Question

Hi can anybody please explain me why is this code snippet giving me StackOverflowError
I really appreciate if you can explain what is happening when instanceObj initializing and calling ObjectTest constructor and java.lang.Object constructor. It seems to me ObjectTest constructor loop over and over.But I don’t know exact reason? So any suggestion…

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public class ObjectTest {

  public ObjectTest() {

   }


  ObjectTest instanceObj = new ObjectTest();


  public static void main(String[] args) {

     ObjectTest localObj = new ObjectTest();
   }
}

Answer 1

Let’s see what will be executed :

1. main() create a new instance of ObjectTest
2. the ObjectTest class has a field instanceObj which will contain an ObjectTest
3. the instanceObj in initialized with a new ObjectTest
4. go to step 2

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I think you wanted something more like this :

public class ObjectTest {
    private static ObjectTest instanceObj;

    private ObjectTest() {
    }

    public static ObjectTest getInstance() {
        if (instanceObj != null) {
            instanceObj = new ObjectTest();
        }
        return instanceObj;
    }

    public static void main(String[] args) {

        ObjectTest localObj = ObjectTest.getInstance();
    }
}

Or this :

public class ObjectTest {
    private static final ObjectTest instanceObj = new ObjectTest();

    private ObjectTest() {
    }

    public static ObjectTest getInstance() {
        return instanceObj;
    }

    public static void main(String[] args) {

        ObjectTest localObj = ObjectTest.getInstance();
    }
}

This is the Singleton pattern.

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