Instance attribute initialization

[maximiliano marasso]

Hi all,
I have a question about instance variables initialization. Hi have the following code

public MyClass{
  ...
 private MyObject var1 = MyClassB.getType();

 private void printOutput(){
       System.out.println(var1.getName());
 }
 ....

}

//in another package
public MyClassB{
    public static MyObject getType(){
         return new MyObject();
    }
}

My doubt is why, even though I'm initializing the 'var1' instance attribute at the declaration point, I get a null at the time the method 'printOutput' is run?
Initialization of type 'var1' is made through a static method ('getType' in the example) in another class from another package.
If I initialize the 'var1' attribute in the same 'printOutput' method just before printing it works fine.



Thanks a lot

[Ralph Cook]

Then at the time you execute printOutput, the either var1 has been set to null after it was initialized, or var1.getName() returns null.

Does the MyObject constructor initialize whatever it needs to in order for getName() to return a value?

rc

[maximiliano marasso]

'var1' attribute is null at the time the code reaches the println method. So it would throw a NullPointerException

[James Boswell]

maximiliano

What you have just said implies the call to getType() is returning null, unless there is other code which could possibly change the value of var1.

[maximiliano marasso]

Well, in fact there is no other code that changes the value of the instance attribute. The attribute is declared and instantiated in one line and in a private method called from the constructor of the class a method of the 'var1' attribute is called. Is there any restriction or problem in that sequence?

Thanks in advance

[Paul Clapham]

Why don't you just show us the real code? Or at least a short version of the real code which actually exhibits the problem? Looking at a bogus code fragment where you assert various things about the code you didn't post is not very practical. Show us the code.

[maximiliano marasso]

Hi guys,
I'm sorry for leaving this thread unattended all these days but I was ill and with a lot of work afterwards.
Anyway I have solved the problem. It was all because of the way constructors were called. Constructors were not finished initialising when I intended to use an instance attribute still not initialised.
In the end it was java basics, but something I did not see at fisrt.
Here is the code so you can comment on it and give me your opinion.

Thanks in advance!

public class Main {
public static void main(String[] args) {
        MyClass myClass = new MyClass();        
    }
}

public class MyObject {
    
    private String label;
    
    public void setLabel(String label){
        this.label = label;
    }
    
    public String getLabel(){
        return this.label;
    }
}

public class MyClass extends MyAbstractClass{
    private MyObject obj = MyStaticClass.getType("test");
    
    public MyClass(){
        super();
    }
    
    @Override
    protected void showMessage(){
        System.out.println(obj.getLabel());
    }
}

abstract class MyAbstractClass {
    
    public MyAbstractClass(){
        showMessage();
    }
    
    protected abstract void showMessage();
}