Week 8 lecture topics

Further examples of methods in Java

Here is another simple example of a class containing several methods and attributes :-

        public class Password {

           private static String key = "XXXXXX";

           public static boolean checkValid(String word){
              return ( key.equals(word) );
           } //end checkValid

           public static boolean changePass(String old, String new) {
              if ( key.equals(old) ){
                 key = new;
                 return true;
              } else
                 return false;              
           } //end changePass

           public static void reset() {
              key = "XXXXXX";
           } //end reset

        } // end class Password

This class could then be used in a main method as follows :-

     public class Application {

        public static void main(String [] args) {
           Password.changePass("XXXXXX","CABLOG");
           if (Password.checkValid("CABLOG"))
              System.out.println("Password correct");
           else
              System.out.println("Password wrong"):
        } //end main

     } //end class Application

Looking at the above code, a number of observations can be made :-

1. static is used to prefix both class methods and attributes.
2. Methods are generally (but not always) prefixed by public
3. Attributes are generally (but not always) prefixed by private
4. The name of a method is immediately preceeded by the type of reply
5. Methods which do not make a reply are prefixed with void
6. Method names are followed by parameter lists in parentheses ()
7. Methods which dont need parameters still have the parentheses.
8. Methods which have a non void reply type must include a return statement.
9. Methods which have a void reply type must not include a return statement.

Formal and actual parameters

When writing sets of instructions which you wish to put inside a procedure it is often required to generalize the procedure so that it can be used in a variety of ways.

For instance a procedure to draw a square is not much use if it can only draw the square in one position, of one size and one colour. It would be better to pass the information regarding colour, position and size into the procedure (method) and write it in such a way as to use this information.

The information is passed to the method through variables called parameters and these are given in a parameter list in the method header.

The following example illustrates their use :-

     public class Interest {

        public static double simple(double amount, int years, float rate) {
           double result;    //somewhere to place the result of calculation
           result = amount * (1 + rate/100) * years;
           return result;
        } //end simple

        public static double compound(double amount, int years, float rate) {
           double result;
           result = amount * Math.pow((1 + rate/100), years);
           return result;
        } //end compound

     } //end class Interest

This would then be used in another program as follows :-

     public class Application {

        public static void main(String [] args) {
           double simp,comp;
           int period = 5;
           simp = Interest.simple(1000.0, period, 15.0);
           comp = Interest.compound(1000.0, period, 15.0);
           System.out.println("Simple interest = " + simp);
           System.out.println("Compound interest = " + comp);
        } //end main

     } //end class Application

Note the following points :-

• The parameters have the syntax type name and are separated by commas.
• These are called formal parameters and represent places in memory on the run-time stack. They are listed primarily to let the compiler know how many data values, of what types and in what order are going to be passed to the method.
• Parameters only exist in memory until the method terminates. The parameter names can only be referred to within the method where they are defined, where they behave like local varaibles.
• The instructions within the method make use of these data values

There are two ways that information can be conveyed to methods - through parameters or through attributes. The previous example could have been designed as follows :-

     public class Interest {

        private static double principal;
        private static int period;

        public static setData(double amount, int years) {
           principal = amount;
           period = years;     //stores the data in class attributes
        } //end setData

        public static double simple(float rate) {
           double result;    //somewhere to place the result of calculation
           result = principal * (1 + rate/100) * period;
           return result;
        } //end simple

        public static double compound(float rate) {
           double result;
           result = principal * Math.pow((1 + rate/100), period);
           return result;
        } //end compound

     } //end class Interest

This would then be used in another program as follows :-

     public class Application {

        public static void main(String [] args) {
           double simp,comp;
           int period = 5;
           Interest.setData(1000.0, period);
           simp = Interest.simple(15.0);
           comp = Interest.compound(15.0);
           System.out.println("Simple interest = " + simp);
           System.out.println("Compound interest = " + comp);
        } //end main

     } //end class Application