Why is sub class' static code getting executed?

Question

I have written the following code and created object for the super class.

class SuperClass{     static int a=2;     static int b(){         return 2;     }     int c(){         return 2;     }      SuperClass(){         System.out.println("Super");     }     static {         System.out.println("super");     } }  public class Sub extends SuperClass{     Sub(){     System.out.println("Sub");     }     static {         System.out.println("sub");     }      static int b(){         return 3;     }     int c(){         return 3;     }     public static void main(String ax[]){         SuperClass f =new SuperClass();         System.out.println(f.c());         System.out.print(SuperClass.b());     }    } 

When I checked the output, it is as follows:

super sub Super 2 2 

I know that static block is executed only when object of the class is initialized or any static reference is made. But here, i did not make any of these to Sub class. then why do i see "sub" i.e. sub class' static block output?

Answer 1

I know that static block is executed only when object of the class is initialized or any static reference is made. But here, i did not make any of these to Sub class.

Your code doesn't, but in order for that main to run, Sub has to be loaded. So the static initializer for it is run.

E.g., I'm assuming you ran it like this:

java Sub 

The java tool has to load Sub to call Sub.main. That's the static reference (access, really) causing the static initializer to run. (If you ran it in an IDE, the IDE will do the java tool part, but the result is the same.)

So here's what happened:

1. java triggers load of Sub

2. JVM has to load SuperClass in order to load Sub

3. So we see their static initializers run, in order (SuperClass, then Sub):

   super sub 

4. java tool calls main

5. Code in main calls new SuperClass:

   Super 

6. Code in main calls f.c()

   2 

7. Code in main calls SuperClass.b:

   2 

As Holger helpfully points out, this is covered by the JVM specification in §5.5 - Initialization and the related §5.2 - Java Virtual Machine Startup:

Initialization of a class or interface consists of executing its class or interface initialization method (§2.9).

A class or interface C may be initialized only as a result of:

• ...

• If C is a class, the initialization of one of its subclasses.

• If C is a class, its designation as the initial class at Java Virtual Machine startup (§5.2).

That second-to-last bullet point covers SuperClass, and the last bullet point covers Sub.

Answer 2

Because your main() method is a member of Sub, that class needs to be loaded for your program to run.