Question

java 空指针异常问题。。。不知道该怎么解决。

import java.awt.BorderLayout;
import java.awt.FlowLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
public class test
{
private JFrame f;
public test()
{
f.setLayout(new BorderLayout());
f.getContentPane().add(new panel1(),"North");
f.getContentPane().add(new panel2(),"South");
f.setSize(50,50);
f.setVisible(true);

}
public static void main(String [] args)
{
new test();
}
}

class panel1 extends JPanel
{
private JButton btn,btn2,btn3;
private JLabel l;
public panel1()
{
this.setLayout(new FlowLayout());
btn = new JButton("OK");
btn2 = new JButton("Cancel");
btn3 = new JButton("NO");
l = new JLabel("hgf");
add(btn);
add(btn2);
add(btn3);
add(l);
}
}
class panel2 extends JPanel
{
private JButton btn;
private JLabel l;
public panel2()
{
this.setLayout(new FlowLayout());
btn = new JButton("WoWo");
l = new JLabel("asas");
add(btn);
add(l);
}
}

Answer 1

public class test
{
private JFrame f;
public test()
{
f.setLayout(new BorderLayout());
f.getContentPane().add(new panel1(),"North");
f.getContentPane().add(new panel2(),"South");
f.setSize(50,50);
f.setVisible(true);

}}

这段代码，请在构造函数内添加 f = new JFrame();

Answer 2

f没有初始化,值为null,引用f.setLayout(new BorderLayout());时导致空指针异常

Answer 3

private JFrame f;

要先实例化

Answer 4

private JFrame f; 没有实例化