else if(token.equals("^"))
{
	val1 = Integer.parseInt(stack.pop().toString());
	val2 = Integer.parseInt(stack.pop().toString());
 
	result = val2 ^ val1;
 
	stack.push(new Integer(result));
}

import javax.swing.*;
 
public class StackArray 
{
    private Object stack[];
    private int sizeIndex = 5;
    private int topPtr;
 
    public StackArray()
    {
        stack = new Object[sizeIndex];	
    }
 
    public void push(Object stackItem)
    { 
        if(!isFull())
        {
        	stack[topPtr] = stackItem;
        	topPtr++;					
        }    
        else
        {
        	JOptionPane.showMessageDialog(null,"Stack is Full!","Error",JOptionPane.ERROR_MESSAGE);
 
        }      
    }
 
    public Object pop()
    {
    	Object x = null;				
    	if(!isEmpty())
        {
            x = stack[--topPtr];		
            stack[topPtr] = null;
        }
        return x;							
    }
 
    public Object top()						
    {	
    	Object ts = null;				
        if(!isEmpty())
        {
        	ts = stack[topPtr - 1];			
        }
        return ts;  					
    }
 
    public boolean isEmpty()
    {
        return topPtr == 0;
    }
 
    public boolean isFull()
    {
		return topPtr == sizeIndex;	 	
    }
 
 
	public boolean contains(Object stackItem)
	{
		 boolean contains = false;			
		 if(!isEmpty())
		 {
		 	for(int i = 0; i < sizeIndex; i++)	
			{
			 if(stack[i].equals(stackItem))
			 {
			 	contains = true;				
				break;
			 }
			}
		 }
		 return contains;						
	}
 
    public String toString()				
    {
        StringBuffer display = new StringBuffer();	
 
        display.append("{");					
            for(int i = 0; i< sizeIndex; i++)
            {
                if(stack[i] != null)
                {
                    display.append(stack[i]);
                    if( i < topPtr - 1)			
                    {
                        display.append(",");
                    }
                }
            }
 
        display.append("}");
        return display.toString();
    }
}

 
public class Numeral
{
	public static boolean isOperand(String o)	
	{	
		boolean operand = true;	//why boolean and why true?
			for(int i = 0; i < o.length(); i++)  //purpose of this: Why o.length and i++? 
			{
				char c = o.charAt(i);			
				if(!Character.isDigit(c))  //purpose of this and how to read this?
				{
					operand = false;  //Why operand is false?
					break;
				}	
			}
		return operand;	 //Why return to operand?
	}
 
	public static boolean isOperator(String o)
	{
		return(o.equals("+") || o.equals("*") ||   //Why return to this?
		       o.equals("-") || o.equals("/"));
	}
}

 
 
import javax.swing.*;
import java.util.*;
 
public class PostfixEvaluator
{
 
	StackArray 	stack = new StackArray();
	StringTokenizer tok;
 
	private int val1,val2;
	private int result;
 
	public PostfixEvaluator(String postfix)
	{
	tok = new StringTokenizer(postfix);
 
		while(tok.hasMoreElements())
		{
			String token = tok.nextToken();
			if(Numeral.isOperand(token))
			{
				stack.push(token);
			}
 
			else if (token.equals("a") || token.equals("b") || token.equals("c"))	
			{
				JOptionPane.showMessageDialog(null,"Invalid Input","Error",JOptionPane.ERROR_MESSAGE);
			}
			else if(Numeral.isOperator(token))
			{
				if(token.equals("+"))
				{
					val1 = Integer.parseInt(stack.pop().toString());
					val2 = Integer.parseInt(stack.pop().toString());
 
					result = val1 + val2;
 
					stack.push(new Integer(result));
				}
				else if(token.equals("-"))
				{
					val1 = Integer.parseInt(stack.pop().toString());
					val2 = Integer.parseInt(stack.pop().toString());
 
					result = val2 - val1;
 
					stack.push(new Integer(result));
				}
				else if(token.equals("*"))
				{
					val1 = Integer.parseInt(stack.pop().toString());
					val2 = Integer.parseInt(stack.pop().toString());
 
					result = val1 * val2;
 
					stack.push(new Integer(result));
				}
				else if(token.equals("/"))
				{
					val1 = Integer.parseInt(stack.pop().toString());
					val2 = Integer.parseInt(stack.pop().toString());
 
					result = val2 / val1;
 
					stack.push(new Integer(result));
				}
 
			}
		}
	}
 
	public String getResult()
	{
		return String.valueOf(stack.top());
	}
}

 
This is the runner.
 
 
 
 
 
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
 
public class Postfix
{
 
JTextField TxtPostfix = new JTextField(15);
JTextField TxtResult = new JTextField(15);
 
private String inputString = null;
	private JFrame frame;
	private JLabel label;
	private JPanel boxLayout,panel1,panel2,panel3;
 
	JButton eval = new JButton("Evaluate");
	JButton clear = new JButton("Clear");
	JButton sample = new JButton("Ex. 2 3 4 * + ");
 
	private Font font;
 
	public Postfix()
	{
		boxLayout = new JPanel();
 
		font = new Font("Serif",Font.BOLD,12);
 
		//For postfix notation
		panel1 = new JPanel(new FlowLayout(FlowLayout.CENTER,5,0));
		panel1.add(label = new JLabel("Postfix : "));
		panel1.add(TxtPostfix);
 
 
		label.setFont(font);
		label.setPreferredSize(new Dimension(49,16));
		label.setHorizontalAlignment(JLabel.RIGHT);
 
 
		panel2 = new JPanel(new FlowLayout(FlowLayout.CENTER,5,0));
		panel2.add(label = new JLabel("Result : "));
		panel2.add(TxtResult);
 
		panel3 = new JPanel(new FlowLayout(FlowLayout.CENTER,5,0));
		panel3.add(eval);
		panel3.add(clear);
		panel3.add(sample);
 
 
		label.setFont(font);
		label.setForeground(Color.red);
		label.setPreferredSize(new Dimension(49,16));
		label.setHorizontalAlignment(JLabel.RIGHT);
 
		TxtResult.setEditable(false);
	}
 
	public Container createContent()
	{
 
		eval.addActionListener ( new ActionListener () {
					public void actionPerformed ( ActionEvent e) {
 
		 		PostfixEvaluator p; 
				p = new PostfixEvaluator(TxtPostfix.getText());				
 
				TxtResult.setText(p.getResult());
 
					}
				});
 
		clear.addActionListener ( new ActionListener () {
					public void actionPerformed ( ActionEvent e) {
						TxtPostfix.setText("");
						TxtResult.setText("");
 
					}
				});
 
		sample.addActionListener ( new ActionListener () {
					public void actionPerformed ( ActionEvent e) {
						TxtPostfix.setText("2 3 4 * +");
 
 
					}
				});		
 
 
		boxLayout.setOpaque(true);
 
		boxLayout.add(panel1);
		boxLayout.add(Box.createRigidArea(new Dimension(0,5)));
		boxLayout.add(panel2);
		boxLayout.add(Box.createRigidArea(new Dimension(0,5)));
		boxLayout.add(panel3);
		boxLayout.add(Box.createRigidArea(new Dimension(0,5)));
 
 
		JPanel layoutPanel = new JPanel();	
 
		layoutPanel.add(boxLayout);
 
		return layoutPanel;
	}	
 
	public void showGUI()
	{
 
		frame = new JFrame("Midterm Project");
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setContentPane(createContent());
 
		frame.pack();
		frame.setVisible(true);
		frame.setResizable(false);	
 
	}
 
	public static void main(String[] args)
	{
		Runnable doRun = new Runnable(){
				public void run()
				{
					new Postfix().showGUI();
				}
			};
		javax.swing.SwingUtilities.invokeLater(doRun);
	}
 
 
}