Java GUI Simple Calculator

package javagui;

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

public class JavaGUI extends JFrame implements ActionListener{
    String nilai1="0",nilai2="0",hasil="";
    Font f=new Font("SansSerif",Font.BOLD, 50);
    JTextField t=new JTextField();
    JButton[] tombol=new JButton[10];
    JButton b=new JButton("C");
    JButton p=new JButton("+");
    JButton m=new JButton("-");
    JButton e=new JButton("=");
    JButton b00=new JButton("00");
    JButton d=new JButton("Del");
    JPanel x= new JPanel(new GridLayout(4, 1, 1, 1));
    JPanel y=new JPanel(new GridLayout(1,4,1,1));
    JPanel y2=new JPanel(new GridLayout(1,4,1,1));
    JPanel y3=new JPanel(new GridLayout(1,4,1,1));
    JPanel x2= new JPanel(new GridLayout(1, 2, 1, 1));
    JPanel x3= new JPanel(new GridLayout(1, 2, 1, 1));
    JPanel xyz= new JPanel(new GridLayout(2, 1, 0, 25));
    int flag=1;

    JavaGUI(){
        fontadj();
        layOut();
        frame();
        pack();
    }

    void fontadj(){
        t.setText("0");
        t.setHorizontalAlignment(JTextField.RIGHT);
        t.setFont(f);
    }

    void frame(){
        setLayout(new GridLayout(1,1,0,75));
        setTitle("CALCULATOR JAVA");
        setResizable(false);
        setLocationRelativeTo(null);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    void layOut(){
        for (int i = 0; i < tombol.length; i++) {
            tombol[i]=new JButton(""+i);
            tombol[i].addActionListener(this);
        }
        y.add(tombol[7]);
        y.add(tombol[8]);
        y.add(tombol[9]);
        y.add(b);b.addActionListener(this);
        x.add(y);
        y2.add(tombol[4]);
        y2.add(tombol[5]);
        y2.add(tombol[6]);
        y2.add(p);p.addActionListener(this);
        x.add(y2);
        y3.add(tombol[1]);
        y3.add(tombol[2]);
        y3.add(tombol[3]);
        y3.add(m);m.addActionListener(this);
        x.add(y3);
        x3.add(tombol[0]);
        x3.add(d);d.addActionListener(this);
        x2.add(x3);
        x2.add(e);e.addActionListener(this);
        x.add(x2);
        xyz.add(t);
        xyz.add(x);
        xyz.setBorder(BorderFactory.createEmptyBorder(10, 25, 10, 25));
        add(xyz);
    }

    void reset(){
        nilai1="0";
        nilai2="0";
        flag=1;
    }

    public void actionPerformed(ActionEvent e) {
        t.setHorizontalAlignment(JTextField.RIGHT);
        System.out.println(e.getActionCommand()+",Flag="+flag);
        if(e.getActionCommand().equals("C")){
            t.setText("0");
            reset();
        }
        else if(e.getActionCommand().equals("Del")&&flag==1&&!nilai1.equals("")){
            int temp;
            System.out.println("masuk");
            if(Integer.parseInt(nilai1)%10!=0)
                temp=(Integer.parseInt(nilai1)-Integer.parseInt(nilai1)%10)/10;
            else
                temp=Integer.parseInt(nilai1)/10;
            nilai1=""+temp;
            if(nilai1.equals("0"))
                nilai1="";
            t.setText(nilai1);
        }
        else if(e.getActionCommand().equals("Del")&&flag!=1&&!nilai2.equals("")){
            int temp;
            if(Integer.parseInt(nilai2)%10!=0)
                temp=(Integer.parseInt(nilai2)-Integer.parseInt(nilai2)%10)/10;
            else
                temp=Integer.parseInt(nilai2)/10;
            nilai2=""+temp;
            if(nilai2.equals("0"))
                nilai2="";
            t.setText(nilai2);
        }
        else if(e.getActionCommand().equals("+")){
            flag=3;
        }
        else if(e.getActionCommand().equals("-")){
            flag=4;

        }
        else if(flag==1&&!e.getActionCommand().equals("=")&&!e.getActionCommand().equals("Del")){
            System.out.println("Masuk samadengan data1");
            if(nilai1.equals("0"))
                nilai1="";
            nilai1+=e.getActionCommand();
            t.setText(nilai1);
        }
        else if(e.getActionCommand().equals("=")&&flag==3){
            int has;
            has=Integer.parseInt(nilai1)+Integer.parseInt(nilai2);
            hasil=""+has;
            t.setText(hasil);
            reset();
        }
        else if(e.getActionCommand().equals("=")&&!nilai1.equals("")){
            int has;
            System.out.println("masuk");
            has=Integer.parseInt(nilai1)-Integer.parseInt(nilai2);
            hasil=""+has;
            t.setText(hasil);
            reset();

        }
        else if (flag!=1){
            if(nilai2.charAt(0)=='0')
                nilai2="";
            nilai2+=e.getActionCommand();
            t.setText(nilai2);
        }
    }

    public static void main(String[] args) {
        new JavaGUI();
    }
}