A类实例化B类实例。 ClassB的实例如何调用A类的方法？

Question

I have 2 classes, and I have made one class (Class A) instantiate a Class B object.

I have a method in Class B that I want to call a method in Class A.

I'm working on a larger project for practicing Java, so I am simplifying things here.

// Class A:

public class ClassA {
  private int number;
  private ClassB instanceOfB = new ClassB();

  public ClassA {
    number = 0;
  }

  public void incrementNumber {
    number++;
  }

  public void incrementNumberLongWay {
    instanceOfB.incrementNumberInA()
  }
} 

// Class B:

public class ClassB {
  public void incrementNumberInA() {
    // My desire: Call Class A's incrementNumber method
    // What should I put here?
   }

}

How do I make sure incrementNumberLongWay works? Class A has been instantiated, and it's method incrementNumberLongWay is called, so this should call ClassB's method incrementNumberInA

I know this seems extremely convoluted, but the reason I'm doing this, is because in my program I'm not incrementing numbers, but instead doing some logic in Class B, and only wanting to affect Class A in certain cases.

Answer 1

You can't do this with the code provided. Relationships are by default one way. B doesn't know about A so cannot access it.

What you can do is pass a reference of A to B in it's construction process and then access A via that reference.

Answer 2

ClassB doesn't know anything about ClassA. So, you couldn't do it. The ugly decision is

public void incrementNumberLongWay() {
        instanceOfB.incrementNumberInA(this);
    }

and in

public class ClassB {
    public void incrementNumberInA(ClassA cl) {
        cl.incrementNumber();
    }
}

Answer 3

You can't call methods from class A from class B as class B has no reference to an object of class a. You could, however, pass class A's current number state to class B as parameter, and return a value from class B which class A can then get and use.

For example:

public class A {
    private int number;
    public A(int number) {
        this.number = number;
    }
    public void incrementNumber(boolean largeIncrement) {
        if(largeIncrement) {
            B bInstance = new this.B();
            number = bInstance.incrementNumberLongWay(number);
        }
        else {
            number++;
        }
    }
    private class B {
        private B() {
            // if some initialization is needed...
        }
        public int incrementNumberLongWay(int num) {
            num += 1000;
            return num;
        }
    }
}

Hope this is what you wanted.

Answer 4

One solution would be to pass a method of A as a callback.

For example:

public class ClassA {
  private int number;
  private ClassB instanceOfB = new ClassB();

  public ClassA {
    number = 0;
  }

  public void incrementNumber {
    number++;
  }

  public void incrementNumberLongWay {
    instanceOfB.incrementNumberInA(this::increment);
    // alternatively
    // instanceOfB.incrementNumberInA(() -> incrementNumber());
  }
} 

public class ClassB {
  public void incrementNumberInA(Runnable callbackMethod) {
    callbackMethod.run();
  }
}

This removes B's dependency on A, and instead allows a general callback mechanism.

However, for such a simple scenario this approach isn't advised.

Answer 5

It's probably a bad idea in general to have a circular dependency in this way. One approach to break the cycle would be to have a third class (classC?) that implements the increment logic (or whatever your real-world equivalent is), and have classA and classB instances each reference classC. That way there's no case where two classes know about each other.