Is it possible to invoke a method before even the object gets created?

[Hsitas Hickory]

Below is the program, i want to understand

class First{
First(){
System.out.println("In first constructor");
func();
}
public void func(){
System.out.println("In First func");
}
}
public class TestSubClass extends First{
TestSubClass(){
System.out.println("In Test SubClass constructor");
func();
}
public void func(){
System.out.println("In TestSubClass func");
}

public static void main(String args[]){
TestSubClass tsc = new TestSubClass();
}

}


The out put is

In first constructor
In TestSubClass func
In Test SubClass constructor
In TestSubClass func


Why is the second line of output "In TestSubClass func", but not "In First func".

[Rob Spoor]

Because the method is overridden. It is calling the func() method of the actual object, even though its constructor has not yet been called. This is why it's usually a bad idea to call methods that can be overridden in the constructor; the instance fields of the subclass have not yet been initialized so if you attempt to use them in that method you will get strange behaviour. Consider:

class Super
{
    public Super()
    {
        method();
    }

    public void method()
    {
        System.out.println("I am super");
    }
}

class Sub extends Super
{
    private int x = 13;

    public void method()
    {
        System.out.println("I am sub, and x is " + x);
    }

    public static void main(String[] args)
    {
        Sub sub = new Sub();
        System.out.println("x is now " + sub.x);
    }
}

The initialization of x to 13 occurs after the Super constructor has finished, so after the call to method(). That's why it's still 0 inside that call.

[Muhammad Khojaye]

Nice and precise explanation by Rob . Quality tools like PMD also report that such behavior can throw NullPointerException.

  
public class SeniorClass {
  public SeniorClass(){
      toString(); //may throw NullPointerException if overridden
  }
  public String toString(){
    return "IAmSeniorClass";
  }
}
public class JuniorClass extends SeniorClass {
  private String name;
  public JuniorClass(){
    super(); //Automatic call leads to NullPointerException
    name = "JuniorClass";
  }
  public String toString(){
    return name.toUpperCase();
  }
}

[roi yal]

beacuse, this func method has been override. and " tsc " is parent class.

[Pushkar Choudhary]

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[Jim Hoglund]

Would it be useful to make all constructor called methods private, or
would this create design problems with subclasses and later changes?

Jim... ...

[Rob Spoor]

In an ideal world they would be private and/or final, but of course this world is not ideal. Sun's API is full of calls to overridable methods inside constructors (e.g. several of JTable's constructors call setModel which is public and non-final).

[Jim Hoglund]

Thanks Rob.
Jim... ...