In constructors of java why this() and super() should be in the first line?

In Java you can put a call to the parent constructor using the keyword super as the first line in a subclass constructor to initialize inherited fields. When to use super keyword :

1) To access the data members of parent class when both parent and child class have member with same name
2) To explicitly call the no-arg and parameterized constructor of parent class) To access the method of parent class when child class has overridden that method.

Example :

//Parent class or Superclass or base class
class Superclass
{
   int num = 100;
}
//Child class or subclass or derived class
class Subclass extends Superclass
{
   /* The same variable num is declared in the Subclass
    * which is already present in the Superclass
    */
    int num = 110;
    void printNumber(){
	System.out.println(num);
    }
    public static void main(String args[]){
	Subclass obj= new Subclass();
	obj.printNumber();	
    }
}

The parent class constructor requires to be called before the subclass' constructor. This will assure you that if you call some methods on the parent class in your constructor, the parent class has already been set up correctly.
    So the this() and super() statements should be the first statements