如何重新设计一个Progression类在java中是抽象的和通用的？

Question

I have the following Progression class:

/** Generates a simple progression. By default: 0,1,2,3...*/
public class Progression {

// instance variable
protected long current;

/** Constructs a progression starting at zero. */
public Progression() { this(0); }

/** Constructs a progression with a given start value. */
public Progression(long start) { current = start; }

/** Returns the next value of the progression.*/
public long nextValue() {
   long answer = current;
   advance();
   return answer;
}

/** Advances the current value to the next value of the progression */
protected void advance() {
   current++;
}

/** Prints the next value of the progression, separated by spaces .*/
public void printProgression(int n) {
   System.out.print(nextValue());
   for(int j = 1; j < n;j++)
      System.out.print(" " + nextValue());
   System.out.println();
  }
}

How do I redesign the above java Progression class to be abstract and generic, producing a sequence of values of generic Type T, and supporting a single constructor that accepts an initial value?

I understand how to make the above class abstract but I don't see or understand how to translate the class to generics. In particular I don't know how to redesign the advance() method so that it uses java generics to produce a sequence of values of generic Type T.

Answer 1

You can only code what you know to hold for all generic instantiations. Everything else remains abstract. This can be seen by looking at the (added) method getInitial: it would return 0 for a Long, but (perhaps) "A" for a String. Also, nextValue is illuminating: it calls advance (no matter how) but advance is left to the implementation of the instantiation.

public abstract class Progression<T> {
    protected T current;

    public Progression() { this( getInitial()); }
    protected abstract T getInitial();
    public Progression(T start) { current = start; }

    public T nextValue() {
         T answer = current;
         advance();
         return answer;
    }

    protected abstract void advance();

    public void printProgression(int n) {
        System.out.print(nextValue());
        for(int j = 1; j < n;j++)
            System.out.print(" " + nextValue());
        System.out.println();
    }
}