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Stack.java

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public class Stack <T> extends Quack<T>{

/**

* push(next)

* ----------

* PRE: stack is not null

* POST: adds next to the top of the stack

*/

public void push(T next) {

super.append(next); // adds next to end of quack

}

/**

* pop()

* -----

* PRE: stack is not null

* POST: if stack is empty, returns null,

* if stack is occupied, removes and returns the object at the top of the stack

*/

public T pop() {

return super.remove(super.size() - 1); // removes the object at the end of the quack

}

/**

* peek()

* ------

* PRE: stack is not null

* POST: if stack is empty, returns null.

* if stack is occupied, returns object at top of stack, without removing it.

*/

public T peek() {

return super.get(super.size() - 1); // gets the object at the end of the quack

}

/**

* main

* ------

* Tests methods in the Stack class

*/

public static void main(String[] args) {

Stack<Integer> empty = new Stack<>();

Stack<Integer> one = new Stack<>();

Stack<Integer> multiple = new Stack<>();

one.push(1);

multiple.push(1);

multiple.push(2);

multiple.push(3);

System.out.println("Empty (should print nothing): " + empty);

System.out.println("One (should print '1': " + one);

System.out.println("Multiple (should print '1 2 3'): " + multiple);

test(empty.peek() == null, "peek() on empty");

test(one.peek() == 1, "peek() on one");

test(multiple.peek() == 3, "peek() on multiple");

System.out.println("After peek(), empty should remain the same (should print nothing): " + empty);

System.out.println("After peek(), one should remain the same (should print '1'): " + one);

System.out.println("After peek(), multiple should remain the same (should print '1 2 3'): " + multiple);

test(empty.pop() == null, "pop() on empty");

test(one.pop() == 1, "pop() on one");

test(multiple.pop() == 3, "pop() on multiple");

System.out.println("After peek(), empty should remain the same (should print nothing): " + empty);

System.out.println("After peek(), one should be empty (should print nothing): " + one);

System.out.println("After peek(), multiple should print '1 2': " + multiple);

}

}