whats this mean

[rohan yadav]

In k&b book of scjp5 page no130
there is one line saying "You cannot make call to an instance method or access instance variable until after the super constructor runs"
whats that mean??
Can anyone explain with example?
Thanks in advance!!!

[Christophe Verré]

I think that the following example illustrates the problem :

public class Parent {

}

public class Child extends Parent{
	private int instanceVar = 0;
	public Child() {
		instanceVar = 1;
		hello();
		super(); // super has to be called first
	}
	
	public void hello() {
		
	}
}

[rohan yadav]

Thanks for replaying Christophe Verré.I got the concept now, i was thinking that once you have called super() method you cant call the instance method.
But i got that super has to call first.

[Seetharaman Venkatasamy]

please give a meaningful subject line http://faq.javaranch.com/java/UseAMeaningfulSubjectLine

[Prithvi Sehgal]

super() should always be the first line in the constructor. If your super-class has a no-arg constructor, you
don't need to put super() yourself, compiler will insert it for you. Look below

  
public class Parent{

}

public class Child extends Parent{

private int age ;

private String name;

public Child(int age, String name){
//Even if you don't put super() here, compiler will implicitly insert for you as your superclass has a by default no-arg constructor

this.age = age;

this.name=name;
}

}

Please do use meaningful subject lines, so that you can get the response quickly.

HTH,

[Nitish Bangera]

"You cannot make call to an instance method or access instance variable until after the super constructor runs"

THis is why if an explicit call for super() is put, it should be put as the first line in the constructor.

[Deepak Bala]

Hi rohan,

Welcome to javaranch. When you post here again, make sure that you provide a subject line that describes your problem. That way more users will reply to your question.

[Vlado Zajac]

It means not only that super(...) needs to be first statement in constructor but also that you cannot use instance variables or methods in the super(...) call.

This is forbidden:

class BadSuper
{
  public BadSuper(int x){}
}

class BadSub extends BadSuper
{
  int aa=2;
  public BadSub()
  {
    super(aa); //this line causes compiler error - cannot access aa before superclass constructor has run
  }
}