add(this);
  }
}