What are the rules dictating the inheritance of static variables in Java?

Question

I have a class, Super:

public class Super {     public static String foo = "foo"; } 

I also have another class, Sub that extends Super:

public class Sub extends Super {     static {         foo = "bar";     }      public static void main (String[] args) {         System.out.println(Super.foo);     } } 

When I run it, it prints out bar.
My third (and last) class is Testing:

public class Testing {     public static void main (String[] args) {         System.out.println(Super.foo);         System.out.println(Sub.foo);         System.out.println(Super.foo);     } } 

This prints:

foo foo foo 

I don't understand why the contents of foo vary depending on what class you're accessing it from. Can anyone explain?

Answer 1

I don't understand why the contents of foo vary depending on what class you're accessing it from.

Basically it's a matter of type initialization. The value of foo is set to "bar" when Sub is initialized. However, in your Testing class, the reference to Sub.foo is actually compiled into a reference to Super.foo, so it doesn't end up initializing Sub, so foo never becomes "bar".

If you change your Testing code to:

public class Testing {     public static void main (String[] args) {         Sub.main(args);         System.out.println(Super.foo);         System.out.println(Sub.foo);         System.out.println(Super.foo);     } } 

Then it would print out "bar" four times, because the first statement would force Sub to be initialized, which would change the value of foo. It's not a matter of where it's accessed from at all.

Note that this isn't just about class loading - it's about class initialization. Classes can be loaded without being initialized. For example:

public class Testing {     public static void main (String[] args) {         System.out.println(Super.foo);         System.out.println(Sub.class);         System.out.println(Super.foo);     } } 

That still prints "foo" twice, showing that Sub isn't initialized - but it's definitely loaded, and the program will fail if you delete the Sub.class file before running it, for example.