爲什麼我的視圖對象引用仍爲空（Android開發）？

Question

擺弄Android開發。我在Java方面有經驗，但對於Android開發人員來說我很新，所以請和我一起裸照。

如果我正確解釋下面的錯誤消息，「視圖」對象由於某種原因似乎爲空。在線觀看時，大多數遇到類似問題的人通常會在設置內容視圖之前創建參考。然而，正如你可以看到我的代碼，這不是我的情況。我不確定問題是什麼。

以下是錯誤消息我在嘗試運行應用程序時（用我自己的手機作爲媒介的調試，如果它的問題）：

04-24 20:54:03.848: E/AndroidRuntime(13773): FATAL EXCEPTION: main 
04-24 20:54:03.848: E/AndroidRuntime(13773): Process: com.example.fiddle, PID: 13773 
04-24 20:54:03.848: E/AndroidRuntime(13773): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.fiddle/com.example.fiddle.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.RelativeLayout.setOnTouchListener(android.view.View$OnTouchListener)' on a null object reference 
04-24 20:54:03.848: E/AndroidRuntime(13773): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3254) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3350) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at android.app.ActivityThread.access$1100(ActivityThread.java:222) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1795) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at android.os.Handler.dispatchMessage(Handler.java:102) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at android.os.Looper.loop(Looper.java:158) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at android.app.ActivityThread.main(ActivityThread.java:7229) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at java.lang.reflect.Method.invoke(Native Method) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 
04-24 20:54:03.848: E/AndroidRuntime(13773): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.RelativeLayout.setOnTouchListener(android.view.View$OnTouchListener)' on a null object reference 
04-24 20:54:03.848: E/AndroidRuntime(13773): at com.example.fiddle.MainActivity.onCreate(MainActivity.java:25) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at android.app.Activity.performCreate(Activity.java:6876) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1135) 
04-24 20:54:03.848: E/AndroidRuntime(13773): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3207) 
04-24 20:54:03.848: E/AndroidRuntime(13773): ... 9 more 

這裏是我的MainActivity.java代碼：

package com.example.fiddle; 

import android.annotation.SuppressLint; 
import android.annotation.TargetApi; 
import android.os.Build; 
import android.os.Bundle; 
import android.support.v7.app.ActionBarActivity; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.MotionEvent; 
import android.view.View.OnTouchListener; 
import android.view.View; 
import android.widget.RelativeLayout; 
import android.widget.Toast; 

@SuppressLint("NewApi") 
public class MainActivity extends ActionBarActivity { 
    @Override 
    protected void onCreate(Bundle savedInstanceState) {   
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

     RelativeLayout view = (RelativeLayout) findViewById(R.layout.activity_main); 

     view.setOnTouchListener (new OnTouchListener() { 
      public boolean onTouch(View v, MotionEvent event) { 
       Toast.makeText(getBaseContext(), String.valueOf(event.getX()) + ", " + String.valueOf(event.getY()), Toast.LENGTH_LONG); 
       return true; 
      } 
     }); 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.main, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 
     if (id == R.id.action_settings) { 
      return true; 
     } 
     return super.onOptionsItemSelected(item); 
    } 
} 

這裏是視圖的XML文檔：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:paddingBottom="@dimen/activity_vertical_margin" 
    android:paddingLeft="@dimen/activity_horizontal_margin" 
    android:paddingRight="@dimen/activity_horizontal_margin" 
    android:paddingTop="@dimen/activity_vertical_margin" 
    android:clickable="true" 
    android:focusable="true" 
    android:focusableInTouchMode="true" 
    tools:context="com.example.fiddle.MainActivity" > 

    <TextView 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:text="@string/hello_world" /> 

</RelativeLayout> 

Answer 1

您應該添加一個id爲您RelativeLayout

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
     ... 
     android:id="@+id/root_relativelayout" 
     ... 
     > 
     ... 
    </RelativeLayout> 

然後在onCreate方法，找到RelativeLayout通過

protected void onCreate(Bundle savedInstanceState) {   
     ... 
     //RelativeLayout view = (RelativeLayout) findViewById(R.layout.activity_main); 
     RelativeLayout view = (RelativeLayout) findViewById(R.id.root_relativelayout); 
     ... 
    } 

Answer 2

您需要在XML文件中添加android:id="@+id/relative_layout"到RelativeLayour。
而替換該行
RelativeLayout view = (RelativeLayout) findViewById(R.layout.activity_main);
與 RelativeLayout view = (RelativeLayout) findViewById(R.id.relative_layout);
順便說一句，你需要仔細閱讀並做更多的例子。