链表堆栈实现-Java

Question

I have to implement a stack in the form of Linked List for one of my assignments. We have to implement the interface DStack , which has the methods push() , pop() , peek() , and isEmpty() in our LinkedList.java file. The list would take a series of doubles as its content.

What I am still struggling with is basically the push() and pop() methods for linked lists. I am not sure about the nodes I have made, but here is the code that I have so far:

public class ListStack implements DStack{

private Node head;
private int size;


private class Node{
    double d;
    Node next;

    public Node(){
        head = null;
        next = null;
    }

    public Node(double data){
        d = data;
    }
}

public boolean isEmpty() {
    return (head == null);
}

@Override
public void push(double d) {
    if(!isEmpty()){
        Node newHead = new Node(d);
        newHead.next = head;
        head = newHead;
        size++;
    }
    else
        throw new EmptyStackException(); //To change body of generated methods, choose Tools | Templates.
}

@Override
public double pop() {
    if(!isEmpty()){
        double d = head.d;
        head = head.next;
        size--;
        return d;
    }
    else
        throw new EmptyStackException(); //To change body of generated methods, choose Tools | Templates.
}

@Override
public double peek() {
    if(!isEmpty()){
        return head.d;
    }
    else
        throw new EmptyStackException(); //To change body of generated methods, choose Tools | Templates.
}

Could someone advise me if I am doing the push() and pop() methods correctly? If not, where exactly do I need to revise it? I'm looking for explanations online but most of the materials don't really make sense to me.

Also, my teacher forbid us to use any other classes from the Java framework or other class library. Does this mean I cannot use Iterable in my code? If so, then creating the additional methods for linked lists such as remove() , next() , etc. are not needed, right?

I'm still tripping over these linked lists concepts so if anyone could clear this up, I would really appreciate it. Thank you!

Answer 1

pop() and peek() look ok, but you shouldn't throw an exception when trying to push() on an empty stack. You'll never be able to use the silly thing if that's the case. Instead, you might want to do something different if it's empty, but under no conditions should push() throw that EmptyStackException .

Answer 2

Looks OK, need to initialize head otherwise push/pop would always throws exception. Consider to throw stack full exception when is not possible instead of empty stack exception. This would be unlimited list capacity, may be you could consider with a fixed capacity then there would be limitation on push.

Answer 3

In addition to the already provided advise - it helped me a lot to draw the state of a stack/list for some basic scenarios: At the beginning, in between with a few elements in it and at the end when you delete the last element. And then going through your code step by step and check what happens to the variables and pointers for push() and pop().

Of course that could also be done with the Debugger- but Pen & Paper sometimes clears things up ;) Good luck with your assignment!