assign instance variable to a static variable

Question

I have something like this:

public class Test {

    public static MyObject o4All = null;

    public MyObject o4This = null;

    public void initialize() {

        // create an instance of MyObject by constructor
        this.o4This = new MyObject(/* ... */);

        // this works, but I am wondering
        // how o4All is internally created (call by value/reference?)
        Test.o4All = this.o4This;

    }
}

I know, I should assign or change a static variable only by a static method. But according to java-docs (http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html), I can use an object reference.

Class methods cannot access instance variables or instance methods directly—they must use an object reference.

What if I change a property of o4This? Will the property of o4All also be changed indirectly?

You don't assign a variable to a variable, but rather an object reference to a variable. That's a big difference and understanding this will help you understand the behavior of your program and Rohit Jain's advice. — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Feb 8, 2013 at 17:35

Rohit Jain · Accepted Answer · 2013-02-08 17:34:15Z

7

What if I change a property of o4This? Will the property of o4All also be changed indirectly?

Yes, it will be changed. Because now, both o4All and o4This are referring to the same instance. You did this by the following assignment: -

Test.o4All = this.o4This;

In the above assignment, you are not creating a copy of the instance referred to by o4This, rather, you are just copying the value of o4This in the o4All reference. Now, since o4This value is a reference to some instance. So, o4All now have reference to the same instance as that of o4This. And hence, any change you make to the instance using a reference, will be reflected in the other reference too.

answered Feb 8, 2013 at 17:34

Rohit Jain

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