I am creating a Simple Java GUI Calculator.

I need to figure out how to solve this errors.

- User should not be able to start with an operator. (Erase if entered before the input.)

- User should not be able to end with an operator. (Erase if entered at the end of the input.)

- User should not be able to use more than one operator for each calculation. (Keep the first operation only.)

-Preceding zeros should be erased.

-Still user can get some error?

- Create a small text area at the bottom of your app, and whenever the input is bad, show a message like "Bad Input!" and erase the user's input.

I am also trying to add an icon to the title bar and it's not working.

This is my code so far.

Calculator class:

package calculator;

import java.awt.BorderLayout;

import java.awt.Color;

import java.awt.FlowLayout;

import java.awt.GridLayout;

import java.awt.Image;

import java.awt.Toolkit;

import java.awt.event.ActionEvent;

import java.awt.event.ActionListener;

import java.io.File;

import java.io.IOException;

import javax.imageio.ImageIO;

import javax.swing.ImageIcon;

import javax.swing.JButton;

import javax.swing.JFrame;

import javax.swing.JPanel;

import javax.swing.JTextArea;

public class calc extends JFrame {

JButton btn_zero = new JButton("0");

JButton btn_one = new JButton("1");

JButton btn_two = new JButton("2");

JButton btn_three = new JButton("3");

JButton btn_four = new JButton("4");

JButton btn_five = new JButton("5");

JButton btn_six = new JButton("6");

JButton btn_seven = new JButton("7");

JButton btn_eight = new JButton("8");

JButton btn_nine = new JButton("9");

JButton btn_clear = new JButton("C");

JButton btn_plus = new JButton(" + ");

JButton btn_minus = new JButton(" - ");

JButton btn_divide = new JButton(" / ");

JButton btn_times = new JButton(" * ");

JButton btn_equals = new JButton(" = ");

JTextArea txt_Area = new JTextArea(2, 5);

JTextArea txt_Area2 = new JTextArea(2, 1);

char operator;

public calc() {

this.createGUI();

this.showFrame();

}

private void createGUI() {

add(txt_Area, BorderLayout.NORTH);

add(txt_Area2, BorderLayout.SOUTH);

JPanel buttonpanel = new JPanel();

buttonpanel.setBackground(Color.GRAY);

buttonpanel.setLayout(new GridLayout(4,3));

btn_zero.setBackground(Color.DARK_GRAY);

btn_zero.setOpaque(true);

btn_one.setBackground(Color.DARK_GRAY);

btn_one.setOpaque(true);

btn_two.setBackground(Color.DARK_GRAY);

btn_two.setOpaque(true);

btn_three.setBackground(Color.DARK_GRAY);

btn_three.setOpaque(true);

btn_four.setBackground(Color.DARK_GRAY);

btn_four.setOpaque(true);

btn_five.setBackground(Color.DARK_GRAY);

btn_five.setOpaque(true);

btn_six.setBackground(Color.DARK_GRAY);

btn_six.setOpaque(true);

btn_seven.setBackground(Color.DARK_GRAY);

btn_seven.setOpaque(true);

btn_eight.setBackground(Color.DARK_GRAY);

btn_eight.setOpaque(true);

btn_nine.setBackground(Color.DARK_GRAY);

btn_nine.setOpaque(true);

btn_plus.setBackground(Color.ORANGE);

btn_plus.setOpaque(true);

btn_minus.setBackground(Color.ORANGE);

btn_minus.setOpaque(true);

btn_equals.setBackground(Color.DARK_GRAY);

btn_equals.setOpaque(true);

btn_divide.setBackground(Color.ORANGE);

btn_divide.setOpaque(true);

btn_times.setBackground(Color.ORANGE);

btn_times.setOpaque(true);

btn_clear.setBackground(Color.DARK_GRAY);

btn_clear.setOpaque(true);

buttonpanel.add(btn_seven);

buttonpanel.add(btn_eight);

buttonpanel.add(btn_nine);

buttonpanel.add(btn_times);

buttonpanel.add(btn_four);

buttonpanel.add(btn_five);

buttonpanel.add(btn_six);

buttonpanel.add(btn_divide);

buttonpanel.add(btn_one);

buttonpanel.add(btn_two);

buttonpanel.add(btn_three);

buttonpanel.add(btn_minus);

buttonpanel.add(btn_zero);

buttonpanel.add(btn_clear);

buttonpanel.add(btn_equals);

buttonpanel.add(btn_plus);

add(buttonpanel, BorderLayout.CENTER);

txt_Area.setForeground(Color.white);

txt_Area.setBackground(Color.GRAY);

txt_Area.setEditable(true);

txt_Area.setFont(txt_Area.getFont().deriveFont(32f));

txt_Area.setLineWrap(true);

txt_Area.setWrapStyleWord(true);

btn_zero.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("0".toString());

System.out.println(txt_Area.getText());

}

});

btn_one.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("1".toString());

System.out.println(txt_Area.getText());

}

});

btn_two.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("2".toString());

System.out.println(txt_Area.getText());

}

});

btn_three.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("3".toString());

System.out.println(txt_Area.getText());

}

});

btn_four.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("4".toString());

System.out.println(txt_Area.getText());

}

});

btn_five.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("5".toString());

System.out.println(txt_Area.getText());

}

});

btn_six.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("6".toString());

System.out.println(txt_Area.getText());

}

});

btn_seven.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("7".toString());

System.out.println(txt_Area.getText());

}

});

btn_eight.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("8".toString());

System.out.println(txt_Area.getText());

}

});

btn_nine.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("9".toString());

System.out.println(txt_Area.getText());

}

});

btn_plus.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("+".toString());

operator = '+';

}

});

btn_minus.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("-".toString());

operator = '-';

}

});

btn_divide.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("/".toString());

operator = '/';

}

});

btn_times.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

txt_Area.append("*".toString());

operator = '*';

}

});

btn_clear.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

Object source = e.getSource();

if (source == btn_clear) {

txt_Area.setText(null);

System.out.println(txt_Area.getText());

}

}

});

btn_equals.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

String LeftOperand;

String RightOperand;

String inputText;

String outputText;

inputText = txt_Area.getText();

System.out.println(inputText);

int operationIndex = inputText.indexOf(operator);

LeftOperand = txt_Area.getText().substring(0, operationIndex);

RightOperand = txt_Area.getText().substring(operationIndex + 1, txt_Area.getText().length());

switch (operator) {

case '+':

outputText = " = " + (Double.parseDouble(LeftOperand) + Double.parseDouble(RightOperand));

System.out.println(outputText);

txt_Area.append(outputText);

break;

case '-':

outputText = " = " + (Double.parseDouble(LeftOperand) - Double.parseDouble(RightOperand));

System.out.println(outputText);

txt_Area.append(outputText);

break;

case '*':

outputText = " = " + (Double.parseDouble(LeftOperand) * Double.parseDouble(RightOperand));

System.out.println(outputText);

txt_Area.append(outputText);

break;

case '/':

outputText = " = " + (Double.parseDouble(LeftOperand) / Double.parseDouble(RightOperand));

System.out.println(outputText);

txt_Area.append(outputText);

break;

}

}

});

}

private void showFrame() {

this.setSize(230, 340);

setTitle("Calculator");

this.setVisible(true);

this.setResizable(false);

this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

}

private void setIcon() {

setIconImage(Toolkit.getDefaultToolkit().getImage(getClass().getResource("calculator.png")));

}

}

Main class:

import calculator.calc;

public class Main {

public static void main(String [] args) {

new calc();

}

}