将具有可变长度的 JRadioButtons 数组添加到 JWindow

Question

大家好，我试图向 JWindow 添加一个可变长度的数组，但出现运行时错误。需要这里的帮助..

这是我的代码：

    package components;

import java.util.Calendar;

public class FullDay extends javax.swing.JWindow {

    public FullDay() {
        initComponents();
    }                         
    private void initComponents() {

        int i;
//to get the no of days of the current month.

        Calendar cal = Calendar.getInstance();
        int maxdays = cal.getActualMaximum(Calendar.DAY_OF_MONTH);
        getContentPane().setLayout(new java.awt.FlowLayout());

//Creating an array of radiobuttons having size equal to the no of days of the current //month

        for(i=0; i<maxdays; i++) {
            jRadioButton[maxdays] = new javax.swing.JRadioButton();
            getContentPane().add(jRadioButton[i]);
        }        

        pack();

    }
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new FullDay().setVisible(true);
            }
        });
    }
    private javax.swing.JRadioButton jRadioButton[];
}

但我得到的错误是：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at components.FullDay.initComponents(FullDay.java:18)
at components.FullDay.<init>(FullDay.java:8)
at components.FullDay$1.run(FullDay.java:30)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

分配前检查jRadioButton数组是否创建。

Answer 2

你需要初始化你的 JRadioButton 数组

JRadioButton jRadioButton[] = new JRadioButton[someValue];

Answer 3

您可能想要初始化数组的第 i 个字段，而不是总是初始化最后一个：

 jRadioButton[i] = new javax.swing.JRadioButton();

代替：

 jRadioButton[maxdays] = new javax.swing.JRadioButton();

此外，正如前面的答案所述，在循环之外创建数组本身。