Calling method in object from object within it

Question

I have the following problem, and no idea how to solve it. Let's say we have a class classA, and in this class is the main method creating an object of the class itself. Now we take another class, classB. In the constructor of classA we make an object of classB. Now in a method of classB we want to call a method of classA.

Let me provide you with an example.

public class classA {
    public classA() {
        //some code
        classB objectB = new classB();
    }

    public static void main(String[] args) {
        classA objectA = new classA();
        //more code
    }

    public void methodA() {
        //even more code
    }
}

public class classB {
    public void someListener() {
        //code needed to call methodA of the object objectA
    }
}

The question is: what would the code be, where there is now just the comment //code needed to call methodA of the object objectA?

The reason I'm in this situation, is that in the code that I'm going to use it for, there are various methods running in objectB controlled by loops, but once a certain thing happens a method in what is shown here as objectA has to be called. How do I do this?

An SO User · Accepted Answer · 2013-05-25 19:02:06Z

Simple ... just pass the reference of class A object to object of class B. Let class B store the reference and call the methods of A whenever necessary.

New Code (commented)

public class classA{
    classA{
        //some code
        classB objectB = new classB(this);  //------------- pass reference here
    }
    public static void main(String[] args){
        classA objectA = new classA();
        //more code
    }
    public void methodA(){
        //even more code
    }
}

public class classB{
    private classA storedReference = null; //------------- you need a variable of classA to store the reference

    public classB(classA passedObject){   //------------- you need an appropriare constructor
        storedReference = passedObject;   //------------- store the classA object reference
    }

    public void someListener(){
        storedReference.methodA();       //------------- call methods whenever necessary
        //code needed to call methodA of the object objectA
    }
}

Thanks, it really helped. I didn't know of references and such, but know I can see how this might look like a stupid question :P. Thank you very much!

Brian Agnew · Accepted Answer · 2013-05-25 19:02:41Z

1

You can pass a reference to ClassA to the constructor in ClassB e.g

class ClassB(ClassA ref) {
    ref.someMethod();
}

However you need to be careful if you're calling a method on an instance of a class that you're constructing. The class may not have completed its construction and may be be in an incomplete state.

answered May 25, 2013 at 19:02

Brian Agnew

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3 Comments

An SO User Over a year ago

Yes, there has to be a better strategy for event handling. OP needs to figure this out :) Maybe have static methods to get appropriate data ? :)

Lara Over a year ago

I know it can be done easier when it's an event. I just used eventhandler as an example, because where the method of objectA has to be called is a method as a result of a method called by the eventhandler, but I thought that would be more infomation than useful :P

An SO User Over a year ago

@user1816721 I am glad I could help. Just consider what Brian Agnew said. Use it only when you are completely sure that the class is fully constructed :)

Losiowaty · Accepted Answer · 2013-05-25 19:06:02Z

0

You need to add constructor to your classB, so that you can pass a classA as reference to it.

So your classB becomes:

public class classB {
    classA cA;

    public classB(classA cA) {
        this.cA = cA;
    }

   public void someListener(){
        cA.methodA();
   }

}

And your classA constructor :

classA{
    //some code
    classB objectB = new classB(this);
}

answered May 25, 2013 at 19:06

Losiowaty

8,0062 gold badges34 silver badges47 bronze badges

Comments

Makoto · Accepted Answer · 2013-05-25 19:13:06Z

In ClassB, you want to call a method of ClassA. That's not difficult to do, but I believe you have your relationships incorrect.

If you're using the current code you have: You need to pass an instance of ClassA to ClassB, which can be accomplished using the this operator:

public classA() {
    //some code
    classB objectB = new classB();
    objectB.someListener(this);
}

In classB:

public void someListener(classA theClassA) {
    //code needed to call methodA of the object objectA
    theClassA.methodA();
}

If you want to reverse the relationships between the two: Instead of having an instance of classB in classA that relies on classA to do its work, have classB be the dominant object, and have it contain and instance of classA.

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