Question

Suppose you have a class Dog, that has

public class Dog {

    private String name;
    private double age;

    // some setters
    // some getters

Additionally, you have a class DogHandler, that makes an instance of the Dog d and passes it to Owner

I suppose, i can

… make a copy of a Dog before passing it to Owner, but that’s an expensive operation and i’d rather avoid it.

… come up with an interface that Dog implements which contains getters only, cast Dog to that interface and pass the result along

… initialize settable variables in a constructor and simply not allow changes for this instance of an object

Are there any other ways to make sure receiver of the object cant modify it?

How do you take a simple bean containing some data and make it read-only?

Answer 1

This can be achieved in few ways, I can propose you 2 of them:

a) interface with getters is good idea

b) create derived class from Dog which has setters method blocked, like this:

class UnmodifiedDog extends Dog {
    public UnmodifiedDog(double age, String name) {
        super.setAge(age);
        super.setName(name);
    }
    @Override
    public void setAge(double age) {
        throw new UnsupportedOperationException();
    }

    @Override
    public void setName(String name) {
         throw new UnsupportedOperationException();
    }   
}

In DogHandler:

Dog createDog() {
    return new UnmodifiedDog(10, "Fido");
}

and you can pass this to the Owner:

owner.receiveDog(dogHandler.createDog());