无法向我的JPanel添加任何内容

时间:2014-04-26 03:17:40

标签: java swing nullpointerexception jpanel

我正在尝试为学校完成一个项目,包括将10个jlabels,10个jtexts和10个jbuttons放入一个jpanel,并将它们的输出放入另一个jpanel。问题是我不知道如何使用jpanels,我无法弄清楚我遇到的这个问题。我试图将jtexts和jlabels添加到jpanel,但我一直收到此错误:

Exception in thread "main" java.lang.NullPointerException
        at CP235_A8_Application_Fitzgerald.create_interface(CP235_A8_Application_Fitzgerald.java:89)
        at CP235_A8_Application_Fitzgerald.<init>(CP235_A8_Application_Fitzgerald.java:253)
        at CP235_A8_Application_Fitzgerald.main(CP235_A8_Application_Fitzgerald.java:263)

我找到了某个地方,因为我添加的内容有一个空值,这就是造成它的原因,所以在将它们添加到jpanel之前我给了我的每个jtexts和jlabels,但是那并没有&#39工作。我读了一些关于不实例化jpanel的其他内容(但我并不是很明白,所以我不确定这是否适用)无论如何,我很难过一些帮助。

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class CP235_A8_Application_Fitzgerald extends JFrame
{

private JTextField text1;
private JLabel label1;
private JTextField text2;
private JLabel label2;
private JTextField text3;
private JLabel label3;
private JTextField text4;
private JLabel label4;
private JTextField text5;
private JLabel label5;
private JTextField text6;
private JLabel label6;
private JTextField text7;
private JLabel label7;
private JTextField text8;
private JLabel label8;
private JTextField text9;
private JLabel label9;
private JTextField text10;
private JLabel label10;

private JButton exit;
private JButton sort;
private JButton clear;
private JButton display;

private JPanel inputpane = new JPanel();

 private void create_interface()
 {
System.out.println("create_interface is running.");

int a;
int b;
int c;
int d;
int e;
int f;
int g;
int h;
int i;
int j;

a = 15;
b = 10;
c = 18;
d = 7;
e = 25;
f = 2;
g = 14;
h = 27;
i = 33;
j = 11;

Container GUI = getContentPane();
GUI.setBackground(Color.red);

System.out.println("still running");

GUI.add(inputpane);
inputpane.setLayout(new BorderLayout());
inputpane.setBackground(Color.green);


System.out.println("still running");

//inputpane.add(sort, BorderLayout.SOUTH);
//inputpane.add(clear, BorderLayout.SOUTH);
//inputpane.add(exit, BorderLayout.SOUTH);
//inputpane.add(display, BorderLayout.SOUTH);

text1.setText(String.valueOf(a));
inputpane.add(text1, BorderLayout.EAST);
text2.setText(String.valueOf(b));
inputpane.add(text2, BorderLayout.EAST);
text3.setText(String.valueOf(c));
inputpane.add(text3, BorderLayout.EAST);
text4.setText(String.valueOf(d));
inputpane.add(text4, BorderLayout.EAST);
text5.setText(String.valueOf(e));
inputpane.add(text5, BorderLayout.EAST);
text6.setText(String.valueOf(f));
inputpane.add(text6, BorderLayout.EAST);
text7.setText(String.valueOf(g));
inputpane.add(text7, BorderLayout.EAST);
text8.setText(String.valueOf(h));
inputpane.add(text8, BorderLayout.EAST);
text9.setText(String.valueOf(i));
inputpane.add(text9, BorderLayout.EAST);
text10.setText(String.valueOf(j));
inputpane.add(text10, BorderLayout.EAST);
label1.setText("Integer 1");
inputpane.add(label1, BorderLayout.WEST);
label2.setText("Integer 2");
inputpane.add(label2, BorderLayout.WEST);
label3.setText("Integer 3");
inputpane.add(label3, BorderLayout.WEST);
label4.setText("Integer 4");
inputpane.add(label4, BorderLayout.WEST);
label5.setText("Integer 5");
inputpane.add(label5, BorderLayout.WEST);
label6.setText("Integer 6");
inputpane.add(label6, BorderLayout.WEST);
label7.setText("Integer 7");
inputpane.add(label7, BorderLayout.WEST);
label8.setText("Integer 8");
inputpane.add(label8, BorderLayout.WEST);
label9.setText("Integer 9");
inputpane.add(label9, BorderLayout.WEST);
label10.setText("Integer 10");
inputpane.add(label10, BorderLayout.WEST);

System.out.println("still running");
System.out.println("still running");
System.out.println("still running");


//exit.setBounds(650, 350, 300, 100);
//inputpane.add(exit);
exit.setText("EXIT NOW!");
exit.addActionListener(new ActionListener()
{   
    public void actionPerformed (ActionEvent event)
    {
        ExitButtonEventHandler(event);
    }
}
);


    /*
SP1.setBounds(220, 350, 300, 100);
GUI.add(SP1);
SP1.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
area51.append("Type stuff here, pal!");
    */

setTitle("Display variables.");
setLocation(100, 100);
setSize(1200, 800);
setVisible(true);

System.out.println(a);
System.out.println(b);
System.out.println(c);
System.out.println(d);
System.out.println(e);
System.out.println(f);
System.out.println(g);
System.out.println(h);
System.out.println(i);
System.out.println(j);


System.out.println("create_interface is ending.");
    }

    private void ExitButtonEventHandler(ActionEvent event)
    {
System.exit(0);
    }

    public CP235_A8_Application_Fitzgerald()
    {
System.out.println("Constructor is running.");

create_interface();

System.out.println("Constructor is running.");
    }


    public static void main(String[] args) 
    {
System.out.println("Hello from Assignment 2.");

CP235_A8_Application_Fitzgerald Peter = new CP235_A8_Application_Fitzgerald(); 

System.out.println("Goodbye from Assignment 2.");

    }

    /*
    private void DisplayListJButtonActionPerformed( ActionEvent event)
    {
int count = 0

RefillAL();

SortedListJTextArea.setText("");
SortedListJTextArea.append("Sorted Integer List\n");

for (IntegersArrayList ) 

if ( count == 0)
{
    SortedListJTextArea.append("No Integers to sort\n");
}

    }


    RefillAL()
    {
Integer i;
String str;

IntegersArrayList.clear();

str = text1.getText();
if ( ! str.equals(""))
{
    try
    {
        i = Integer.parseInt(str);
        IntegersArrayList.add(i);
    }
    catch (NumberFormatException exception)
    {

    }
 }
 */
     }

P.S。

正如您所看到的,许多代码已被注释掉且不完整,我现在并不太关心,现在我专注于JPanel不工作(当然除非它jpanel不起作用的部分原因)。提前感谢帮助我的人。

1 个答案:

答案 0 :(得分:1)

您在使用它们之前声明了一堆乱七八糟的组件,但没有初始化它们。您必须初始化变量,在尝试使用变量之前创建WhatNot whatnot = new WhatNot()。否则你将使用空变量。

或者更具体地说,在使用它们之前,必须为text1和所有其他文本和标签组件分配有效的JTextField对象。

例如,更改:

private JTextField text1;

// 10 just represents whatever column size you prefer
private JTextField text1 = new JTextField(10); 
// ...
// the same with all the other components. 

这与Swing无关,与基本Java无关,在尝试创建GUI之前,您需要先阅读Java文本的介绍,因为所有这些都在前几个中得到了很好的解释。章节。


根据我的评论,您需要学习如何调试NPE(NullPointerException)的一般概念。您应仔细检查抛出它的行,找出哪个变量为null,然后追溯到您的代码以查看原因。你会一次又一次地遇到这些,相信我。