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.

Answer 2

You can have static initialisation, but you cannot have a static block. The fact the static initialisation needs a static code block to implement does change the Java syntax.

The point is you are not meant to have code in an interface (before Java 8) but you are allowed to initialise fields.

BTW you can have a nested class or enum which has as much code as you like and you can call this while initialising a field. ;)