Outline of Class 5: Constructors

Miscellaneous

• So, how is the first assignment going?
• Has the book arrived (sorry, didn't get over to the bookstore today)?
• How many of you use the outlines? Any opinions on how they should change?

The Command Line

• You may have observed that the main method takes an array of strings as a parameter.
• Where does that array of strings come from?
• Traditionally, Unix commands (which you've typed already) can have a number of arguments, which follow them on the command line.
• You can provide similar arguments to your Java program by following the class name with the arguments when you call the Java interpreter, as in

  % javac MyClass alpha beta gamma
  

• Within your main method, you refer to those arguments as args[0] ... args[args.length] (assuming, of course, that you've used args as the name of the one parameter to main.

Exercise

• Write a simple class, Echo, that echos the first argument given on the command line.
• What happens if you don't give your program any arguments?

Constructors

• Along with creating new objects, we may need to initialize the objects (assigning values to particular fields, allocating new objects for some fields, etc.)
• Classes can include constructors that describe how to create such objects.
• Constructors can have parameters that are used in the initialization of the object.

• Constructors look very much like methods, with the difference that they have the same name as the class.
• Java is something of a polymorphic language: it permits you to define multiple methods or constructors with the same name, as long as they have different parameter types.
• This is particularly important for constructors, as there are many different ways that we may want to describe a new object.

• For our sample Complex class, we'll have three constructors:
  • A default constructor, with no parameters
  • A constructor used for converting doubles to complex numbers (so the imaginary part should be 0)
  • A constructor that takes both a real and imaginary part

       /**
        * Initialize to 0.0 + 0.0i
        */
       public Complex() {
         real_part = 0.0;
         imaginary_part = 0.0;
       } // Complex
       /**
        * Initialize to x + y*i
        */
       public Complex(double x,double y) {
         real_part = x;
         imaginary_part = y;
       }
       /**
        * Initialize to x + 0.0i
        */
       public Complex(double x) {
          real_part = x;
          imaginary_part = 0.0;
       }
    

• If you don't declare any constructors, Java uses a default constructor with no parameters.
• If you do declare constructors, that default constructors is no longer available.