Java Notes

Constructors - super

Every object contains the instance variables of its class. What isn't so obvious is that every object also has all the instance variables of all super classes (parent class, grandparent class, etc). These super class variables must be initialized before the class's instances variables.

Automatic insertion of super class constructor call

When an object is created, it's necessary to call the constructors of all super classes to initialize their fields. Java does this automatically at the beginning if you don't.

For example, the first Point constructor (see Constructors) could be be written

public Point(int xx, int yy) {
    super();  // Automatically inserted
    x = xx;
    y = yy;
}

Explicit call to superclass constructor

Normally, you don't explicitly write the constructor for your parent class, but there are two cases where this is necessary:

• Passing parameters. You want to call a parent constructor which has parameters (the default construct has no parameters). For example, if you are defining a subclass of JFrame you might do the following.

  class MyWindow extends JFrame {
      . . .
      //======== constructor
      public MyWindow(String title) {
          super(title);
          . . .
  

  In the above example you wanted to make use of the JFrame constructor that takes a title as a parameter. It would have been simple to let the default constructor be called and use a setter method as an alternative.

  class MyWindow extends JFrame {
      . . .
      //======== constructor
      public MyWindow(String title) {
          // Default superclass constructor call automatically inserted.
          setTitle(title);  // Calls method in superclass.
          . . .
  

• No parameterless constructor. There is no parent constructor with no parameters. Sometimes is doesn't make sense to create an object without supplying parameters. For example, should there really be a Point constructor with no parameters? Altho the previous example (see Constructors) did define a parameterless constructor to illustrate use of this, it probably isn't a good idea for points.