一个加法计算器的小程序。有三个小疑点

import java.awt.Button;
import java.awt.Frame;
import java.awt.GridLayout;
import java.awt.Panel;
import java.awt.TextField;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;


public class Calculator extends WindowAdapter implements ActionListener{
    private double result = 0;
    private double data1 = 0;
    private boolean resultIndicate = false;
    private Frame f;
    private Button b1, b2, b3, b4, b5, b6, b7, b8, b9;
    private TextField t;
    private Panel p,p1,p2;
    private char prec='+';
    private int zhi=0;
    private int nextzhi=0;
    private int sum=0;
    static public void main(String args[]) {
        Calculator cc = new Calculator();
        cc.go();
    }
    public void go()
    {
        f = new Frame("Calculator");
        p = new Panel();
        p1 = new Panel();
        p2 = new Panel();
        p.setLayout(new GridLayout(3,3));
        p2.setLayout(new GridLayout(1, 1));
        p1.setLayout(new GridLayout(2, 1));
        b1 = new Button("1");
        b2 = new Button("2");
        b3 = new Button("3");
        b4 = new Button("4");
        b5 = new Button("5");
        b6 = new Button("6");
        b7 = new Button("7");
        b8 = new Button("8");
        b9 = new Button("9");
        t = new TextField("0",30);
        f.add(t,"South");
        f.add(p,"Center");
        f.add(p1,"East");
        f.add(p2,"North");
        p.add(b1);
        p.add(b2);
        p.add(b3);
        p.add(b4);
        p.add(b5);
        p.add(b6);
        p.add(b7);
        p.add(b8);
        p.add(b9);
        b1.addActionListener(this);
        b2.addActionListener(this);
        b3.addActionListener(this);
        b4.addActionListener(this);
        b5.addActionListener(this);
        b6.addActionListener(this);
        b7.addActionListener(this);
        b8.addActionListener(this);
        b9.addActionListener(this);
        f.setSize(250, 190);
        f.setVisible(true);
    }
    public void actionPerformed(ActionEvent es) {
        // TODO Auto-generated method stub
        if(es.getSource()==b1){t.setText(t.getText()+1);}
        if(es.getSource()==b2){t.setText(t.getText()+2);}
        if(es.getSource()==b3){t.setText(t.getText()+3);}
        if(es.getSource()==b4){t.setText(t.getText()+4);}
        if(es.getSource()==b5){t.setText(t.getText()+5);}
        if(es.getSource()==b6){t.setText(t.getText()+6);}
        if(es.getSource()==b7){t.setText(t.getText()+7);}
        if(es.getSource()==b8){t.setText(t.getText()+8);}
        if(es.getSource()==b9){t.setText(t.getText()+9);}
        zhi=Integer.parseInt(t.getText());
        StringBuffer sb=new StringBuffer(t.getText());
        int length=sb.length();
        sb.deleteCharAt(length-1);
        t.setText(sb.toString());
        nextzhi=Integer.parseInt(t.getText());
        sum=zhi+nextzhi;
        t.setText(sum+"");
    }
        public void windowClosing(WindowEvent e) {
            System.exit(0);
        }
}