Implementing stack in java please help

[Sandeep Kumar B]

Hi All
I am trying to implement stack in java but getting some problems

Here is the class file I wrote

package mypackage;

public class MyStackClass
{
	
	//
	MyStackClass prev;
	MyStackClass next;  // Next is enough for stack as of now
	private int data;
	
	//Constructor
	 MyStackClass()
	 {
	 	//this.next = null; //
	 }
	 
		public void push(int data)
		{
			MyStackClass tmpStack = new MyStackClass();
			tmpStack = this;
			try
			{
			
				do
				{
					//First reach the end of the stack
					tmpStack = tmpStack.next;
					//System.out.println("Yes"+tmpStack.data);
				}while(tmpStack.next!=null);
			}
			catch(NullPointerException e)
			{
				//If its the first element in the stack
				System.out.println("null");
				MyStackClass newStack = new MyStackClass();
				newStack.setData(data);
				//this = newStack; // Doesnt works
				this.next = newStack;
			}
			
					
			//Once we reach end, just push the data
			//New node to be added to the stack
			MyStackClass newStack = new MyStackClass();
			newStack.setData(data);
			tmpStack.next = newStack; // This line is throwing null pointer exception
			newStack.prev = tmpStack;
			//tmpStack1.setData(data);
			
			//this.next = st;
			//this.next.setData(data);		
		}
		
		public int pop()
		{
			//First reach the end
			MyStackClass tmpStack = new MyStackClass();
			MyStackClass newStack = new MyStackClass();
			tmpStack = this;
			do
			{
				//First reach the end
				newStack = tmpStack;
				tmpStack = tmpStack.next;
				//System.out.println("Yes"+tmpStack.data);
			}while(tmpStack.next!=null);
			//Now remove the last element
			int iLastElement = tmpStack.data;
			newStack.next= null;
			tmpStack = null;
			return iLastElement;
			
		}
		public void displayData()
		{
			MyStackClass varStack= new MyStackClass();
			varStack = this;
			System.out.println("Printing Stack Data: ");
			//System.out.println("Printintg");			
			do
			{
				System.out.println(varStack.data);
				//increment by going to next 
				//varStack = varStack.prev;
				varStack = varStack.next;
			}while(varStack!=null);
		}
		
		private void setData(int data)
		{
			this.data = data;
		}
	 
}

And the main program invoking it

package mypackage;
public class TestStack
{

	public static void main(String[] args)
	{
		MyStackClass myStack = new MyStackClass();
		for(int i =0; i<30; i++)
		{
			myStack.push(i);
		}
		myStack.displayData();
		
	}
}

Exception in thread "main" java.lang.NullPointerException
at mypackage.MyStackClass.push(MyStackClass.java:45) === newStack.setData(data);
at mypackage.TestStack.main(TestStack.java:10) === myStack.push(i);

I am a little confused on how to assign the element first time


For some reasons, when I wrote almost the same code in the same class file as main it works(see the code below)

package mypackage;

public class mystack
{
	mystack prev;
	mystack next;  // Next is enough for stack as of now
	private int data;
	
	//Constructor should create a new object in stack
	
	mystack()
	{
		//Constructor by default is not pointing to any other element
		next = null; // 
	}
	
	public int pop()
	{
		//First reach the end
		mystack tmpStack = new mystack();
		mystack tmpStack1 = new mystack();
		tmpStack = this;
		do
		{
			//First reach the end
			tmpStack1 = tmpStack;
			tmpStack = tmpStack.next;
			//System.out.println("Yes"+tmpStack.data);
		}while(tmpStack.next!=null);
		//Now remove the last element
		int iLastElement = tmpStack.data;
		tmpStack1.next= null;
		tmpStack = null;
		return iLastElement;
		
	}
	public void push(int data, mystack st)
	{
		this.next = st;
		st.prev = this;
		this.next.setData(data);		
	}
	public void push(int data)
	{
		mystack tmpStack = new mystack();
		tmpStack = this;
		do
		{
			//First reach the end
			tmpStack = tmpStack.next;
			//System.out.println("Yes"+tmpStack.data);
		}while(tmpStack.next!=null);
				
		//Once we reach end, just push the data
		mystack tmpStack1 = new mystack();
		tmpStack1.setData(data);
		tmpStack.next = tmpStack1;
		tmpStack1.prev = tmpStack;
		//tmpStack1.setData(data);
		
		//this.next = st;
		//this.next.setData(data);		
	}
	public void peek()
	{
		
	}
	private void setData(int data)
	{
		this.data = data;
	}
	private int getData()
	{
		return data;
	}
    public static void main(String[] args)
	{
		System.out.println("Program starting");
		mystack st = new mystack();
		st.setData(5);
		mystack st1 = new mystack();
		st.push(4,st1);
		mystack st2 = new mystack();
		st1.push(7,st2);
		System.out.println("Next item in the stack is : " + st.getData());
		System.out.println("Next item in the stack is : " + st1.getData());
		System.out.println("Next item in the stack is : " + st2.getData());
		
		//Print the whole stack given the class st
		mystack varStack;
		varStack = st;
		
		//Push into a stack
		mystack newStack = new mystack();
		newStack.setData(0);
		mystack tmpStack[] =new mystack[11];
		tmpStack[0] = newStack;
		for(int i = 1; i<10;i++)
		{
			tmpStack[i] = new mystack();
			tmpStack[i-1].push(i,tmpStack[i]);
		}
		tmpStack[0].push(14);
		tmpStack[0].push(17);
		tmpStack[0].push(19);
		//tmpStack[0].push(18);
		//varStack = tmpStack[10];
		varStack = tmpStack[0];
		do
		{
			System.out.println(varStack.data);
			//increment by going to next 
			//varStack = varStack.prev;
			varStack = varStack.next;
		}while(varStack!=null);
		
		System.out.println("Element removed is : " + tmpStack[0].pop());
		System.out.println("Element removed is : " + tmpStack[0].pop());
		System.out.println("Element removed is : " + tmpStack[0].pop());
		System.out.println("Element removed is : " + tmpStack[0].pop());
		
	}
}

Please help me so that my concepts are clear and I can proceed to implementing other data strcutures

[Greg Brannon]

The two approaches to implementing a Stack you've shown are fundamentally different. In your example that "worked", you used an array to store and manage your Stack contents. In your latest attempt, each push creates a new instance of your StackClass. How do you keep track of what's 'in' your stack in the new approach?

As for your error, review your code to determine what 'next' and 'tmpStack.next' are or should be. I'm (easily) confused, but so is the compiler. While you're at it, do the same for 'prev'. I think that's an error in waiting.

[Campbell Ritchie]

As GB has hinted, a Stack can be implemented atop an array, or a linked lists. Both have advantages and disadvantages. I would suggest you draw the operation of your stack on a sheet of paper. Then you can work out how to implement the push pop peek and isEmpty methods.
Also don't write 222 lines of code and expect it to work. Write the first 5 lines, compile, execute, test, and then try the next 5 lines.

[Daniel Marti]

I'm going to propose a slightly radical change on your code. As you know, you can only see the top of the stack, so exactly why do you want to contain the stack somewhere?
All you need is to envelop each object on the stack with something that makes it behave like a stack, and a single object that represents the top of said stack:

public class MyStackManager
{
private static StackObject top;
public static push(stackObject obj){
obj.setNext(top);
top=obj;
}//pop is the reverse
.
.
.
}


public class StackObject{
private StackObject next;
private Object containedObject;
.
.
.
}

With these 2 very simple classes you can create any stack you want. To improve on it, use templating. You can add a count to MyStackManager to provide the amount of objects in the stack without having to go through all of the elements and count. Also you do not need to make it static, i am just showing the simplest form of it.

[Sandeep Kumar B]

Thanks
I will work on these