What is difference between references and objects in java? [duplicate]

Answer 1

What is difference between references and objects in java?

A reference is an entity which provides a way to access object of its type. An object is an entity which provides a way to access the members of it's class or type.

Generally, You can't access an object without a reference to it.

class GUI
{
    void aMethod()
    {
        // some business logic.
    }
}

You can call aMethod with or without a reference. but you definitely need an object.

Without Reference:

new GUI().aMethod(); 
// you can't reuse the object
// bad way to code.

With Reference:

GUI aGUIReference = new GUI();
aGUIReference.aMethod();
// Now, the object can be reused.
// Preferred way to code

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Now a little explanation to your code lines:

GUI g1 = new GUI();
// g1 is a reference to an object of GUI class.

GUI g2;
// g2 is a reference that can point to an object of GUI class
// but currently not pointing to any.

The only difference b/w g1 and g2 is that g1 is initialized with an object but g2 points to null

g2 = g1;
// it means g2 will point to the same object g1 is pointing to
// only one object but two references.

Answer 2

Both, g1 and g2 are references. But g2 is not initialized, so couldn't be used in code.

//Tells that g1 g2 should hold Gui object. Currently they are not holding any object.
Gui g1;
Gui g2;

//Now g1 reference holds object. g2 reference still doesn't point to any object.
g1 = new GUI();

Answer 3

Objects are like bubbles floating through a space called heap. Variables are merely links to these bubbles. The expression new GUI() creates an object and = operator links it to the variable g1. g2 is also a variable but it's not linked to any object. You can easily swap them by writing

g2 = g1;
g1 = null;

Now g2 refers to the object that g1 was referring to in the beginning, and g1 refers to nothing. The object is intact, only the variables was changed.