How do I do this in java since pointers don't exist?

Question

Let's say I've made an instance of a class A which includes a field int value.

int D;
A test = new A();
test.value = 3;

I want to make a variable D which will be linked to this test value of the class instance. Whenever this value changes my variable D will change to. I can do that if I know the link. I could simply do the following when a change occurs :

D = test.value;

However, let's say that I want D to be linked to different values(from other instances, let's say test2, test3) as the program runs. In that case I could use switch statements and have a different command depending on the link.

switch (help_var):
    case 1:
        D = test.value;
        break;
    case 2:
        D = test2.value;
        break;
    case 3:
        D = test3.value;
        break;

A changing pointer would be very useful. As the program runs, my pointer will change. In C, I would do the following in every case:

D = *pointer;

Is there a way to put the (test.value) reference in a variable and use something like this in Java?

Answer 1

It turns out, that since version 1.8 java already has this functionality built-in with the IntegerProperty class, but since its API somehow outscopes this question, let me give a minimalist example, too:

public class IntContainer {
    private int i;

    public int getValue();
    public void setValue(int value);

}

Such a container class will do about the same as a C pointer, though comparing them does not make sense in the first place.
However, when you replace int value with IntContainer pointer (or IntegerProperty) and change the IntContainer's value, the modification will be visible for every instance holding a reference to the same IntContainer.

Answer 2

You approach violates the encapsulation or information hiding principle by accessing an internal property of class A.

To come around this you should move the calculation working with that value to either a separate method in class A (if it somehow depends on A or its subclasses ) or another class that you can pass to a method of class A so that A can pass its value in:

interface MyCalculation{
  int calculateWith(inf value);
}

class A {
  inf test = 42;

  int doThis(MyCalculation calculation){
    return calculation.calculateWith(test);
  }
}

// somewhere else:

A a = new A();
int D = a.doThis(value->42*value);

Answer 3

As you already noticed there aren't any pointers in Java.

So you need to create some wrappers to achieve the desired behavior, especially for primitive types like int.

One option is as @Izruo already answered to embed the values into an wrapper, with a custom class or with something like IntegerProperty.

Another option is to create a create a callback which returns the value of the desired variable. (Which is basically the idea behind @Timothy Truckle answer.)

For example with Supplier<T>:

public class Callback<T> {
    private Supplier<T> supplier;

    public void link(Supplier<T> to) {
        supplier = to;
    }

    public T get() {
        if(supplier == null)
            return null;
        return supplier.get();
    }
}

To link the values you can then use:

Callback<Integer> d = new Callback<>();
d.link(() -> test.value);
System.out.println(d.get());

Answer 4

Simple answer : you cannot do that in Java.

If you want to emulate it, you will have to emulate create "mutable" Integer object.

public class MutableInteger {
  private int value=0;
  public void setValue(int i){
      value=i;
  }
  public int getValue(){
     return value;
  }
}

You then can write :

MutableInteger D;
A test = new A();
test.value = new MutableInteger() ;
D=test.value;

That way, modify test.value will impact D and vice-versa

Answer 5

In Java, you can have only references to objects. int is a primitive type and not an object. This means you can't have reference to a field of int type.

Sometimes people do a workaround of using an array of size 1 (int[1]) which is an object and can be referred to by reference.