static initialization in interface

Question

When I tried to write something like this:

public interface MyInterface {
    static {
        System.out.println("Hello!");
    }
}

the compiler could not compile it.

But when I wrote something like this:

interface MyInterface {
    Integer iconst = Integer.valueOf(1);
}

and decompiled it, I saw static initialization:

public interface MyInterface{
    public static final java.lang.Integer i;

    static {};
      Code:
      0:   iconst_1
      1:   invokestatic    #1; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      4:   putstatic       #2; //Field i:Ljava/lang/Integer;
      7:   return
}

Could you please explain this behavior to me?

Answer 1

Interfaces should not have side-effects and that even applies to static intializers. They would have highly JVM-implementation dependent behavior. Look at the following code

public class InterfaceSideEffects {
  public static void main(String[] args) {
    System.out.println("InterfaceSideEffects.main()");
    Impl i=new Impl();
    System.out.println("Impl initialized");
    i.bla();
    System.out.println("Impl instance method invoked");
    Foo f=new Impl();
    System.out.println("Impl initialized and assigned to Foo");
    f.bla();
    System.out.println("Foo interface method invoked");
  }
}
interface Foo {
  int dummy=Bar.haveSideEffect();
  void bla();
}
class Bar {
  static int haveSideEffect() {
    System.out.println("interface Foo initialized");
    return 0;
  }
}
class Impl implements Foo {
  public void bla() {
  }
}

What do you think, when will interface Foo initialized be printed? Try to guess and run code afterwards. The answer might surprise you.