Java setContentPane()和addActionListener获取NullPointerException
该错误与main方法中的Java setContentPane()和addActionListener获取NullPointerException,java,swing,nullpointerexception,Java,Swing,Nullpointerexception,该错误与main方法中的setContentPane有关,然后与actionListener有关。我已经在下面公布了确切的错误。为什么会这样?我使用IntelliJ IDEA GUI表单创建者创建了它 import javax.swing.*; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; public class Form1 { private JPanel panel1; pri
setContentPane
有关,然后与actionListener
有关。我已经在下面公布了确切的错误。为什么会这样?我使用IntelliJ IDEA GUI表单创建者创建了它
import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class Form1 {
private JPanel panel1;
private JButton button1;
public Form1() {
button1.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
System.out.println("Button clicked");
}
});
}
public static void main(String args[]){
JFrame frame = new JFrame("Form 1");
frame.setContentPane(new Form1().panel1);
frame.pack();
frame.setVisible(true);
}
}
线程“main”java.lang.NullPointerException中的异常
在Form1。(Form1.java:12)
位于Form1.main(Form1.java:22)
在Java中,不能使用未初始化的变量,否则将收到NPE。请参阅以下代码并阅读Java书籍:)
因此,您将遇到两个导致错误的问题。创建对象后(
JButton
,JPanel
,在本例中),必须实例化它们。有很多不同的方法可以做到这一点,这取决于您试图做什么,但最简单的解决方法是在构造函数中添加以下行:
public class Form1 {
private JPanel panel1;
private JButton button1;
public Form1() {
panel1 = new JPanel();
button1 = new JButton1("Press Me");
panel1.add(button1);
button1.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
System.out.println("Button clicked");
}
});
}
public static void main(String args[]){
JFrame frame = new JFrame("Form 1");
frame.setContentPane(new Form1().panel1);
frame.pack();
frame.setVisible(true);
}
}
在此之后,您的代码应该可以正常运行。您没有初始化panel1。我猜您想创建一个panel1并将button1(也未初始化)添加到其中。请参阅&
public class Form1 {
private JPanel panel1;
private JButton button1;
public Form1() {
panel1 = new JPanel();
button1 = new JButton1("Press Me");
panel1.add(button1);
button1.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
System.out.println("Button clicked");
}
});
}
public static void main(String args[]){
JFrame frame = new JFrame("Form 1");
frame.setContentPane(new Form1().panel1);
frame.pack();
frame.setVisible(true);
}
}
panel1 = new JPanel();
button1 = new JButton();