When will a static class initialize?

Question

Consider a Java class with static fields only and no constructor:

public class OnlyStatic {
   static O1 o1 = new o1();
   static O2 o2 = new o2();

   public static int compute(int whatever) {
       return o1.foo+o2.bar+whatever;
   }
}

In a different class, the method compute is used, either by static import:

static import OnlyStatic.compute
int a = OnlyStatic.compute(3);

Or directly, assuming the caller is in the same package:

int a = OnlyStatic.compute(3);

When are o1 and o2 initialized? At the import, or when compute() is called for the first time?

Answer 1

The objects o1 and o2 are not available to your static context without making them static also.

JVMS states that

Any static initializers declared in a class are executed when the class is initialized

Further

A class or interface type T will be initialized immediately before the first occurrence of any one of the following:

• T is a class and an instance of T is created.
• T is a class and a static method declared by T is invoked.
• A static field declared by T is assigned.
• A static field declared by T is used and the field is not a constant variable (§4.12.4).
• T is a top-level class, and an assert statement (§14.10) lexically nested within T is executed.

So in your case, when the static method compute() is first executed.