Cfrtjryk

I have created my first Java app. I am very familiar with C++ and thus without reading any books (just introduction, googling and IntelliJ auto-completion) I directly jumped into Java.
So I made a calculator.

Main.java:

import javax.script.ScriptEngine;

import javax.script.ScriptEngineManager;

import javax.script.ScriptException;

import javax.swing.*;

import javax.swing.text.PlainDocument;

import java.awt.*;

import java.awt.event.KeyEvent;



class Main extends JFrame {



    public static void main(String args) throws AWTException {

        System.out.println("init");

        var app = new Main();

        app.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        app.setVisible(true);

    }



    private JButton numButtons = new JButton[10];

    private JButton opeButtons = new JButton[5];

    private JButton lb = new JButton("(");

    private JButton rb = new JButton(")");

    private JButton cb = new JButton("C");

    private JButton decb = new JButton(".");

    private JTextField text = new JTextField();

    private Robot keyboard = new Robot();

    private ScriptEngine jsEng;



    public Main() throws AWTException {

        super();



        jsEng = (new ScriptEngineManager()).getEngineByName("js");



        var textDoc = (PlainDocument)text.getDocument();

        textDoc.setDocumentFilter(new EquationFilter());

        initGui();





        for(JButton i : numButtons) {

            i.addActionListener(e -> simKey(i.getText().charAt(0)));

        }



        for(int i = 0;i<4;i++) {

            int finalI = i;

            opeButtons[i].addActionListener(e -> simKey(opeButtons[finalI].getText().charAt(0)));

        }

        decb.addActionListener(e -> simKey(decb.getText().charAt(0)));

        rb.addActionListener(e -> simKey(rb.getText().charAt(0)));

        lb.addActionListener(e -> simKey(lb.getText().charAt(0)));



        cb.addActionListener(e-> text.setText(""));

        opeButtons[4].addActionListener(e -> {

            try {

                text.setText(jsEng.eval(text.getText()).toString());

            } catch (ScriptException e1) {

                e1.printStackTrace();

            }

        });





    }



    private void simKey(char code){

        text.requestFocus();

        keyboard.keyPress(KeyEvent.VK_SHIFT);

        switch (code) {

            case '+':

                keyboard.keyPress(KeyEvent.VK_EQUALS);

                keyboard.keyRelease(KeyEvent.VK_EQUALS);

                break;

            case '(':

                keyboard.keyPress(KeyEvent.VK_LEFT_PARENTHESIS);

                keyboard.keyRelease(KeyEvent.VK_LEFT_PARENTHESIS);

                break;

            case ')':

                keyboard.keyPress(KeyEvent.VK_RIGHT_PARENTHESIS);

                keyboard.keyRelease(KeyEvent.VK_RIGHT_PARENTHESIS);

                break;

            case '*':

                keyboard.keyPress('8');

                keyboard.keyRelease('8');

                break;

            default:

                keyboard.keyRelease(KeyEvent.VK_SHIFT);

                keyboard.keyPress(code);

                keyboard.keyRelease(code);

                return;

        }

        keyboard.keyRelease(KeyEvent.VK_SHIFT);



    }



    private void initGui(){

        setTitle("Calculator");

        setSize(400,500);

        var grid = new GridBagLayout();

        setLayout(grid);

        GridBagConstraints gridCon = new GridBagConstraints();

        gridCon.weightx = gridCon.weighty = 1.0;



        gridCon.gridy = 0;

        gridCon.gridx = 0;

        gridCon.gridwidth = 4;

        gridCon.fill = gridCon.BOTH;

        add(text,gridCon);



        gridCon.gridwidth = 1 ;



        String names = {"+","-","/","*","="};

        for(int i = 0;i < 5; i++){

            opeButtons[i] = new JButton(names[i]);

        }



        gridCon.gridx = 3;

        for( int y = 1; y < 6; y++ ){

            gridCon.gridy = y;

            add(opeButtons[y-1],gridCon);

        }



        for(int y = 2, i = 1; y < 5; y++ ){

            for(int x = 0 ; x < 3; x++,i++) {

                gridCon.gridx = x;

                gridCon.gridy = y;

                numButtons[i] = new JButton(Integer.toString(i));

                add(numButtons[i],gridCon);

            }

        }



        gridCon.gridx = 0;

        gridCon.gridy = 1;

        add(lb,gridCon);

        gridCon.gridx = 1;

        add(rb,gridCon);

        gridCon.gridx = 2;

        add(cb,gridCon);



        numButtons[0] = new JButton("0");

        gridCon.gridx = 0;

        gridCon.gridy = 5;

        gridCon.gridwidth = 2;

        add(numButtons[0],gridCon);



        gridCon.gridwidth = 1;

        gridCon.gridx = 2;

        add(decb,gridCon);

    }

}

EquationFilter.java:

import javax.swing.text.AttributeSet;

import javax.swing.text.Document;

import javax.swing.text.DocumentFilter;

import javax.swing.text.BadLocationException;

import java.awt.event.KeyEvent;

import java.security.Key;



public class EquationFilter extends DocumentFilter {



    @Override

    public void insertString(FilterBypass fb, int offset, String string,

                             AttributeSet attr) throws BadLocationException {



        Document doc = fb.getDocument();

        StringBuilder sb = new StringBuilder();

        sb.append(doc.getText(0, doc.getLength()));

        sb.insert(offset, string);



        if (test(sb.toString())) {

            super.insertString(fb, offset, string, attr);

        }

    }



    private boolean test(String text) {

        char validInput = {'1','2','3','4','5','6','7','8','9','0','.','/','*','-','+','(',')',' ','','b'};



        if(text.isEmpty())

            return true;



        for(char i : validInput){

            if(i == text.charAt(text.length()-1))

                return true;

        }

        return false;



    }



    @Override

    public void replace(FilterBypass fb, int offset, int length, String text,

                        AttributeSet attrs) throws BadLocationException {



        Document doc = fb.getDocument();

        StringBuilder sb = new StringBuilder();

        sb.append(doc.getText(0, doc.getLength()));

        sb.replace(offset, offset + length, text);



        if (test(sb.toString())) {

            super.replace(fb, offset, length, text, attrs);

        }



    }



    @Override

    public void remove(FilterBypass fb, int offset, int length)

            throws BadLocationException {

        Document doc = fb.getDocument();

        StringBuilder sb = new StringBuilder();

        sb.append(doc.getText(0, doc.getLength()));

        sb.delete(offset, offset + length);



        try {

            if (test(sb.toString())) {

                super.remove(fb, offset, length);

            }

        }

        catch (StringIndexOutOfBoundsException e) {

            super.remove(fb,0,1);

        }



    }

}

https://gitlab.com/SmitTheTux/javacalculator

Please suggest me improvments.