Java Notes

Constructors

When you create a new instance (a new object) of a class using the new keyword, a constructor for that class is called. Constructors are used to initialize the instance variables (fields) of an object. Constructors are similar to methods, but with some important differences.

Example of explicit this constructor call

public class Point {
    int m_x;
    int m_y;

    //============ Constructor
    public Point(int x, int y) {
        m_x = x;
        m_y = y;
    }

    //============ Parameterless default constructor
    public Point() {
        this(0, 0);  // Calls other constructor.
    }
    . . .
}

super(...) - The superclass (parent) constructor

An object has the fields of its own class plus all fields of its parent class, grandparent class, all the way up to the root class Object. It's necessary to initialize all fields, therefore all constructors must be called! The Java compiler automatically inserts the necessary constructor calls in the process of constructor chaining, or you can do it explicitly.

The Java compiler inserts a call to the parent constructor (super) if you don't have a constructor call as the first statement of you constructor. The following is the equivalent of the constuctor above.

    //============ Constructor (same as in above example)
    public Point(int x, int y) {
        super();  // Automatically done if you don't call constructor here.
        m_x = x;
        m_y = y;
    }

Why you might want to call super explicitly

Normally, you won't need to call the constructor for your parent class because it's automatically generated, but there are two cases where this is necessary.

  1. You want to call a parent constructor which has parameters (the automatically generated super constructor call has no parameters).
  2. There is no parameterless parent constructor because only constructors with parameters are defined in the parent class.