Is it possible in java to check has-a relationship?

Question

In java we can use instanceOf keyword to check the isA relationship. But is it possible to check hasA relationship too?

Answer 1

If you write your own method to do it.

public class Human {

    private Human parent;

    ..
    public boolean hasParent() {
         return parent!=null;
    }

}

Answer 2

I suppose that you could do this with Reflection, but I cannot see how this would be a useful thing to do in the context of Java. Java is designed to be a programming language, not an object modeling language.

EDIT - I'm assuming that you want to do this at a linguistic level ... rather than just coding a whole bunch of 'hasA' methods.

Answer 3

The plain association/aggregation constructs are not language elements, you write them in the program code. So you write the checker methods too.

Answer 4

Do you mean you want to check if an object has a property of a particular type? There's no built-in way to do that - you'd have to use reflection.

An alternative is to define an interface which has the relevant property - then check whether the object implements that interface using instanceof.

Why do you want to do this though? Is it just speculation, or do you have a specific problem in mind? If it's the latter, please elaborate: there may well be a better way of approaching the task.

Answer 5

The hasA relationship is often modeled as a private variable in a class:

Public class AClass {
  private AnotherClass reference;

  public AClass() {
    reference = null;
  }

  public boolean hasAnotherClass(){
    return reference != null;
  }

  // getters and setters are left out for readability

}

You can view and modify the reference using a getter and a setter. You can check for the relation using the method hasAnotherClass().