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Lets say I have Object X and Object B. X is an object of a random class, which in turn creates B, and passes itself on to be stored as a variable in object B.

Now object B, wants to run a method in object X, without knowing what class it is.

Is it possible to call a method in any object, and if a method with that name exist, it runs, and if not, it doesn't. I assume some try/catch can work around that part.

But lets say i have something like this:

public class ObjectB {

  public Object parentX;

  public ObjectB(Object x) {
    parentX = x;
  }

  public void anyMethod() {
    x.runMyMethod();
  }
}

In another program I worked with, this was possible with interfaces, and therefore have an idea that it is possible with Java too. However, i cant seem to find the way to set this up. So if the above was the case, how would it be set up, such that ObjectB can call the method on all the classes in my program if that was what i wanted?

GhostCat
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Danfjo
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    If you want to invoke that method, `x` has to be an instance of a class implementing that interface. (And do you mean `parentX`?) – Andy Turner Jul 04 '17 at 18:59
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    No, the only way to do that would be using reflection, but reflection is most of the time not the right answer. You should probably do that by fixing your design. But without knowing what you want to achieve, at a higher level, it's hard to answer. – JB Nizet Jul 04 '17 at 19:00
  • Use the interface, instead of `Object` as the type of `x` and `parentX`. – Dawood ibn Kareem Jul 04 '17 at 19:06
  • Some other languages are interpreted (or partly interpreted), which means that when you call a method on an object, the program can search the object when the call occurs, to see if the object defines a method with that name. Java isn't like that. It wants to know at _compile time_ whether the object has the needed method. – ajb Jul 04 '17 at 19:30

3 Answers3

2

Now object B, wants to run a method in object X, without knowing what class it is.

Simply doesn't make sense. In order to invoke a method, you need to know its name and the expected parameters. In that sense, class B needs some knowledge. Thus, you would/could do something like

if (parentX instanceof ClassA) {
 ( (ClassA) parentX ) . someAMethod(bla, blub); 
} 
if (parentX instanceof InterfaceC) {
 ( (InterfaceC) parentX ) . someCMethod(bla, blub); 
}

In that sense: you might use an interface here; but classes work as well. The key thing is: you need a certain piece of information; otherwise you are simply stuck.

The only other alternative is to look into reflection - then you would not need to know the specific type; as you can inspect parentX to find out which methods could be called on that object. But reflection is A) an advanced concept and b) intricate in its usage.

GhostCat
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2

Yes its definitely possible.

Define interface MyIntf, and your Class X implements it. now in Class B you can assign the object of Class X (or any class that implements MyIntf) to parent which of type MyIntf the you can invoke method with that parent

    interface MyIntf
    {
       void runMyMethod()
    }

    public class ObjectX implements MyIntf {
           void runMyMethod()
           {
           }
    }

    public class ObjectB { 

      public MyIntf parentX;

      public ObjectB(MyIntf x) {
        parentX = x;
      } 

      public void anyMethod() { 
        parentX.runMyMethod(); 
      } 
    }
Pavan
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0

If you look at the end of this documentation, it tells you about casting. Casting allows the program to assume the object is of a type and attempt to call a method within it.

public void anyMethod() {
     ((MethodParentObject) this.x).runMyMethod();
}
Cardinal System
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