java calc example: This is an example of java.swing.* usage. Non-GUI features are intentionally poorly implemented. Don't plagiarizer.

Calculator.java

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

class Calculator extends JFrame implements ActionListener {
    
    int cols = 6;
    JPanel[] row = new JPanel[cols];
    JButton[] button = new JButton[20];
    String[] buttonString = {"7", "8", "9", "+",
                             "4", "5", "6", "-",
                             "1", "2", "3", "*",
                             ".", "/", "C", "",
                             "", "=", "0", ""};

    Dimension text_area_dim = new Dimension(300, 40); // text box
    Dimension normal_button_dim = new Dimension(40, 40); // regular column buttons
    boolean[] function = new boolean[4];
    double[] temporary = {0, 0};

    JTextArea display = new JTextArea(1,20);
    //JTextArea display2 = new JTextArea(1,20);
    Font font = new Font("Times new Roman", Font.BOLD, 14);
    
    Calculator() {
        //super("Calculator");
        setDesign();

        setSize(380, 250);
        setResizable(false);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        GridLayout grid = new GridLayout(5,6); // rowsXcols
        setLayout(grid);
        
        for(int i = 0; i < cols; i++)
            row[i] = new JPanel(); //add jpanels to our jfram indieces

        for(int i = 0; i < 20; i++) {
            button[i] = new JButton();
            button[i].setText(buttonString[i]);
            button[i].setFont(font);
            button[i].addActionListener(this);
        }
        
        display.setFont(font);
        display.setEditable(false);
        display.setComponentOrientation(ComponentOrientation.RIGHT_TO_LEFT);
        display.setPreferredSize(text_area_dim);

        for(int i = 0; i < 20; i++) {
            button[i].setPreferredSize(normal_button_dim);
            if( buttonString[i] == "" ) {
                button[i].setOpaque(false);
                button[i].setContentAreaFilled(false);
                button[i].setBorderPainted(false);
            }

        }

        int thisRow = 0;

        row[thisRow].add(display);
        add(row[thisRow]);
        
        thisRow++;
        for(int i = 0; i < 4; i++)
            row[thisRow].add(button[i]);
        row[thisRow].add(button[14]);
        add(row[thisRow]);

        thisRow++;
        for(int i = 4; i < 8; i++)
            row[thisRow].add(button[i]);
        row[thisRow].add(button[15]);
        add(row[thisRow]);

        thisRow++;
        for(int i = 8; i < 12; i++)
            row[thisRow].add(button[i]);
        row[thisRow].add(button[16]);
        add(row[thisRow]);

        thisRow++;  
        row[thisRow].add(button[18]);
        for(int i = 12; i < 14; i++)
            row[thisRow].add(button[i]);
        row[thisRow].add(button[17]);
        add(row[thisRow]);
        
        setVisible(true);
    }
    
    public void clear() {
        try {
            display.setText("");
            for(int i = 0; i < 4; i++)
                function[i] = false;
            for(int i = 0; i < 2; i++)
                temporary[i] = 0;
        } catch(NullPointerException e) {  
        }
    }
    
    public void getSqrt() {
        try {
            double value = Math.sqrt(Double.parseDouble(display.getText()));
            display.setText(Double.toString(value));
        } catch(NumberFormatException e) {
        }
    }
    
    public void getPosNeg() {
        try {
            double value = Double.parseDouble(display.getText());
            if(value != 0) {
                value = value * (-1);
                display.setText(Double.toString(value));
            }
            else {
            }
        } catch(NumberFormatException e) {
        }
    }
    
    public void getResult() {
        double result = 0;
        temporary[1] = Double.parseDouble(display.getText());
        String temp0 = Double.toString(temporary[0]);
        String temp1 = Double.toString(temporary[1]);
        try {
            if(temp0.contains("-")) {
                String[] temp00 = temp0.split("-", 2);
                temporary[0] = (Double.parseDouble(temp00[1]) * -1);
            }
            if(temp1.contains("-")) {
                String[] temp11 = temp1.split("-", 2);
                temporary[1] = (Double.parseDouble(temp11[1]) * -1);
            }
        } catch(ArrayIndexOutOfBoundsException e) {
        }
        try {
            if(function[2] == true)
                result = temporary[0] * temporary[1];
            else if(function[3] == true)
                result = temporary[0] / temporary[1];
            else if(function[0] == true)
                result = temporary[0] + temporary[1];
            else if(function[1] == true)
                result = temporary[0] - temporary[1];
            display.setText(Double.toString(result));
            for(int i = 0; i < 4; i++)
                function[i] = false;
        } catch(NumberFormatException e) {
        }
    }
    
    public final void setDesign() {
        try {
            UIManager.setLookAndFeel(
                    "com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel");
        } catch(Exception e) {   
        }
    }
    
    @Override
    public void actionPerformed(ActionEvent ae) {

        

        if(ae.getSource() == button[0])
            display.append("7");
        if(ae.getSource() == button[1])
            display.append("8");
        if(ae.getSource() == button[2])
            display.append("9");
        if(ae.getSource() == button[3]) {
            //add function[0]
            temporary[0] = Double.parseDouble(display.getText());
            function[0] = true;
            display.setText("");
        }
        if(ae.getSource() == button[4])
            display.append("4");
        if(ae.getSource() == button[5])
            display.append("5");
        if(ae.getSource() == button[6])
            display.append("6");
        if(ae.getSource() == button[7]) {
            //subtract function[1]
            temporary[0] = Double.parseDouble(display.getText());
            function[1] = true;
            display.setText("");
        }
        if(ae.getSource() == button[8])
            display.append("1");
        if(ae.getSource() == button[9])
            display.append("2");
        if(ae.getSource() == button[10])
            display.append("3");
        if(ae.getSource() == button[11]) {
            //multiply function[2]
            temporary[0] = Double.parseDouble(display.getText());
            function[2] = true;
            display.setText("");
        }
        if(ae.getSource() == button[12])
            display.append(".");
        if(ae.getSource() == button[13]) {
            //divide function[3]
            temporary[0] = Double.parseDouble(display.getText());
            function[3] = true;
            display.setText("");
        }
        if(ae.getSource() == button[14])
            clear();
        if(ae.getSource() == button[15])
            getSqrt();
        if(ae.getSource() == button[16])
            getPosNeg();
        if(ae.getSource() == button[17])
            getResult();
        if(ae.getSource() == button[18])
            display.append("0");
    }
    
    public static void main(String[] arguments) {
        Calculator c = new Calculator();
    }
}