Question

Just getting my feet wet with Java so I apologize if this seems a bit naive. I’m trying to get a better understanding of the conventions for accessing instance / member variables in practice.

Can a non-static instance variable be manipulated from a
non-static context?

For instance, How could one modify the following class definition to allow the id and version variables to increment.

    class Foo {
    
        private int id;
        private int version;
        public String product;
        public String model;
    
        private Foo( ) {
    
            // Can these variables be accessed from a non-static context?
            id++;
            version++;
    
        }
        ...

In comparison with static fields …

      class Foo {

        private static int id;
        private static int version;
        public String product;
        public String model;

        private Foo( ) {

            id++;
            version++;

        }
        ...

        

First Example …

1
1
Model One
First

1
1
Model Two
Second

Second Example …

1
1
Model One
First

2
2
Model Two
Second

Answer 1

You can access a static-variable from a non-static context, because static variables are bound to a class. So, even if you access it within a non-static context, it is accessed just like in static context.

For e.g: –

MyClass obj = null;
obj.staticVar = 10;

Surprisingly the above code does not throw NPE, as the static varible is accessed on class name regardless of how you are accessing it.

So, obj.staticVar is actually replaced with MyClass.staticVar.

But the reverse is not true. You cannot access a non-static variable from a static context. Because, in a static context, you don’t have any reference to an instance. And you can’t access instance variable without any reference to an instance of that class.

---

Having said that, also note that, just because you can do something, doesn’t mean that it is a good idea. Modifying a static variable inside a constructor or in any non-static context is a terrible idea. Because, any change you make to those variables, will be reflected for all the instances of that class.

Rather, you should use a static-initialization block to initialize the static variables. Static initialization block is executed at class-loading time, to initialize all the static variables.