Kalkulator (dasar)

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

class Kalkulator implements ActionListener{

    //=====================================
    private final byte NULL=9;
    private final byte TAMBAH=0;
    private final byte KURANG=1;
    private final byte KALI=2;
    private final byte BAGI=3;
    //=====================================
    private final JFrame mainF;
    private JPanel io;
    private JPanel inputP;
    private JPanel operatorP;
    private JButton[]angka;
    private JButton[]operator;
    private JTextField in;
    private JTextField out;
    //=====================================
    private double inNum=0.0;
    private double hasil=0.0;
    private byte lastOperator=NULL;

    Kalkulator(){
        setup();
        mainF=new JFrame("Kalkulator");
        mainF.add(inputP);
        mainF.add(operatorP);
        mainF.add(io);
        mainF.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainF.setSize(500, 500);
        mainF.setLayout(null);
        mainF.setResizable(false);
        mainF.setLocationByPlatform(true);
        mainF.setVisible(true);
    }

    private void setup(){
        //Setup Panel
        inputP=new JPanel();
        inputP.setBounds(5,50,180,115);
        inputP.setLayout(null);
        inputP.setBorder(BorderFactory.createLineBorder(Color.LIGHT_GRAY, 3));
        operatorP=new JPanel();
        operatorP.setBounds(190,50,125,115);
        operatorP.setLayout(null);
        operatorP.setBorder(BorderFactory.createLineBorder(Color.LIGHT_GRAY, 3));
        io=new JPanel();
        io.setBounds(5,5,(190+120),40);
        io.setLayout(null);
        io.setBorder(BorderFactory.createLineBorder(Color.LIGHT_GRAY, 3));
        /*Setup input*/{
            String[]txt={
                " 1 "," 2 "," 3 "," 4 "," 5 ",
                " 6 "," 7 "," 8 "," 9 "," 0 "
            };
            angka=new JButton[10];
            Rectangle pos=new Rectangle(10,10,50,20);
            for(byte i=0;i<10;i++){
                angka[i]=new JButton(txt[i]);
                angka[i].setBounds(pos);
                if((i+1)%3==0){
                    pos.y+=25;
                    pos.x-=(55*3);
                }
                pos.x+=55;
                inputP.add(angka[i]);
                angka[i].addActionListener(this);
            }
        }
        /*setup operator*/{
            String[]txt={
                " + "," - "," * "," / ",
                " = ","bs"," C "
            };
            operator=new JButton[7];
            Rectangle pos=new Rectangle(10,10,50,20);
            for(byte i=0;i<4;i++){
                operator[i]=new JButton(txt[i]);
                operator[i].setBounds(pos);
                if((i+1)%2==0){
                    pos.y+=25;
                    pos.x-=(55*2);
                }
                if(i==6)pos.width+=100;
                pos.x+=55;
                operatorP.add(operator[i]);
                operator[i].addActionListener(this);
            }
            pos=new Rectangle(operator[2].getBounds());
            pos.width=105;
            pos.y+=25;
            operator[4]=new JButton(txt[4]);
            operator[4].setBounds(pos);
            operator[4].addActionListener(this);
            operatorP.add(operator[4]);
            pos.y+=25;
            pos.width=operator[0].getBounds().width;
            for(byte i=5;i<7;i++){
                operator[i]=new JButton(txt[i]);
                operator[i].setBounds(pos);
                operatorP.add(operator[i]);
                operator[i].addActionListener(this);
                pos.x+=55;
            }
        }
        /*setup field text*/{
            Rectangle pos=new Rectangle(5,10,170,20);
            in=new JTextField(100);
            out=new JTextField(100);
            in.setBounds(pos);
            pos=new Rectangle(190,10,115,20);
            out.setBounds(pos);
            io.add(in);
            io.add(out);
            in.setForeground(Color.red);
            //=======================================
            // * Sementara
            //in.setEnabled(false);
            //out.setEnabled(false);
        }
    }

    @Override
    public void actionPerformed(ActionEvent e){
       if(in.getText().equals(""))in.setText("0");
       String input_num=e.getActionCommand();
       //==============================================
       if(input_num.charAt(1)>='0'&&input_num.charAt(1)<='9'){
           in.setText(in.getText()+input_num.charAt(1));
       }
       //==============================================
       if(input_num.equals("bs"))
           in.setText(in.getText().substring(0, in.getText().length()-1));
       //==============================================
       if(input_num.equals(" C ")){
           in.setText("");
           hasil=0;
           lastOperator=NULL;
       }
       //==============================================
       if(input_num.charAt(1)=='+'){
           if(lastOperator==NULL){
               hasil=Double.parseDouble(in.getText());
           }else{
               inNum=Double.parseDouble(in.getText());
               hitung();
           }
           lastOperator=TAMBAH;
       }else
       if(input_num.charAt(1)=='-'){
           if(lastOperator==NULL){
               hasil=Double.parseDouble(in.getText());
           }else{
               inNum=Double.parseDouble(in.getText());
               hitung();
           }
           lastOperator=KURANG;
       }else
       if(input_num.charAt(1)=='*'){
           if(lastOperator==NULL){
               hasil=Double.parseDouble(in.getText());
           }else{
               inNum=Double.parseDouble(in.getText());
               hitung();
           }
           lastOperator=KALI;
       }else
       if(input_num.charAt(1)=='/'){
           if(lastOperator==NULL){
               hasil=Double.parseDouble(in.getText());
           }else{
               inNum=Double.parseDouble(in.getText());
               hitung();
           }
           lastOperator=BAGI;
       }else
       if(input_num.charAt(1)=='='){
           if(lastOperator==NULL)
               hasil=Double.parseDouble(in.getText());
           else
               inNum=Double.parseDouble(in.getText());
           //-----------------------------------------
           hitung();
           Double h=hasil;
           //-----------------------------------------
           out.setText(h.toString());
           hasil=inNum=0;
           in.setText("");
           lastOperator=NULL;
       }
    }

    private void hitung(){
       switch(lastOperator){
           case TAMBAH:
               hasil+=inNum;
               break;
           case KURANG:
               hasil-=inNum;
               break;
           case BAGI:
               hasil/=inNum;
               break;
           case KALI:
               hasil*=inNum;
               break;
       }
    }

    public static void main(String[]args){
        Kalkulator k=new Kalkulator();
    }
}