Question

The program below prints:

my name is:null

my name is:null

Someclass static init

AFAIK when a class is first loaded static blocks and fields are always initialized first, instance blocks and fields second. Therefore variables “objectName1” and “objectName2” should be initialized first, instance variable “list” second…but the ouput obviously contradicts this theory… Can anyone explain the program behavior (I’m not looking for a critique of the design in itself btw) ?

import java.util.ArrayList;
import java.util.List;

public class Main2{
    public static void main (String[] args){
        SomeClass.getInstance();
    }
}

class SomeClass {
    private static final SomeClass instance = new SomeClass();

    public static SomeClass getInstance(){
        return instance;
    }

    static {
        System.out.println ("Someclass static init");
    }
    private  static String objectName1  ="test1";
    private  static String objectName2  ="test2";

    @SuppressWarnings("serial")
    private  List<SomeObject> list=
        new ArrayList<SomeObject> ()  { {
 add (new SomeObject(objectName1));
 add (new SomeObject(objectName2));
    }};
}

class SomeObject {
    String name;
    SomeObject (String name){
        this.name = name;
        System.out.println ("my name is:" +name);
    }
}

Answer 1

Static blocks are initialized in order (so you can rely on the ones above in the ones below). By creating an instance of SomeClass as your first static initializer in SomeClass, you’re forcing an instance init during the static init phase.

So the logical order of execution of your code is:

• Load class SomeClass, all static fields initially defaults (0, null, etc.)
• Begin static inits
• First static init creates instance of SomeClass
• Begin instance inits for SomeClass instance, using current values for static fields (so objectName1 and objectName2 are null)
• Load SomeObject class, all static fields initially default (you don’t have any)
• Do SomeObject static inits (you don’t have any)
• Create instances of SomeObject using the passed-in null values
• Continue static inits of SomeClass, setting objectName1 and objectName2

To make this work as you may expect, simply put the inits for objectName1 and objectName2 above the init for instance.