將可變長度的JRadioButtons數組添加到JWindow中

Question

大家好，我試圖給JWindow添加一個可變長度的數組，但我得到一個運行時錯誤。想從這裏幫助..

這裏是我的代碼：

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[]; 
} 

但我得到的錯誤是：jRadioButton陣列

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

的陣列，您可能需要初始化數組的第i個場，而不是總是初始化最後一個：

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

代替：

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

另外，正如前面的答案所述，創建數組本身之外的循環。

Answer 2

檢查是否被分配之前創建的。

Answer 3

你需要初始化你一個JRadioButton

JRadioButton jRadioButton[] = new JRadioButton[someValue];