我将Button变量的值分配给在我的XML布局中创建的按钮之一，我已经在XML中给出了一个合适的id按钮，并且在将值分配给Button时我会参考这个空。
此方法出现问题：
public void addListenerOnButtons() {
updateInfoButton = (Button) findViewById(R.id.updateWalkInfoButton);
updateInfoButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
Intent resultIntent = new Intent();
resultIntent.putExtra("newName", "");
resultIntent.putExtra("newSDesc", "");
resultIntent.putExtra("newLDesc", "");
setResult(RESULT_OK, resultIntent);
finish();
}
});
helpButton = (ImageButton) findViewById(R.id.helpButton);
helpButton.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View v){
Intent intent = new Intent(context, HelpScreen.class);
startActivity(intent);
}
});
}
updateInfoButton在此方法后仍为null，这会导致我的应用程序崩溃，此页面的XML如下所示：
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/createScrollView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@color/black"
tools:context=".EditWalksInfoActivity" >
<RelativeLayout
android:id="@+id/formLayout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical" >
<TextView
android:id="@+id/editWalkInfoHeaderTextView"
style="@android:style/TextAppearance.Large"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="15dp"
android:text="@string/editWalkInfoHeaderText"
android:textColor="@color/light_gray" />
<EditText
android:id="@+id/editWalkNameEditText"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/editWalkTitleTextView"
android:layout_marginTop="15dp"
android:ems="6"
android:hint="@string/createWalkNameHint"
android:inputType="textPersonName"
android:textColor="@color/light_gray" >
<requestFocus />
</EditText>
<EditText
android:id="@+id/editWalkShortDescEditText"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/editWalkShortDescTextView"
android:layout_marginTop="15dp"
android:ems="6"
android:hint="@string/createWalkShortDescHint"
android:inputType="text"
android:textColor="@color/light_gray" >
<requestFocus />
</EditText>
<EditText
android:id="@+id/editWalkLongDescEditText"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/editWalkLongDescTextView"
android:layout_marginTop="15dp"
android:ems="6"
android:hint="@string/createWalkLongDescHint"
android:inputType="textMultiLine"
android:textColor="@color/light_gray" >
<requestFocus />
</EditText>
<Button
android:id="@+id/updateWalkInfoButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/editWalkLongDescEditText"
android:layout_centerHorizontal="true"
android:layout_marginTop="15dp"
android:text="@string/updateWalkInfoButtonText"
android:textColor="@color/light_gray" />
<TextView
android:id="@+id/editWalkShortDescTextView"
style="@android:style/TextAppearance.Medium"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_below="@+id/editWalkNameEditText"
android:layout_marginLeft="18dp"
android:layout_marginTop="15dp"
android:text="@string/createWalkShortDescTextView"
android:textColor="@color/light_gray" />
<TextView
android:id="@+id/editWalkTitleTextView"
style="@android:style/TextAppearance.Medium"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/editWalkShortDescTextView"
android:layout_below="@+id/editWalkInfoHeaderTextView"
android:layout_marginTop="15dp"
android:text="@string/createWalkTitleTextView"
android:textColor="@color/light_gray" />
<TextView
android:id="@+id/editWalkLongDescTextView"
style="@android:style/TextAppearance.Medium"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/editWalkShortDescTextView"
android:layout_below="@+id/editWalkShortDescEditText"
android:layout_marginTop="15dp"
android:text="@string/createWalkLongDescTextView"
android:textColor="@color/light_gray" />
<ImageButton
android:id="@+id/editWalkHelpButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/editWalkTitleTextView"
android:layout_alignTop="@+id/editWalkInfoHeaderTextView"
android:background="@color/black"
android:contentDescription="@string/helpIconAltText"
android:src="@drawable/help" />
</RelativeLayout>
</ScrollView>
的manifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="xxxxxx"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="22" />
<permission
android:name="xxxxxx".MAPS_RECEIVE"
android:protectionLevel="signature" />
<!-- Accessing camera hardware -->
<!-- putting android.hardware.camera prevents non-camera devices using this app -->
<uses-feature android:name="android.hardware.camera" />
<uses-feature
android:glEsVersion="0x00020000"
android:required="true" />
<uses-permission android:name="xxxxxx".MAPS_RECEIVE" />
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_MOCK_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="com.google.android.providers.gsf.permission.READ_GSERVICES" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:logo="@drawable/ic_launcher"
android:theme="@style/AppTheme" >
<uses-library
android:name="com.google.android.maps"
android:required="true" />
<activity
android:name="xxxxxx.StartScreen"
android:label="@string/app_name"
android:screenOrientation="landscape" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name="xxxxxx.CreateWalkActivity"
android:label="@string/title_activity_create_walk"
android:screenOrientation="landscape" >
</activity>
<activity
android:name="xxxxxx.HelpScreen"
android:label="@string/title_activity_help_screen"
android:screenOrientation="landscape" >
</activity>
<activity
android:name="xxxxxx.WalkRecording"
android:label="@string/title_activity_walk_recording"
android:screenOrientation="landscape" >
</activity>
<activity
android:name="xxxxxx.CreateNewPOIActivity"
android:label="@string/title_activity_create_new_poi"
android:screenOrientation="landscape" >
</activity>
<meta-data
android:name="com.google.android.maps.v2.API_KEY"
android:value="AIzaSyBriHozWKei0lNm0XP2SR3hz8GW790L1eA" />
<meta-data
android:name="com.google.android.gms.version"
android:value="@integer/google_play_services_version" />
<activity
android:name="xxxxxx.ConfrimUploadActivity"
android:label="@string/title_activity_confrim_upload"
android:screenOrientation="landscape" >
</activity>
<activity
android:name="xxxxxx.ServerResponse"
android:label="@string/title_activity_server_response"
android:screenOrientation="landscape" >
</activity>
<activity
android:name="xxxxxx.EditWalksInfoActivity"
android:label="@string/title_activity_edit_walks_info" >
</activity>
</application>
</manifest>
这是logcat输出：
01-06 19:09:27.640: E/AndroidRuntime(6958): FATAL EXCEPTION: main
01-06 19:09:27.640: E/AndroidRuntime(6958): Process: dcs.aber.ac.uk.cs211.group02, PID: 6958
01-06 19:09:27.640: E/AndroidRuntime(6958): java.lang.RuntimeException: Unable to start activity ComponentInfo{dcs.aber.ac.uk.cs211.group02/dcs.aber.ac.uk.cs211.group02.EditWalksInfoActivity}: java.lang.NullPointerException
01-06 19:09:27.640: E/AndroidRuntime(6958): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
01-06 19:09:27.640: E/AndroidRuntime(6958): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
01-06 19:09:27.640: E/AndroidRuntime(6958): at android.app.ActivityThread.access$800(ActivityThread.java:135)
01-06 19:09:27.640: E/AndroidRuntime(6958): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
01-06 19:09:27.640: E/AndroidRuntime(6958): at android.os.Handler.dispatchMessage(Handler.java:102)
01-06 19:09:27.640: E/AndroidRuntime(6958): at android.os.Looper.loop(Looper.java:136)
01-06 19:09:27.640: E/AndroidRuntime(6958): at android.app.ActivityThread.main(ActivityThread.java:5017)
01-06 19:09:27.640: E/AndroidRuntime(6958): at java.lang.reflect.Method.invokeNative(Native Method)
01-06 19:09:27.640: E/AndroidRuntime(6958): at java.lang.reflect.Method.invoke(Method.java:515)
01-06 19:09:27.640: E/AndroidRuntime(6958): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
01-06 19:09:27.640: E/AndroidRuntime(6958): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
01-06 19:09:27.640: E/AndroidRuntime(6958): at dalvik.system.NativeStart.main(Native Method)
01-06 19:09:27.640: E/AndroidRuntime(6958): Caused by: java.lang.NullPointerException
01-06 19:09:27.640: E/AndroidRuntime(6958): at dcs.aber.ac.uk.cs211.group02.EditWalksInfoActivity.addListenerOnButtons(EditWalksInfoActivity.java:78)
01-06 19:09:27.640: E/AndroidRuntime(6958): at dcs.aber.ac.uk.cs211.group02.EditWalksInfoActivity.onCreate(EditWalksInfoActivity.java:30)
01-06 19:09:27.640: E/AndroidRuntime(6958): at android.app.Activity.performCreate(Activity.java:5231)
01-06 19:09:27.640: E/AndroidRuntime(6958): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
01-06 19:09:27.640: E/AndroidRuntime(6958): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
01-06 19:09:27.640: E/AndroidRuntime(6958): ... 11 more
编辑：
订购方法：
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_edit_walks_info);
context = this;
locateUIElements();
addListenerOnButtons();
Bundle bundle = getIntent().getBundleExtra("Walk info");
if (bundle != null){
name = bundle.getString("walkTitle");
sDesc = bundle.getString("walkSDesc");
lDesc = bundle.getString("walkLDesc");
setEditTextValues();
}
}
答案 0 :(得分：4)
您的问题中的问题是您的XML中没有id/helpButton。
helpButton = (ImageButton) findViewById(R.id.helpButton);
但
<ImageButton
android:id="@+id/editWalkHelpButton"
R.id.helpButton可能来自其他资源文件，因此它是有效的标识符。
您看到的后续问题是因为生成的R.java和二进制XML文件中的标识符不同步。在编辑布局XML文件和使用@+id标识符重新排序元素时，这是一个常见问题。解决方案是清理和重建项目。
答案 1 :(得分：2)
您可能尝试在活动视图层次结构膨胀之前获取按钮。在Activity.setContentView。
之后调用addListenerOnButtons方法protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// call sequence is important
setContentView(R.layout.your_layout);
addListenerOnButtons();
}
希望这有帮助。
答案 2 :(得分：1)
这里有一个潜在的灾难：
updateInfoButton =（Button）findViewById（R.id.updateWalkInfoButton）;
有可能在代码执行时，View还没有准备好但是非常高，并且保持空引用，确保在执行setContentView之后将代码移动到某处，也可以在setContentView之后的onCreate中或之后的某处视图已设置...
问候！
答案 3 :(得分：0)
是否设置了错误的上下文？ activity_edit_walks_info是您发布的ScrollView Layout XML的名称吗？因为XML中定义的上下文是EditWalksInfoActivity。
此外，addListenerOnButtons（）中的updateInfoButton是全局设置的吗？如果是这样，你可以发布定义的地方吗？
答案 4 :(得分：0)
解决这个问题的方法是简单地清理项目：
日食中的转到：
项目＆gt;干净，似乎解决了许多关于应用程序与其资源不同步的问题。
特别感谢用户：laalto的建议。