Question

I have a socket which is currently connected.

Socket s; //Connected socket

If I use:

 Socket d = s;

Will socket d be the same, connected socket? It depends if java is assigning by value or reference, in this case I would like it to assign by reference. I’m not sure what it’s going to do exactly so I’m asking here.

Answer 1

Will socket d be the same, connected socket?

Yes

It depends if java is assigning by value or reference

Java is entirely pass-by/assign-by value. The value being assigned here is a reference to the object. Think of it as an int that tells us what slot in memory the object is in.

Continuing the int analogy:

int a;
int b;

a = 5;
b = a;
a = 6;

What’s b‘s value? 5, of course, the later assignment of 6 to a has nothing to do with the value stored in b.

And so similarly:

Socket d;
Socket s;

d = /* ... create and connect a socket... */;
s = d;
d = null;

What’s the value of s? Right! It’s the reference to the connected socket object. The value in s (a reference to the socket) is completely unaffected by your assigning a different value to d.

Now, of course, when d and s are pointing (referring) to the same object, naturally any time that object changes, you can see the change via d or s. All that d and s do is tell us where the object is, they don’t contain the object at all. They point to the memory that contains the object.

Once you grasp that object references are primitives just like ints and have exactly the same behavior, understanding Java code gets a lot easier.