Static initializer block in derived nested class

Question

assuming the following example:

public abstract class A {

  public void readList() {
    for (Integer o : InnerA.INT_LIST) {
      System.out.println(o);
    }
  }

  public static class InnerA {

    protected static List<Integer> INT_LIST;

    static {
      INT_LIST = new ArrayList<Integer>();
      INT_LIST.add(1);
    }
  }
}

public class B extends A {      

  public static class InnerB extends InnerA {
    static {
      INT_LIST.add(2);
    }
  }
}

My assumtion was, that when I call

new B().readList();

the output would be

1
2

but instead it is

Adding the constructor

public B() {
    new InnerB();
}

leads to the expected behaviour. I thought since the nested class is static, it gets initialized when a new B object is created (it obviously is when A is initialized).

Can someone please explain this to me?

Thanks.

chartsai · Accepted Answer · 2015-03-18 11:27:33Z

1

Let's see the implementation of readList() first:

public void readList() {
  for (Integer o : InnerA.INT_LIST) {
    System.out.println(o);
  }
}

It is obviously that it print all elements in the InnerA.
Class B extends Class A and doesn't override readList() function. So the behavior of new B().readList() is print all elemtns in the InnerA, too.

Since the InnerA.INT_LIST only has a elements added in static block of InnerA, it only has 1 elements.

Actually, your code never use class InnerB. JVM do not load it so its static block is not executed.

edited Mar 18, 2015 at 11:27

answered Mar 18, 2015 at 10:56

chartsai

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2 Comments

Jilles van Gurp Over a year ago

Yes correct, class B never loads because of lazy loading. So, the static initializer never executes.

s1ck Over a year ago

Ok, did not know about this lazy loading. So I either override readList() or add the new InnerB() in the constructor of B. Thanks EDIT overriding readList() and iterating through InnerB.INT_LIST does not work either. The only thing that works is calling new InnerB() in the constructor of B.

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