问开始java swing，给出按钮功能

EN

import javax.swing.*;
import java.awt.event.*;
import java.awt.BorderLayout;
import java.awt.GridLayout;
import javax.swing.JPanel;


public class GUI extends JFrame implements ItemListener{ //ActionListener
    //JTextArea info = new JTextArea("this will calculate numbers");
    JTextField f1 = new JTextField(10); //param = width
    protected String input;

    //*****************************************************************************************************************




    //constructor
    //@ args String, Int, Int
    public GUI(String title, int width, int height ){
        //gui.pack() *try this later*

        setBounds(500, 170, width, height);// sets location x,y on screen at start of program also sets size of GUI x,y

        setTitle(title); //sets title of

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); //needed else x wont close properly



        createGUI();

    }

    //separate gui logic from other logic
    public void createGUI(){

        //set layout you plan to use
        setLayout(new BorderLayout());
        //add(info, BorderLayout.NORTH);
        add(f1, BorderLayout.NORTH);

        //CENTER********************************************************************************************************
        JPanel center = new JPanel();
        center.setLayout(new GridLayout(4,3));
        center.add(new JButton("1"));
        center.add(new JButton("2"));
        center.add(new JButton("3"));
        center.add(new JButton("4"));
        center.add(new JButton("5"));
        center.add(new JButton("6"));
        center.add(new JButton("7"));
        center.add(new JButton("8"));
        center.add(new JButton("9"));
        center.add(new JButton("0"));
        center.add(new JButton("."));
        center.add(new JButton("C"));
        add(center, BorderLayout.CENTER);
        //EAST**********************************************************************************************************
        JPanel east = new JPanel();
        east.setLayout(new GridLayout(6,1));
        east.add(new JButton("+"));
        east.add(new JButton("-"));
        east.add(new JButton("x"));
        east.add(new JButton("/"));
        east.add(new JButton("%"));
        east.add(new JButton("="));
        add(east, BorderLayout.EAST);
    }


    //main
    public static void main(String[] args){
        GUI calculator = new GUI("GUI Calculator",300,250);

        calculator.setVisible(true);//makes calculator gui visible *make this line last or close to last*

    }

    //Must do this method or else class must be made abstract
    public void itemStateChanged(ItemEvent event){

    }
}