這是怎麼回事？它的投擲錯誤

Question

這是我的動態添加按鈕的代碼。運行時拋出錯誤。任何想法？ :)

DatabaseHandler db; 
private RelativeLayout relativeLayout; 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_settings); 
    boolean userCounts = db.getUserExistance(); 
    if(userCounts == false){    
     Button button= new Button(this); 
     button.setText("Add password"); 
     relativeLayout.addView(button); 
    } 
    else if(userCounts == true){    
     Button button2 = new Button(this); 
     button2.setText("Change password"); 
     relativeLayout.addView(button2);    
    } 

} 

錯誤日誌：

09-17 11:44:34.658: D/AndroidRuntime(655): Shutting down VM 09-17 11:44:34.658: W/dalvikvm(655): threadid=1: thread exiting with uncaught exception (group=0x40a13300) 
09-17 11:44:34.699: E/AndroidRuntime(655): FATAL EXCEPTION: main 
09-17 11:44:34.699: E/AndroidRuntime(655): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.projects.myworldsafe/com.projects.myworldsafe.Settings}: java.lang.NullPointerException 
09-17 11:44:34.699: E/AndroidRuntime(655): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059) 

Answer 1

您還沒有初始化relativeLayout。如果要設置relativeLayout編程嘗試：

relativeLayout = new RelativeLayout(this); 

如果它在你的佈局文件首先你需要找到它，如：

relativeLayout = (RelativeLayout) this.findViewById(R.id.your_layout_id); 

與DatabaseHandler db

Answer 2

同樣的事情，我想你在第一次使用之前忘記初始化db和relativeLayout。

Answer 3

據我，我想通是你需要給佈局爲Relative layout

RelativeLayout = (RelativeLayout) this.findViewById(R.id.relativeID); 

Answer 4

試試這個，

DatabaseHandler db; 
    private RelativeLayout relativeLayout; 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_settings); 
     boolean userCounts = db.getUserExistance(); 
     relativeLayout = (RelativeLayout) this.findViewById(R.id.myRelativeLayout); 
     if(userCounts == false){    
      Button button= new Button(this); 
      button.setText("Add password"); 
      relativeLayout.addView(button); 
     } 
     else if(userCounts == true){    
      Button button2 = new Button(this); 
      button2.setText("Change password"); 
      relativeLayout.addView(button2);    
     } 

    }