Quiz 7 Solutions

1. public class StackBasedQueueOfInts {
   
      private StackOfInts stack;
   
      public StackBasedQueueOfInts() {
        stack = new StackOfInts();
      }
   
      public void enqueue(int n) {
         stack.push(n);
      }
   
      public int dequeue() {
         if (isEmpty()) throw new Error("Empty queue on dequeue");
         StackOfInts temp = new StackOfInts();
         while (!stack.isEmpty())  temp.push(stack.pop());
         int ans = temp.pop();
         while (!temp.isEmpty())  stack.push(temp.pop());
         return ans;
      }
   
      public boolean isEmpty() { return stack.isEmpty(); }
   
   }
   
   

2. public class QueueBasedStackOfInts {
   
      private QueueOfInts queue;
   
      public QueueBasedStackOfInts() {
         queue = new QueueOfInts();
      }
   
      public void push(int n) {
         queue.enqueue(n);
      }
   
      public int pop() {
         if (isEmpty()) throw new Error("Stack underflow");
         int ans = 0;  // bogus initialization
         QueueOfInts temp = new QueueOfInts();
         while (!isEmpty()) {
            int n = queue.dequeue();
            if (isEmpty()) ans = n;
            else temp.enqueue(n);
         }
   
         while (!temp.isEmpty()) queue.enqueue(temp.dequeue());
   
         return ans;
      }
   
      public boolean isEmpty() { return queue.isEmpty(); }
   }
   
   

3. Neither would be a real implementation: each would be abstractly based on the other.

4. public class Square extends Rectangle {
      public Square(int length) {
         this.length = length;
         this.width  = length;
      }
   }