Thread: cannot referenced <instance data member> before super type constructr has been called

From the JLS: Classes

An explicit constructor invocation statement in a constructor body may not refer to any instance variables or instance methods declared in this class or any superclass, or use this or super in any expression; otherwise, a compile-time error occurs.

For example, if the first constructor of ColoredPoint in the example above were changed to:

ColoredPoint(int x, int y) {
this(x, y, color);
}

then a compile-time error would occur, because an instance variable cannot be used within a superclass constructor invocation.

That's not my English, I just copied it from the JLS directly.

But basically what it's saying is that you can't use an instance variable before the super constructor has been called. Which means you can't use an instance variable when invoking another constructor. The basic idea is that the instance doesn't really exist before the super constructor is called (that's not exactly true but it gets the point across), so you can't use instance variables. You can use static variables though.

Originally Posted by chronoz13

wow thats it!,

questions:
1.) - does this refer to my construtor(s)? the two constructors?

Sort of. Remember, in every constructor (that doesn't invoke another constructor like your no-args constructor does), there is an invisible call to super. So your code is really doing this:

public class MyClass {
 
    private final int DEFAULT_VALUE = 10;
    private int instanceValue;
 
    public MyClass() {
 
        this(DEFAULT_VALUE);
    }
 
    public MyClass(int value) {
        super()
        instanceValue = value;
    }
}

So basically, the instance doesn't yet exist until that call to super() happens.

Originally Posted by chronoz13

2.) so it means that in my code above , the instance data member (DEFAULT_VALUE), somehow doesnt "YET" exist when that constructor was calling it?

i just want to clarify these thing, if im right with my understanding

Yeah, pretty much. It's not entirely true, but it's how I look at it.

ok now, i think im getting to the point, just like what youve said, i tried using a static variable

public class NewClass {
 
    private static int CLASS_DATA_MEMBER = 1;
 
    public NewClass() {
 
        this(CLASS_DATA_MEMBER);
    }
 
    public NewClass(int x){
 
    }
}

what i notice, in my own opinion, this works fine because, when the class was loaded(class loader thing), the CLASS_DATA_MEMBER data member, was created before all things happened, so thats why i dont get the error that i got from the DEFAULT_VALUE(instance data member) or from the first code i posted

is my assumption right? or is there any more better explanation with this?

Originally Posted by chronoz13

ok now, i think im getting to the point, just like what youve said, i tried using a static variable

public class NewClass {
 
    private static int CLASS_DATA_MEMBER = 1;
 
    public NewClass() {
 
        this(CLASS_DATA_MEMBER);
    }
 
    public NewClass(int x){
 
    }
}

what i notice, in my own opinion, this works fine because, when the class was loaded(class loader thing), the CLASS_DATA_MEMBER data member, was created before all things happened, so thats why i dont get the error that i got from the DEFAULT_VALUE(instance data member) or from the first code i posted

is my assumption right? or is there any more better explanation with this?

That's pretty much exactly how I look at it. That might not be technically true (depending on when you decide that an instance "exists"), but it makes sense to me.

Yep! Maybe this will clear everything up:

public class Main {
 
    public Main() {
    	System.out.println("In super constructor.");
    }
 
 
    public static class InnerMain extends Main{
 
    	int i = getDefaultValue();
 
        public int getDefaultValue(){
        	System.out.println("Getting default value.");
        	return 10;
        }
 
    	public InnerMain(){
 
    	}
    }
 
    public static void main(String... args){
    	InnerMain im = new InnerMain();
    }
 
}

What do you think that program prints? This is its output:

In super constructor.
Getting default value.

So, you can see that the super constructor happens before the initialization of the instance variables. And that's why you can't use the instance variables until the super constructor has been called.

No problem. Glad you got it figured out!