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這是activity_main.xml中的文件：爲什麼自定義控件拋出NullPointerException？
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/layoutRoot"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<LinearLayout
android:id="@+id/L1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical" >
<Button
android:id="@+id/addButton"
android:layout_width="74dp"
android:layout_height="wrap_content"
android:text="Add" />
<EditText
android:id="@+id/newEditText"
android:layout_width="174dp"
android:layout_height="wrap_content"
android:layout_weight="3.24"
android:ems="10"
android:inputType="text" />
</LinearLayout>
<ListView
android:id="@+id/List"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
</ListView>
</LinearLayout>
這是命名爲second.xml定製化合物控制xml文件：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/jokeView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal" >
<Button
android:id="@+id/expandButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="0.44"
android:text = "One Click"/>
<TextView
android:id="@+id/txtView"
android:layout_width="253dp"
android:layout_height="wrap_content"
android:layout_weight="0.56"
android:ems="10"
android:text = "LOL This is a sample code"/>
</LinearLayout>
<RadioGroup
android:id="@+id/ratingRadioGroup"
android:layout_width="321dp"
android:layout_height="wrap_content"
android:orientation="horizontal" >
<RadioButton
android:id="@+id/likeButton"
android:layout_width="145dp"
android:layout_height="wrap_content"
android:checked="true"
android:text="Like" />
<RadioButton
android:id="@+id/dislikeButton"
android:layout_width="147dp"
android:layout_height="wrap_content"
android:text="Dislike" />
</RadioGroup>
</LinearLayout>
這是膨脹second.xml java文件：
public class Second extends View
{
private Button mExpandButton;
private RadioButton mLikeButton;
private RadioButton mDislikeButton;
private TextView mText;
private RadioGroup mLikeGroup;
private ViewGroup vGroup;
OnClickListener myClickListener = new OnClickListener()
{
@Override
public void onClick(View v)
{
String s= mExpandButton.getText().toString();
if (s.equals("One Click"))
mExpandButton.setText("Other Click");
else
mExpandButton.setText("One Click");
}
};
OnCheckedChangeListener myCkeckListener = new OnCheckedChangeListener()
{
@Override
public void onCheckedChanged(RadioGroup group, int checkedId)
{
if (mLikeButton.isChecked())
mText.setText("I Like it");
if (mDislikeButton.isChecked())
mText.setText("I Don't Like it");
}
};
public Second(Context context)
{
super(context);
mExpandButton = (Button) this.findViewById(R.id.expandButton);
mText = (TextView)this.findViewById(R.id.txtView);
mLikeGroup = (RadioGroup)this.findViewById(R.id.ratingRadioGroup);
mExpandButton.setOnClickListener(myClickListener);
mLikeGroup.setOnCheckedChangeListener(myCkeckListener);
LayoutInflater inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
inflater.inflate(R.layout.second, null);
}
}
最後，這是activity_main.xml中的Java代碼：
public class MainActivity extends Activity
{
protected ListView mLayout;
protected EditText mEditText;
protected Button mButton;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mLayout = (ListView)findViewById(R.id.List);
mEditText = (EditText)findViewById(R.id.newEditText);
mButton = (Button)findViewById(R.id.addButton);
for (int i =0; i< 4; i++)
{
Second sec = new Second(this);
mLayout.addView(sec);
setContentView(sec);
}
}
@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;
}
}
這是生成的logcat（更新。我的包的名字是nullextry）：
04-24 16:33:30.072: E/AndroidRuntime(1759): FATAL EXCEPTION: main
04-24 16:33:30.072: E/AndroidRuntime(1759): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.nullextry/com.example.nullextry.MainActivity}: java.lang.NullPointerException
04-24 16:33:30.072: E/AndroidRuntime(1759): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
04-24 16:33:30.072: E/AndroidRuntime(1759): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
04-24 16:33:30.072: E/AndroidRuntime(1759): at android.app.ActivityThread.access$600(ActivityThread.java:141)
04-24 16:33:30.072: E/AndroidRuntime(1759): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
04-24 16:33:30.072: E/AndroidRuntime(1759): at android.os.Handler.dispatchMessage(Handler.java:99)
04-24 16:33:30.072: E/AndroidRuntime(1759): at android.os.Looper.loop(Looper.java:137)
04-24 16:33:30.072: E/AndroidRuntime(1759): at android.app.ActivityThread.main(ActivityThread.java:5041)
04-24 16:33:30.072: E/AndroidRuntime(1759): at java.lang.reflect.Method.invokeNative(Native Method)
04-24 16:33:30.072: E/AndroidRuntime(1759): at java.lang.reflect.Method.invoke(Method.java:511)
04-24 16:33:30.072: E/AndroidRuntime(1759): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
04-24 16:33:30.072: E/AndroidRuntime(1759): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
04-24 16:33:30.072: E/AndroidRuntime(1759): at dalvik.system.NativeStart.main(Native Method)
04-24 16:33:30.072: E/AndroidRuntime(1759): Caused by: java.lang.NullPointerException
04-24 16:33:30.072: E/AndroidRuntime(1759): at com.example.nullextry.Second.<init>(Second.java:58)
04-24 16:33:30.072: E/AndroidRuntime(1759): at com.example.nullextry.MainActivity.onCreate(MainActivity.java:31)
04-24 16:33:30.072: E/AndroidRuntime(1759): at android.app.Activity.performCreate(Activity.java:5104)
04-24 16:33:30.072: E/AndroidRuntime(1759): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
04-24 16:33:30.072: E/AndroidRuntime(1759): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
04-24 16:33:30.072: E/AndroidRuntime(1759): ... 11 more
04-24 16:33:30.192: D/dalvikvm(1759): GC_CONCURRENT freed 110K, 9% free 2858K/3112K, paused 79ms+5ms, total 232ms
04-24 16:33:32.793: I/Process(1759): Sending signal. PID: 1759 SIG: 9
調試時，我發現second.xml在second.java構造，其中聽衆被分配到各自的控制膨脹後的狀態則拋出該異常。 我錯過了什麼嗎？ 我還沒有宣佈什麼？
愚蠢的錯誤：我沒有申報mExpandButton和mLikeGroup。我修復了second.java代碼，（謝謝鄒祖！）。但我得到同樣的例外（logcat已經更新）。
我編輯了我的答案 – 2013-04-25 08:11:01