create new instance from existing instance

Question

For some reason when I create a new instance of an object from an existing instance and change values in the new instance, it also changes the values in the existing instance. I would prefer if it only changed the state of the values in the new instance though. I'm not sure why this is happening.

Foo existing = new Foo(1, "foo");

for (int i = 0; i < 10; i++) {
   Foo newFoo = existing;
   System.out.println(newFoo.getName()); //Prints "foo" as expected
   newFoo.setName("bar");
   System.out.println(existing.getName()); //This prints out "bar"?
}

Neither of the objects are static.

Answer 1

You can create a new instance at Runtime with:

Foo newFoo = existing.getClass().newInstance();

Note that the newly created instance will not be a copy of the existing one.

However, if you do need to a have a copy of the existing object, you can play a bit with the Clonable interface.

First, you have to make the Foo class implement the Clonable interface:

public class Foo implements Clonable { ... }

Then, you need to provide an implementation of the clone() method that will return a copy of you object:

public Foo clone() {
    return new Foo(this.id, this.name); //for example
}

And finally, you can trigger the newly introduced clone() method like this:

Foo newFoo = existing.clone();

This will give you a fresh Foo object, but with a copied properties from the existing one.

Answer 2

Note that when you do

Foo newFoo = existing;

You are not creating a new instance. Instead, you are simply pointing the newFoo variable to the same object existing points to.

What you're doing is basically this:

Foo existing = new Foo(1, "foo")       <=> existing -> Foo(1, "foo")
Foo newFoo = existing                  <=> existing -> Foo(1, "foo") <- newFoo
newFoo.setName("bar")                  <=> existing -> Foo(1, "bar") <- newFoo
System.out.println(existing.getName()) <=> System.out.println(Foo(1, "bar").getName())

If you want to create a new instance you can do:

Foo newFoo = new Foo(existing.getId(), existing.getName());