Calculator.java

package com.company;

import javax.swing.*;
import javax.swing.text.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Stack;

public class Calculator {

    private JTextPane textArea;

    private boolean resultCalculated;

    public static void main(String[] args) {
        Calculator gui = new Calculator();
        gui.setup();
    }

    private void setup(){
        JFrame frame = new JFrame();
        frame.setLayout(null);

        JPanel topPanel = new JPanel();
        textArea = new JTextPane();

        StyledDocument doc = textArea.getStyledDocument();
        SimpleAttributeSet center = new SimpleAttributeSet();
        StyleConstants.setAlignment(center, StyleConstants.ALIGN_CENTER);
        doc.setParagraphAttributes(0, doc.getLength(), center,false);

        topPanel.add(textArea);
        topPanel.setLayout(null);
        textArea.setBounds(10,10,460,50);
        topPanel.setBounds(0,0,500,70);
        topPanel.setBackground(Color.darkGray);

        frame.getContentPane().add(topPanel);

        JPanel buttonsPanel = new JPanel();
        buttonsPanel.setBackground(Color.GRAY);
        buttonsPanel.setBounds(0,70,500,230);
        buttonsPanel.setLayout(null);
        //y14 x13 слева (25 между 9)

        String[][] buttonsValues = {{"1","2","3","+"},
                {"4","5","6","-"},
                {"7","8","9","*"},
                {".","0","C","/"}};


        for(int i = 0; i < 4; i++)
            for (int j = 0; j < 4; j++){
                addButton(buttonsValues[i][j], buttonsPanel, 13 + (100 + 20) * j,10 + (30 + 5) * i); //373 115
            }

        JButton resultButton = new JButton("=");
        resultButton.setBounds(373,150,100,30);
        resultButton.addActionListener(new ResultAction());
        buttonsPanel.add(resultButton);

        frame.getContentPane().add(buttonsPanel);

        frame.setSize(500,300);
        frame.setResizable(false);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    private void addButton(String value, JPanel panel, int x, int y){
        JButton button = new JButton(value);
        button.setBounds(x,y, 100,30);
        button.addActionListener(new InputAction());
        panel.add(button);
    }

    public static double RPNtoAnswer(String rpn){
        String operand = new String();
        Stack<Double> stack = new Stack<>();

        for (int i = 0; i < rpn.length(); i++){
            if (getP(rpn.charAt(i)) == ' ') continue;

            if (getP(rpn.charAt(i)) == 0){
                while(rpn.charAt(i) != ' ' && getP(rpn.charAt(i)) == 0){
                    operand += rpn.charAt(i++);
                    if(i == rpn.length()) break;
                }
                stack.push(Double.parseDouble(operand));
                operand = new String();
            }

            if (getP(rpn.charAt(i)) > 1){
                double a = stack.pop(), b = stack.pop();

                if(rpn.charAt(i) == '+') stack.push(b+a);
                if(rpn.charAt(i) == '-') stack.push(b-a);
                if(rpn.charAt(i) == '*') stack.push(b*a);
                if(rpn.charAt(i) == '/') stack.push(b/a);
            }
        }

        return stack.pop();
    }

    public static String ExpressionToRPN(String Expr){
        String current = "";
        Stack<Character> stack = new Stack<>();

        int priority;
        for (int i = 0; i < Expr.length();i++){
            priority = getP(Expr.charAt(i));

            if (priority == 0) current += Expr.charAt(i);
            if(priority == 1) stack.push(Expr.charAt(i));

            if (priority > 1){
                current += ' ';
                while(!stack.empty()){
                    if(getP(stack.peek()) >= priority)current += stack.pop();
                    else break;
                }
                stack.push(Expr.charAt(i));
            }

            if (priority == -1){
                current+= ' ';
                while(getP(stack.peek()) != 1)current += stack.pop();
                stack.pop();
            }
        }
        while(!stack.empty())current += stack.pop();

        return current;
    }

    private static int getP(char token){
        if (token == '*' || token == '/') return 3;
        else if (token == '+' || token == '-') return 2;
        else if (token == '(') return 1;
        else if (token == ')')return -1;
        else return 0;
    }

    public class InputAction implements ActionListener{

        @Override
        public void actionPerformed(ActionEvent e) {
            if (!resultCalculated){
                String input = e.getActionCommand();
                textArea.setText(textArea.getText() + input);
            }
            else{
                String input = e.getActionCommand();
                textArea.setText("");
                textArea.setText(textArea.getText() + input);
                resultCalculated = false;
            }

            if (e.getActionCommand().equals("C"))
                textArea.setText("");

        }
    }

    public class ResultAction implements ActionListener{

        @Override
        public void actionPerformed(ActionEvent e) {
            String action = textArea.getText();
            textArea.setText(String.valueOf(RPNtoAnswer(ExpressionToRPN(action))));
            resultCalculated = true;
        }
    }
}