Question

Stack<Animal> myStack = new Stack<Animal>();
Animal a = new Animal();
a.setValueOfAnimal(5);
myStack.push(a);
a.setValueOfAnimal(15);
System.out.println(myStack.pop().getValueOfAnimal());

Above you see a sample main method.

The output will be : 15.

Is there anyway to make the object in the stack safe? I do not want the object in the stack to be modified when I modify the value of “Animal a”.

Is this supported by any built – in Java class?

Answer 1

I do not want the object in the stack to be modified when I modify the
value of “Animal a”.

Then you would have to create new instance of Animal.

When you add the Animal reference in the Stack, you are just storing a copy of the reference, which points to the same Animal. So, if you modify your Animal instance using original reference, it is also modified for the reference in the Stack.

So, you need to modify your code like this: –

myStack.push(a);
a = new Animal(); // Make the reference `a` point to a new instance.
a.setValueOfAnimal(15);