3.2. Subranges.   

We have seen integer= -Integer.MAX_VALUE to Integer.MAX_VALUE.
We may only want a small range of this : posint = 1 .. Integer.MAX_VALUE EX: 


      type      less = 1..31

      var       j:less

               i:integer

      begin

            j  34   //  compile error  - not in range

            i  34   //  OK

            j  i    //  OK at compile

      end               //  Run time error - Scalar out of range

Surely these two are of different types? i is of type integer and j is of type 1..31 ! 

            Very nearly the same!!

Contention as to whether the assignment should be picked

up as a compile time error or not.

We could create a new type in Java via the class mechanism: 

   class IntegerSubRange

   {

   // First the data and the constructor

   //   of this class


     private int lowerBound;

     private int upperBound;


     private int value;


     public IntegerSubRange(int lower, int upper)

     {

        lowerBound=lower;

        upperBound=upper;

     }




  // methods for manipulating value indirectly.

     public assignValue(int val)

     {

        if ( (val >=lowerBound) &&

             (val <=upperBound) )

                value = val;

        else        /* error */;

     }

     public int returnValue()

     {

        return(value);

     }

   } // end of class with data and methods.

   and declare variable as:

        IntegerSubRange j =

           new IntegerSubRange(lower,upper);

Now we can ONLY use or look at the IntegerSubRange value via the methods - the actual value is private and we cannot see it via j.value. The value has been hidden - we have data hiding.

What should we do with the error? What kind of thing can we do? 

     public assignValue(int val) throws IllegalAccessException

     {

        if ( (val >=lowerBound) && (val <=upperBound) )

           value = val;

        else

           throw new IllegalAccessException(

          "Assigned value must be between " + lowerBound + " and " + upperbound + "inclusive");

     }

3.2.1. Subranges of defined types.   

We can use subranges of user defined or enumerated types as below: 


      type  colour = (red,orange,yellow,green,blue,indigo,violet);

            roy    = red .. yellow;

            gbiv   = green .. violet;

            roygb  = red .. blue;


            workday = monday..friday;

       Must define type before using