StackList.java

public class StackList {
	
	StackListNode first;
	int size;

	public static void main(String[] args) {
		
		StackList list = new StackList();
		
		list.push(1);
		list.push(2);
		list.push(3);
		System.out.println(list.peek());
		System.out.println(list.pop());
		System.out.println(list.peek());
		System.out.println(list.pop());
		System.out.println(list.peek());
		System.out.println(list.pop());
		System.out.println(list.peek());
		System.out.println(list.pop());
	}
	
	public StackList()
	{
		this.size = 0;
	}
	
	public void push(Object o)
	{
		StackListNode node = new StackListNode(o, first);
		first = node;
		this.size++;
	}
	
	public Object pop()
	{
		if (!isEmpty()) {
			Object info = first.info;
			first = first.next;
			this.size--;
			return info;
		}
		return null;
	}
	
	public Object peek()
	{
		if (first != null) {
			return first.info;
		}
		return null;
	}
	
	public boolean isEmpty()
	{
		return size == 0;
	}
	
	class StackListNode {
		
		Object info;
		StackListNode next;
		
		StackListNode(Object info, StackListNode next) {
			this.info = info;
			this.next = next;
		}
	}

}