LinkedListStacks.java

/*
*
* This code is written for Stacks tutorial
* The comment won't make sense unless you read the slide that goes with it
*
* Go to Vumari.com to find the slide
*
*/

public class LinkedListStacks {
	
	//node
	private static class Node {
		int data;
		Node previous;
		
		Node(int data){
			this.data = data;
			previous = null; // newly create node points to nothing
		}
	}
	
	private Node pointer;
	private int size = 0;
	
	public void push(int aData){
		
		Node aNewNode = new Node(aData);
		
		if(pointer == null){//first node
			pointer = aNewNode;
		}else{
			aNewNode.previous = pointer; //talk to the old guy
			pointer = aNewNode; //pointer goes to the top
		}
		size++;
	}
	
	
	public int pop(){
		
		if(pointer == null){
			throw new NullPointerException("There is nothing to pop from the stack.");
		}
		
		int temp = pointer.data;
		
		pointer = pointer.previous;//go to the previous guy
		size--;
		return temp;
		
	}
	
	public int getSize(){
		return size;
	}
	
	public boolean isEmpty(){
		return pointer == null;
	}
	
	
	@Override
	public String toString() {
		
		Node printer = pointer; //start from top
		StringBuilder allData = new StringBuilder();
		
		while(printer != null){
			allData.append(printer.data + ", ");
			printer = printer.previous;
		}
		
		return allData.toString().replaceAll(", $",  "");//remove , followed by space in the end for nice printing
	}
}