object a = object b; what happens to object a?

Question

One of my professor has given us a few practice questions for an exam, and one of the questions is something like the below (psuedocode):

a.setColor(blue);
b.setColor(red);
a = b;
b.setColor(purple);
b = a;
//what color is a, and what color is b?

It seems extremely rudimentary to me, so I've proposed the answer that a is 'red', and b is 'red', but I've been told this is incorrect. I've broken down my answer as I would a math problem:

a = 15;
b = 12;
a = b; //a becomes 12
b = 13;
b = a; //b becomes 12

But my thought process is through the mind of C, not Java. I figured there was some universal method for both, but perhaps I am wrong? Is my answer wrong or my professor is wrong? I'm very new to Java, although I have some command of C, Python, and web logic (PHP, Ruby), so I apologize if this is something trivial (which it is).

Answer 1

Assuming you've already created two objects, and made variables a and b refer to them, you've initially got something like this.

a -->  [ white ]     b --> [ white ]

Your first two lines change the colours of the objects, to give you

a -->  [ blue  ]     b --> [  red  ]

Then, you point the variable a to the object referred to by b, so that they both refer to the same object. You now have

       [ blue  ]     b --> [  red  ] <-- a

Then you change the colour of the object referred to by b.

       [ blue  ]     b --> [ purple ] <-- a

Lastly, the line b=a; does nothing, because b already refers to the same object as a.

Answer 2

It's because the fact that in first example, a and b are objects so this is what happens in each of the steps:

a <-- is an object

b <-- is an object

a.setColor(blue); <-- a becomes blue

b.setColor(red); <-- b becomes red

a = b; <-- IMPORTANT:: original a object is released and available for garbage collection and now a holds the reference of b object, which mean a and b are referring the same object now, which is b.

b.setColor(purple); <-- b is purple now. Since a points to b only, a is also purple

Answer: Both a and b are purple at this point.