什么是 Java NullPointerException? [英] What is a Java NullPointerException?

问题描述

我创建了一个 Java 应用程序并收到此异常:

I created a Java Application and get this Exception:

Exception in thread "main" java.lang.NullPointerException
    at java.awt.Container.addImpl(Unknown Source)
    at java.awt.Container.add(Unknown Source)
    at javax.swing.JFrame.addImpl(Unknown Source)
    at java.awt.Container.add(Unknown Source)
    at Executer.<init>(Executer.java:21)
    at Executer.main(Executer.java:14 

代码如下:

import javax.swing.*;
import java.awt.*;

public class Executer {

private JLabel lblCommand;
private JTextField txtEnter;
private JButton btNext, btPrevious;
private JPanel panel;

public static void main(String[] args) {
    new Executer();
}
public Executer() {
    JFrame frame = new JFrame("Execute Script");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(900,400);
    frame.setVisible(true);
    frame.add(panel); 
    frame.setVisible(true);
    MyPanel();
    Text();
    Buttons();
    Fields();
}
public void MyPanel() {
    panel = new JPanel();
    panel.setLayout(null);
}
public void Text(){
    lblCommand = new JLabel("Enter Here");
    lblCommand.setBounds(135, 50, 150, 20);
    Font styleOne = new Font("Arial", Font.BOLD, 13);
    lblCommand.setFont(styleOne);
    panel.add(lblCommand);
}

public void Fields () {
    txtEnter = new JTextField();
    txtEnter.setBounds(210, 50, 150, 20);
    panel.add(txtEnter);
}
public void Buttons() {
    btNext = new JButton ("Next");
    btNext.setBounds(380,325,100,20);
    panel.add(btNext);

    btPrevious = new JButton ("Previous");
    btPrevious.setBounds(260,325,100,20);
    panel.add(btPrevious);
}}

什么是空指针异常?我怎么知道?

What is a NullPointerException? How would I find out?

推荐答案

您需要在添加之前实例化 panel.如果在调用 MyPanel() 之前使用面板，panel 仍然是 null，因此 NullPointerException.

You need to instantiate panel before adding it. If you use panel before calling MyPanel(), panel is still null, hence the NullPointerException.

当你在这里时，看一眼.http://geosoft.no/development/javastyle.html

While you're here, give this a glance. http://geosoft.no/development/javastyle.html

Java 中的方法名称应以小写字母开头，混合大小写，例如myPanel() 而不是 MyPanel().对我们大多数人来说，MyPanel() 乍一看就像一个构造函数，因为您对其进行了不正确的样式设置.

Method names in Java should be mixed case starting with a lower case letter, e.g. myPanel() instead of MyPanel(). To most of us, MyPanel() looks like a constructor at first glance because you improperly styled it.

此外，MyPanel、Text、Fields 和 Buttons 都应该是私有方法，因为它会不适合外部类调用它们.

Additionally, MyPanel, Text, Fields, and Buttons should all be private methods, as it would be improper for an external class to call them.

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