import java.awt.*;
import java.awt.event.*;
public class Test45 {
public static void main(String args[]){
new MyFrame().launch();
}
}class MyFrame extends Frame{
public void launch(){
FlowLayout f = new FlowLayout();
TextField tf1 = new TextField(10);
TextField tf2 = new TextField(10);
TextField tf3 = new TextField(15);
Label l1 = new Label("+");
Button b = new Button("=");
setLayout(f);
add(tf1);
add(l1);
add(tf2);
add(b);
add(tf3);
b.addActionListener(new Listener(this));
pack();
setVisible(true);
}
}class Listener implements ActionListener {
MyFrame mf = null;
Listener(MyFrame mf){
this.mf= mf;
}
public void actionPerformed(ActionEvent w){
int num1 = Integer.parseInt(mf.tf1.getText());
int num2 = Integer.parseInt(mf.tf2.getText());
mf.tf3.setText(" "+ (num1+num2));
}
}为什么mf.tf1.getText()
mf.tf2.getText()
mf.tf3.getText()会报错 我哪里写错了 谢谢啊
import java.awt.event.*;
public class Test45 {
public static void main(String args[]){
new MyFrame().launch();
}
}class MyFrame extends Frame{
public void launch(){
FlowLayout f = new FlowLayout();
TextField tf1 = new TextField(10);
TextField tf2 = new TextField(10);
TextField tf3 = new TextField(15);
Label l1 = new Label("+");
Button b = new Button("=");
setLayout(f);
add(tf1);
add(l1);
add(tf2);
add(b);
add(tf3);
b.addActionListener(new Listener(this));
pack();
setVisible(true);
}
}class Listener implements ActionListener {
MyFrame mf = null;
Listener(MyFrame mf){
this.mf= mf;
}
public void actionPerformed(ActionEvent w){
int num1 = Integer.parseInt(mf.tf1.getText());
int num2 = Integer.parseInt(mf.tf2.getText());
mf.tf3.setText(" "+ (num1+num2));
}
}为什么mf.tf1.getText()
mf.tf2.getText()
mf.tf3.getText()会报错 我哪里写错了 谢谢啊
package com.yxk.test;/**
* ClassName:MyFrame
* @Description: TODO(这里用一句话描述这个类的作用)
* @author yxk
* @version 1.0 2011-9-13
* @since 1.0
* @see
*/
import java.awt.*;
import java.awt.event.*;public class MyFrame extends Frame implements ActionListener{
/**
* @Description: TODO(添加属性注释)
*/
private static final long serialVersionUID = 1L;
public FlowLayout f = new FlowLayout();
public TextField tf1 = new TextField(10);
public TextField tf2 = new TextField(10);
public TextField tf3 = new TextField(15);
public Label l1 = new Label("+");
public Button b = new Button("="); public MyFrame() {
this.addWindowListener(new WindowAdapter() {
@Override
public void windowClosing(WindowEvent e) {
System.exit(0);
}
}); } public void launch() { setLayout(f);
add(tf1);
add(l1);
add(tf2);
add(b);
add(tf3);
b.addActionListener(this);
pack();
setVisible(true);
} @Override
public void actionPerformed(ActionEvent e) {
int num1 = Integer.parseInt(tf1.getText());
int num2 = Integer.parseInt(tf2.getText());
tf3.setText(" " + (num1 + num2));
} public static void main(String[] args) {
new MyFrame().launch();
}}
你把他改成成员变量,然后在方法里面实例化;但注意的是,这些成员变量一定要在点用前实例化,否着会报nullclassexception