Objects and Strings equality and memory digrams

Question

I have having a bit of trouble understanding the difference between these two:

Suppose we have:

 String x = "Test";
 String y = x;
 y = "Not Test"; //x is not modified here

The memory diagram for these two steps is the following:

1. x-> "Test"
2. x-> "Test" <-y
3. x-> "Test" y-> "Not Test"

But, consider class A with field 'int var':

A a = new A();
A b = a;
b.var = 5; 

Here, if we modify b.var, a.var also changes to 5. Why is this different from the above case with strings?

Additionally, I would appreciate it if someone wouldn't mind explaining these memory diagrams for strings and objects. Would the result be different if a and b were objects of different classes set equal? Thanks.

Answer 1

There's a difference between

b.var = 5;

and

b = new A();

The second statement is similar to what you do in your first snippet, since y = "Not Test"; makes y refer to a new instance.

The first statement updates the state of the instance referred by both a and b, so both are affected.

Here's a diagram that shows your second snippet :

a -> object of class A <- b
     containing an int
     variable `var`
        var <- 5

Both a.var and b.var contain 5, since they are the same variable.