android AlertDialog顯示錯誤 Unable to add window token null is not for an application解決辦法

 
          public  
          void onCreate(Bundle savedInstanceState) { 
          
         
          super.onCreate(savedInstanceState); 
          
        setContentView(R.layout.activity_main); 
          
         
          
        findViewById(R.id.button1).setOnClickListener( 
          new Button.OnClickListener() { 
          
             
          
             
          public  
          void onClick(View v) { 
          
                 
          new AlertDialog.Builder(getApplicationContext()) 
          
                .setTitle(R.string.dialogTitle) 
          
                .setMessage(R.string.dialogMessage) 
          
                .setPositiveButton("確定",  
          new DialogInterface.OnClickListener() { 
          
                     
          public  
          void onClick(DialogInterface dialog,  
          int which) { 
          
                        Toast.makeText(getApplicationContext(), "你按了確定。", Toast.LENGTH_SHORT).show(); 
          
                    } 
          
                }).show(); 
          
            } 
          
        }); 
          
    } 
         

報錯：Unable to add window token null is not for an application

  

異常原因：AlertDialog創建語句public AlertDialog.Builder (Context context)中，不能使用getApplicationContext()得到的context，而必須使用Activity，所以解決如下

 

解決辦法：

將 new AlertDialog.Builder(getApplicationContext()) 改為 new AlertDialog.Builder(MainActivity.this)

其中MainActivity 為 當前Activity的名稱