Create Calculator in Java Swing

In this blog post, we'll walk through the creation of calculator application using Java's Swing library. The calculator performs basic arithmetic operations, including addition, subtraction, multiplication, and division, as well as exponentiation. We'll use inFix to postFix conversion and postFix evaluation using stack for result calculation.

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Create a new project in Intellij with the following directory structure. directory structure
Our project consists of three main classes:

CalculatorApp
The main entry point of the application.
CalculatorGui
This class handles the graphical user interface (GUI) of the calculator.
EventListeners
This class manages the events generated by user interactions with the calculator buttons.

CalculatorApp

This is the entry point of our application. It initializes the GUI and sets up the event listeners for the buttons. 

                    

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                        package org.example;

import javax.swing.*;
import java.util.Arrays;


public class CalculatorApp {

    private CalculatorGui calculatorGui;

    private EventListeners eventListeners;

    CalculatorApp(){
        calculatorGui = new CalculatorGui();
        calculatorGui.initCalculatorGui();
        calculatorGui.setLocationRelativeTo(null);
        calculatorGui.setVisible(true);

        eventListeners = new EventListeners(calculatorGui);
        // attach event listeners to all buttons in buttons panel
        Arrays.stream(calculatorGui.getBtnsPanel().getComponents()).forEach(
                btn -> {
                    if (btn instanceof JButton jButton){
                        jButton.addActionListener(eventListeners);
                    }
                }

        );
    }

    public static void main(String[] args){
        CalculatorApp calculatorApp = new CalculatorApp();
    }
}

CalculatorGui

This class constructs the GUI for the calculator, including the display field and buttons for various operations. Components are added using a GridBagLayout, which allows flexible positioning. 

                    

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                    package org.example;

import javax.swing.*;
import java.awt.*;

public class CalculatorGui extends JFrame {
    private JTextField displayField;
    private JPanel btnsPanel;
    private JButton zeroBtn;
    private JButton oneBtn;
    private JButton twoBtn;
    private JButton threeBtn;
    private JButton fourBtn;
    private JButton fiveBtn;
    private JButton sixBtn;
    private JButton sevenBtn;
    private JButton eightBtn;
    private JButton nineBtn;
    private JButton tenBtn;
    private JButton pointBtn;
    private JButton exponentBtn;
    private JButton plusBtn;
    private JButton minusBtn;
    private JButton multiplyBtn;
    private JButton divideBtn;
    private JButton deleteBtn;
    private JButton clearAllBtn;
    private JButton equalBtn;

    // constants
    private final Dimension guiDimension = new Dimension(350, 500);
    private final Dimension displayDimension = new Dimension(250, 50);
    private final Dimension btnPanelDimension = new Dimension(250, 250);
    private final Dimension btnDimension = new Dimension(55, 30);

    private final Insets leftBtnInsets = new Insets(7, 0, 7, 5);
    private final Insets midBtnInsets = new Insets(7, 5, 7, 5);
    private final Insets rightBtnInsets = new Insets(7, 5, 7, 0);

    private final Color bg = new Color(216, 226, 220);
    private final Color displayColor = new Color(23, 48, 28);
    private final Color operandBtnColor = new Color(74, 37, 69);
    private final Color operatorBtnColor = new Color(247, 157, 92);
    private final Color equalBtnColor = new Color(5, 140, 66);
    private final Color delBtnColor = new Color(231, 29, 54);

    private void addDisplayField(){
        displayField = new JTextField();
        displayField.setPreferredSize(displayDimension);
        displayField.setHorizontalAlignment(SwingConstants.RIGHT);
        displayField.setBackground(displayColor);
        displayField.setForeground(Color.WHITE);
        displayField.setFont(new Font("Arial", Font.PLAIN, 24));
        displayField.setBorder(BorderFactory.createEmptyBorder());
        GridBagConstraints gbc = new GridBagConstraints();
        // place in 1st col and 1st row
        gbc.gridx = 0;
        gbc.gridy = 0;
        getContentPane().add(displayField, gbc);
    }

    private JButton createBtn(String btnText, Dimension preferredSize, Color bgColor){
        JButton btn = new JButton(btnText);
        btn.setPreferredSize(preferredSize);
        btn.setBackground(bgColor);
        btn.setForeground(Color.WHITE);
        btn.setFont(new Font("Arial", Font.PLAIN, 20));
        btn.setBorder(BorderFactory.createEmptyBorder());
        return btn;
    }

    private void addBtns(){
        GridBagConstraints gbc = new GridBagConstraints();

        zeroBtn = createBtn("0", btnDimension, operandBtnColor);
        gbc.gridx = 0;
        gbc.gridy = 0;
        gbc.insets = leftBtnInsets;
        btnsPanel.add(zeroBtn, gbc);

        pointBtn = createBtn(".", btnDimension, operandBtnColor);
        gbc.gridx = 1;
        gbc.insets = midBtnInsets;
        btnsPanel.add(pointBtn, gbc);

        exponentBtn = createBtn("^", btnDimension, operatorBtnColor);
        gbc.gridx = 2;
        gbc.insets = midBtnInsets;
        btnsPanel.add(exponentBtn, gbc);

        plusBtn = createBtn("+", btnDimension, operatorBtnColor);
        gbc.gridx = 3;
        gbc.insets = rightBtnInsets;
        btnsPanel.add(plusBtn, gbc);

        oneBtn = createBtn("1", btnDimension, operandBtnColor);
        gbc.gridx = 0;
        gbc.gridy = 1;
        gbc.insets = leftBtnInsets;
        btnsPanel.add(oneBtn, gbc);

        twoBtn = createBtn("2", btnDimension, operandBtnColor);
        gbc.gridx = 1;
        gbc.insets = midBtnInsets;
        btnsPanel.add(twoBtn, gbc);

        threeBtn = createBtn("3", btnDimension, operandBtnColor);
        gbc.gridx = 2;
        gbc.insets = midBtnInsets;
        btnsPanel.add(threeBtn, gbc);

        minusBtn = createBtn("-", btnDimension, operatorBtnColor);
        gbc.gridx = 3;
        gbc.insets = rightBtnInsets;
        btnsPanel.add(minusBtn, gbc);

        fourBtn = createBtn("4", btnDimension, operandBtnColor);
        gbc.gridx = 0;
        gbc.gridy = 2;
        gbc.insets = leftBtnInsets;
        btnsPanel.add(fourBtn, gbc);

        fiveBtn = createBtn("5", btnDimension, operandBtnColor);
        gbc.gridx = 1;
        gbc.insets = midBtnInsets;
        btnsPanel.add(fiveBtn, gbc);

        sixBtn = createBtn("6", btnDimension, operandBtnColor);
        gbc.gridx = 2;
        gbc.insets = midBtnInsets;
        btnsPanel.add(sixBtn, gbc);

        multiplyBtn = createBtn("*", btnDimension, operatorBtnColor);
        gbc.gridx = 3;
        gbc.insets = rightBtnInsets;
        btnsPanel.add(multiplyBtn, gbc);

        sevenBtn = createBtn("7", btnDimension, operandBtnColor);
        gbc.gridx = 0;
        gbc.gridy = 3;
        gbc.insets = leftBtnInsets;
        btnsPanel.add(sevenBtn, gbc);

        eightBtn = createBtn("8", btnDimension, operandBtnColor);
        gbc.gridx = 1;
        gbc.insets = midBtnInsets;
        btnsPanel.add(eightBtn, gbc);

        nineBtn = createBtn("9", btnDimension, operandBtnColor);
        gbc.gridx = 2;
        gbc.insets = midBtnInsets;
        btnsPanel.add(nineBtn, gbc);

        divideBtn = createBtn("/", btnDimension, operatorBtnColor);
        gbc.gridx = 3;
        gbc.insets = rightBtnInsets;
        btnsPanel.add(divideBtn, gbc);

        deleteBtn = createBtn("Del", btnDimension, delBtnColor);
        gbc.gridx = 0;
        gbc.gridy = 4;
        gbc.insets = leftBtnInsets;
        btnsPanel.add(deleteBtn, gbc);

        clearAllBtn = createBtn("CA", btnDimension, delBtnColor);
        gbc.gridx = 1;
        gbc.insets = midBtnInsets;
        btnsPanel.add(clearAllBtn, gbc);

        equalBtn = createBtn("=", btnDimension, equalBtnColor);
        gbc.gridx = 2;
        gbc.insets = rightBtnInsets;
        gbc.gridwidth = 3;
        gbc.fill = 1;
        btnsPanel.add(equalBtn, gbc);

    }

    private void addBtnsPanel(){
        btnsPanel = new JPanel();
        btnsPanel.setPreferredSize(btnPanelDimension);
        btnsPanel.setBackground(null);
        btnsPanel.setLayout(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        // place in 1st col and 2nd row
        gbc.gridx = 0;
        gbc.gridy = 1;
        getContentPane().add(btnsPanel, gbc);

        addBtns();
    }

    public void initCalculatorGui(){
        setSize(guiDimension);
        getContentPane().setBackground(bg);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        GridBagLayout gridBagLayout = new GridBagLayout();
        setLayout(gridBagLayout);

        addDisplayField();
        addBtnsPanel();
    }

    public JTextField getDisplayField() {
        return displayField;
    }

    public void setDisplayField(JTextField displayField) {
        this.displayField = displayField;
    }

    public JPanel getBtnsPanel() {
        return btnsPanel;
    }

    public void setBtnsPanel(JPanel btnsPanel) {
        this.btnsPanel = btnsPanel;
    }

    public JButton getZeroBtn() {
        return zeroBtn;
    }

    public void setZeroBtn(JButton zeroBtn) {
        this.zeroBtn = zeroBtn;
    }

    public JButton getOneBtn() {
        return oneBtn;
    }

    public void setOneBtn(JButton oneBtn) {
        this.oneBtn = oneBtn;
    }

    public JButton getTwoBtn() {
        return twoBtn;
    }

    public void setTwoBtn(JButton twoBtn) {
        this.twoBtn = twoBtn;
    }

    public JButton getThreeBtn() {
        return threeBtn;
    }

    public void setThreeBtn(JButton threeBtn) {
        this.threeBtn = threeBtn;
    }

    public JButton getFourBtn() {
        return fourBtn;
    }

    public void setFourBtn(JButton fourBtn) {
        this.fourBtn = fourBtn;
    }

    public JButton getFiveBtn() {
        return fiveBtn;
    }

    public void setFiveBtn(JButton fiveBtn) {
        this.fiveBtn = fiveBtn;
    }

    public JButton getSixBtn() {
        return sixBtn;
    }

    public void setSixBtn(JButton sixBtn) {
        this.sixBtn = sixBtn;
    }

    public JButton getSevenBtn() {
        return sevenBtn;
    }

    public void setSevenBtn(JButton sevenBtn) {
        this.sevenBtn = sevenBtn;
    }

    public JButton getEightBtn() {
        return eightBtn;
    }

    public void setEightBtn(JButton eightBtn) {
        this.eightBtn = eightBtn;
    }

    public JButton getNineBtn() {
        return nineBtn;
    }

    public void setNineBtn(JButton nineBtn) {
        this.nineBtn = nineBtn;
    }

    public JButton getTenBtn() {
        return tenBtn;
    }

    public void setTenBtn(JButton tenBtn) {
        this.tenBtn = tenBtn;
    }

    public JButton getPointBtn() {
        return pointBtn;
    }

    public void setPointBtn(JButton pointBtn) {
        this.pointBtn = pointBtn;
    }

    public JButton getExponentBtn() {
        return exponentBtn;
    }

    public void setExponentBtn(JButton exponentBtn) {
        this.exponentBtn = exponentBtn;
    }

    public JButton getPlusBtn() {
        return plusBtn;
    }

    public void setPlusBtn(JButton plusBtn) {
        this.plusBtn = plusBtn;
    }

    public JButton getMinusBtn() {
        return minusBtn;
    }

    public void setMinusBtn(JButton minusBtn) {
        this.minusBtn = minusBtn;
    }

    public JButton getMultiplyBtn() {
        return multiplyBtn;
    }

    public void setMultiplyBtn(JButton multiplyBtn) {
        this.multiplyBtn = multiplyBtn;
    }

    public JButton getDivideBtn() {
        return divideBtn;
    }

    public void setDivideBtn(JButton divideBtn) {
        this.divideBtn = divideBtn;
    }

    public JButton getDeleteBtn() {
        return deleteBtn;
    }

    public void setDeleteBtn(JButton deleteBtn) {
        this.deleteBtn = deleteBtn;
    }

    public JButton getClearAllBtn() {
        return clearAllBtn;
    }

    public void setClearAllBtn(JButton clearAllBtn) {
        this.clearAllBtn = clearAllBtn;
    }

    public JButton getEqualBtn() {
        return equalBtn;
    }

    public void setEqualBtn(JButton equalBtn) {
        this.equalBtn = equalBtn;
    }

    public Dimension getGuiDimension() {
        return guiDimension;
    }

    public Dimension getDisplayDimension() {
        return displayDimension;
    }

    public Dimension getBtnPanelDimension() {
        return btnPanelDimension;
    }

    public Dimension getBtnDimension() {
        return btnDimension;
    }

    public Insets getLeftBtnInsets() {
        return leftBtnInsets;
    }

    public Insets getMidBtnInsets() {
        return midBtnInsets;
    }

    public Insets getRightBtnInsets() {
        return rightBtnInsets;
    }

    public Color getBg() {
        return bg;
    }

    public Color getDisplayColor() {
        return displayColor;
    }

    public Color getOperandBtnColor() {
        return operandBtnColor;
    }

    public Color getOperatorBtnColor() {
        return operatorBtnColor;
    }

    public Color getEqualBtnColor() {
        return equalBtnColor;
    }

    public Color getDelBtnColor() {
        return delBtnColor;
    }
}

EventListeners

This class handles the logic for button presses, including calculating results based on user input. It converts infix expressions (like 1 + 2) to postfix notation (like 1 2 +) for easier evaluation. It uses a stack-based approach to evaluate the postfix expression and provides exception handling. 

                    

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                    package org.example;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

public class EventListeners implements ActionListener {
    private CalculatorGui calculatorGui;

    EventListeners(CalculatorGui calculatorGui){
        this.calculatorGui = calculatorGui;
    }

    private int precedence(char ch){
        if(ch == '^')
            return 3;
        else if(ch == '/' || ch == '*')
            return 2;
        else if(ch == '+' || ch == '-')
            return 1;
        else return 0;
    }

    private List<String> infixToPostfix(String infix){
        List<String> postFixList = new ArrayList<>();
        Stack<Character> operatorStack = new Stack<>();
        // split input string with operators and also keep operators to simplify parsing of decimal numbers
        String[] infixArray = infix.split("(?<=[-+*/^])|(?=[-+*/^])");

        for(int i = 0; i< infixArray.length; i++){
            // add numbers in postFixList
            if(!infixArray[i].matches("[+\\-*/^]")){
                postFixList.add(infixArray[i]);
            }else {
                // operator will always be a char at index 0
                char operator = infixArray[i].charAt(0);
                // push operator in stack only if its precedence is higher than top operator in stack else pop operators until top operator has less precedence
                while(!operatorStack.isEmpty() && (precedence(operator) < precedence(operatorStack.peek()) || precedence(operator) == precedence(operatorStack.peek()))){
                    postFixList.add(operatorStack.pop().toString());
                }
                operatorStack.push(operator);
            }
        }

        // after parsing whole string empty the stack and add in postFix
        while(!operatorStack.isEmpty()){
            postFixList.add(operatorStack.pop().toString());
        }
        return postFixList;
    }

    private String evaluatePostFix(List<String> postFixList){
        Stack<Double> operandStack = new Stack<>();

        try{
            postFixList.forEach(p -> {
                // push operands in stack
                if(!p.matches("[+\\-*/^]")){
                    operandStack.push(Double.parseDouble(p));
                }else{
                    // for operator: pop 2 operands and push result in stack
                    Double operand1 = operandStack.pop();
                    Double operand2 = operandStack.pop();

                    if(p.equals("+"))
                        operandStack.push(operand1 + operand2);
                    else if(p.equals("-"))
                        operandStack.push(operand2 - operand1);
                    else if(p.equals("*"))
                        operandStack.push(operand2 * operand1);
                    else if(p.equals("/"))
                        operandStack.push(operand2 / operand1);
                    else if(p.equals("^"))
                        operandStack.push(Math.pow(operand2, operand1));
                }
            });
            return operandStack.pop().toString();
        }catch (Exception e) {
            return "Error";
        }
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        String displayText = calculatorGui.getDisplayField().getText();
        if(e.getActionCommand().equals("Del")){
            // remove last character
            if(!displayText.isEmpty())
                calculatorGui.getDisplayField().setText(displayText.substring(0, displayText.length() -1 ));
        }
        else if(e.getActionCommand().equals("CA")){
            // clear all text
            calculatorGui.getDisplayField().setText("");
        }
        else if(e.getActionCommand().equals("=")){
            // calculate result
            calculatorGui.getDisplayField().setText(evaluatePostFix(infixToPostfix(displayText)));
        }
        else{
            // when operand is pressed append to display text if text does not contain Error
            if(displayText.contains("Error"))
                calculatorGui.getDisplayField().setText(e.getActionCommand());
            else calculatorGui.getDisplayField().setText(displayText + e.getActionCommand());
        }
    }
}

This simple calculator application is a great way to understand the basics of Java Swing for GUI development, as well as fundamental concepts in parsing and evaluating mathematical expressions.