Which of this keyword can be used in a subclass to call the constructor of parent class *?

Subclasses inherit public methods from the superclass that they extend, but they cannot access the private instance variables of the superclass directly and must use the public accessor and mutator methods. And subclasses do not inherit constructors from the superclass.

So, how do you initialize the superclass’ private variables if you don’t have direct access to them in the subclass? In Java, the superclass constructor can be called from the first line of a subclass constructor by using the special keyword super() and passing appropriate parameters, for example super(); or super(theName); as in the code below. The actual parameters given to super() are used to initialize the inherited instance variables, for example the name instance variable in the Person superclass.

public class Employee extends Person
{
    public Employee()
    {
        super(); // calls the Person() constructor
    }
    public Employee(String theName)
    {
        super(theName); // calls Person(theName) constructor
    }
}

The super(theName) in the Employee constructor will call the constructor that takes a String object in the Person class to set the name.

Try creating another Employee object in the main method that passes in your name and then use the get methods to print it out. Which class constructor sets the name? Which class constructor sets the id?

If a class has no constructor in Java, the compiler will add a no-argument constructor. A no-argument constructor is one that doesn’t have any parameters, for example public Person().

If a subclass has no call to a superclass constructor using super as the first line in a subclass constructor then the compiler will automatically add a super() call as the first line in a constructor. So, be sure to provide no-argument constructors in parent classes or be sure to use an explicit call to super() as the first line in the constructors of subclasses.

Regardless of whether the superclass constructor is called implicitly or explicitly, the process of calling superclass constructors continues until the Object constructor is called since every class inherits from the Object class.

9-2-2: Given the class definitions of MPoint and NamedPoint below, which of the constructors that follow (labeled I, II, and III) would be valid in the NamedPoint class?

class MPoint
{
   private int myX; // coordinates
   private int myY;

   public MPoint( )
   {
      myX = 0;
      myY = 0;
   }

   public MPoint(int a, int b)
   {
      myX = a;
      myY = b;
   }

   // ... other methods not shown

}

public class NamedPoint extends MPoint
{
   private String myName;
   // constructors go here
   // ... other methods not shown
}

//  Proposed constructors for this class:
I.   public NamedPoint()
     {
        myName = "";
     }
II.  public NamedPoint(int d1, int d2, String name)
     {
        myX = d1;
        myY = d2;
        myName = name;
     }
III. public NamedPoint(int d1, int d2, String name)
     {
        super(d1, d2);
        myName = name;
     }

• I only
• I is okay but III is also okay.
• I and III
• The MPoint variables are private and they can not be directly accessed in NamedPoint. You can use super as the first line in a constructor to initialize them. If you don't use super as the first line in a constructor one will be put there by the compiler that will call the parent's no argument constructor.
• II only
• II is invalid. Children do not have direct access to private fields. You can use super in a constructor to initialize these by calling the parent's constructor with the same parameter list.
• III only
• I is also okay

You can step through this code using the Java Visualizer by clicking the following link Named Point.