A postfix calculator

Calculator.java

		import java.awt.*;
		import java.awt.event.*;

		public class Calculator {

		private DoubleStack memory;
		private CharStack operators;
		private String postfix;
		private double numbers[];

		public Calculator(){
			memory = new DoubleStack();
			operators = new CharStack();
			numbers = new double[100];
			postfix = "";

		}

		public double getValue(String postfix, double x){
			int i = 0;
			memory.clear();
			for(int n=0;n<postfix.length();n++){
				char ch = postfix.charAt(n);
				if('0'<=ch && ch<='9')
					memory.push(ch-'0');
				else {
					switch(ch) {
						case 'x':
							memory.push(x);
							break;
						case '+':
							double b = memory.pop();
							double a = memory.pop();
							memory.push(a+b);
							break;
						case '$':
							memory.push(-memory.pop());
							break;
						case '-':
							b = memory.pop();
							a = memory.pop();
							memory.push(a-b);
							break;
						case '*':
							b = memory.pop();
							a = memory.pop();
							memory.push(a*b);
							break;
						case '/':
							b = memory.pop();
							a = memory.pop();
							memory.push(a/b);
							break;
						case '^':
							b = memory.pop();
							a = memory.pop();
							memory.push(Math.pow(a,b));
							break;
						}
					}
				}
				return memory.pop();
			}
			public String infixToPostfix(String infix){
				int n = 0;
				postfix = "";
				infix+=" ";
				operators.clear();
				boolean firstTime = true;
				for(int i=0; i<infix.length();i++){
					char ch = infix.charAt(i);
					if(ch =='-' && firstTime)
						ch='$';
					if(ch!=' ')
						firstTime = false;
					if(isADigit(ch))
						postfix += ch;
					if(ch == 'x')
						postfix += ch;
					else if("+-*/^$".indexOf(ch)>=0){
						while(leftFirst(operators.peek(),ch))
							postfix += operators.pop();
						operators.push(ch);
					}
					else if(ch == '('){
						operators.push(ch);
						firstTime = true;
					}
					else if(ch == ')') {
						while(operators.peek()!='(')
							postfix += operators.pop();
							operators.pop();

					}
				}
				while(!operators.empty())
					postfix += operators.pop();
				return postfix;

		}

			private boolean leftFirst(char a, char b)
			{
				if(a =='^' && b=='^')
					return false;
				int r = rank(a);
				int s = rank(b);
				return r>=s;
			}

			private int rank(char ch){
				switch(ch) {
					case'+': case'-':
						return 1;
					case'*': case'/':
						return 2;
					case'^':
						return 3;
					default:
						return 0;
			}
		}
		private boolean isADigit(char ch){
			if((ch>='0' && ch<='9') || ch=='.')
				return true;
			return false;
		}
	}

CharStack.java

public class CharStack {
	private char data[];
	private int top;

	public CharStack(){
		data = new char[100];
		top = -1;
	}
	public void push(char c){
		data[++top] = c;
	}

	public char pop(){
		if(top<0)
		return'(';
		return data[top--];
	}

	public boolean empty(){
		if(top==-1)
			return true;
		return false;
	}
	public char peek(){
		if(top<0)
			return '(';
			return data[top];

	}
	public void clear(){
		top=-1;
	}
}

DoubleStack.java

public class DoubleStack {
	private double data[];
	private int top;

	public DoubleStack(){
		data = new double[100];
		top = -1;
	}
	public void push(double c){
		data[++top] = c;
	}

	public double pop(){
		return data[top--];

	}

	public boolean empty(){
		if(top==-1)
			return true;
		return false;
	}

	public void clear(){
		top=-1;
	}
}

PanelApplet.java

import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;

public class PanelApplet extends Applet
{
	PostfixPanel panel;

	public void init()
	{
		setLayout(null);
		panel = new PostfixPanel();
		add(panel);
		panel.setSize(getSize().width, getSize().height);
		panel.setLocation(0,0);
		panel.setVisible(true);
		panel.requestFocus();
		if (panel instanceof KeyListener)
			addKeyListener((KeyListener)panel);
	}
}

PostfixPanel.java

import java.awt.*;
import java.awt.event.*;


public class PostfixPanel extends Panel implements ActionListener {

	private Label postfixLabel, valueLabel, xLabel, infixLabel;
	private TextField postfixField, valueField, xField, infixField;
	private Calculator calculator;

	public PostfixPanel(){
		setBackground(new Color(100,0,50));
		setLayout(null);

		calculator = new Calculator();

		postfixLabel = new Label("postfix:");
		postfixLabel.setLocation(10,150);
		postfixLabel.setSize(100,20);
		postfixLabel.setForeground(Color.white);
		add(postfixLabel);

		postfixField = new TextField("");
		postfixField.setLocation(10,170);
		postfixField.setSize(200,20);
		postfixField.setBackground(Color.white);
		add(postfixField);

		xLabel = new Label("x:");
		xLabel.setLocation(10,60);
		xLabel.setSize(20,20);
		xLabel.setForeground(Color.white);
		add(xLabel);

		infixLabel = new Label("infix:");
		infixLabel.setLocation(10,10);
		infixLabel.setSize(100,20);
		infixLabel.setForeground(Color.white);
		add(infixLabel);

		infixField = new TextField("");
		infixField.setLocation(10,30);
		infixField.setSize(200,20);
		infixField.setBackground(Color.white);
		add(infixField);

		xField = new TextField("");
		xField.setLocation(10,80);
		xField.setSize(200,20);
		xField.setBackground(Color.white);
		add(xField);

		valueLabel = new Label("value:");
		valueLabel.setLocation(10,200);
		valueLabel.setSize(100,20);
		valueLabel.setForeground(Color.white);
		add(valueLabel);

		valueField = new TextField("");
		valueField.setLocation(10,220);
		valueField.setSize(200,20);
		valueField.setBackground(Color.white);
		add(valueField);

		infixField.addActionListener(this);
		xField.addActionListener(this);
}
	public void actionPerformed(ActionEvent e){
	postfixField.setText(calculator.infixToPostfix(infixField.getText()));
	String postfix = postfixField.getText();
	double x = convert(xField.getText());
	double value = calculator.getValue(postfix, x);
		valueField.setText("" + value);
	}
	public static double convert(String s)
		{
		try
		{
		return (new Double(s)).doubleValue();
		}
		catch (NumberFormatException nfe)
		{
		return -9876.54;
		}
	}
}