java"计算器"问题

两项都做了，只是提供了一个做法，具体的可能还有不完善的地方。

import java.awt.Button;
import java.awt.Color;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import java.awt.TextArea;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class Cal extends JFrame implements ActionListener, KeyListener {
private JTextField tf1;
private JTextField tf2;
private TextArea ta;
private StringBuffer str;
private double x;
private double y;
private int z;// Z表示单击了那一个运算符.0表示"+",1表示"-",2表示"*",3表示"/"
private double m;// 记忆的数字
private JButton b0, b1, b2, b3, b4,