Here is the new definition of UTC2.

The extends UTC1 declares UTC1 to be the superclass, so all of the fields and methods defined in UTC1 will be inherited by UTC2.

There might be some methods in UTC1 that rely on the h field being the UTC hour rather than the EST hour. We don't want to break those methods, so we define our own h field in UTC2.

The call to super inside the Java constructor invokes the Java constructor for the superclass, which initializes the h and m fields of the superclass. The assignment to this.h then initializes the h field of UTC2.

      // Constructor template for UTC2:
      //     new UTC2 (h, m)
      // Interpretation:
      //     h is the UTC hour (between 0 and 23, inclusive)
      //     m is the UTC minute (between 0 and 59, inclusive)
      // Representation:
      //     Internally, the hour is represented in Eastern Standard Time,
      //     which is five hours behind UTC.
      
      class UTC2 extends UTC1 implements UTC {
      
          // the h field inherited from UTC1 will not be used
          // we will use the following h field instead
      
          int h;    // the hour in EST, limited to [0,23]
      
          // the Java constructor
      
          UTC2 (int h, int m) {
              super (h, m);    // invokes the constructor for UTC1
              this.h = (h >= 5) ? (h - 5) : h + 19;
          }
      
          // public methods
      
          // Returns the hour, between 0 and 23 inclusive.
      
          @Override
          public int hour () {
              if (h < 19)
                  return h + 5;
              else
                  return h - 19;
          }
      
          // The minute, isEqual, equals, hashCode, and toString
          // methods are inherited from UTC1.
      
          // a main method for unit testing
      
          public static void main (String[] args) {
              UTC2tests.main(args);
          }
      }