Question

我有一个名为Information的菜单按钮，当我触摸它时，我想要一个名为＆＃34; InformationActivity＆＃34;推出。我已经验证了代码，但似乎没有任何效果。我得到一个NullPointer异常和一个FATAL EXCEPTION：Main。为什么会这样？查看代码以查看失败的位置。谢谢。

这里有与菜单相关的所有内容;

@Override
    public boolean onCreateOptionsMenu(Menu menu){

        menu.add(1,INFORMATION,1,"Information");
        Log.w("I AM HERE ?!", "YES01");
        return true;

        }
    public boolean onOptionsItemSelected (MenuItem item){
        switch (item.getItemId()){
        case INFORMATION:
            Log.w("I AM HERE ?!", "YES02");
            Toast.makeText(getApplicationContext(), "This works - tested.", Toast.LENGTH_LONG).show();
            Intent infoIntent = new Intent(this,InformationActivity.class);
            Log.w("I AM HERE ?!", infoIntent.toString());

//此日志显示为＆＃34;包名称/ .InformationActivity＆＃34; -为什么？！这是正确的格式吗？ startActivity（infoIntent）; //在这里失败 return true;

        default:
            return super.onOptionsItemSelected(item);

        }

这是AndroidManifest.xml中的相关代码;

<activity
            android:name="com.example.pingtest.InformationActivity"
            android:label="@string/title_activity_information" >
        </activity>

这里是activity_information.xml文件。请注意，我在InformationActivity类的Java代码中使用线性布局。但是，它并没有出现在这里。当我使用Java在那里添加新的TextView时，我不需要它。希望有道理。

<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"
    tools:context=".InformationActivity" >

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        />

</RelativeLayout>

这里是Logcat输出

11-29 16:55:04.223: I/ViewRootImpl(2740): ViewRoot's KeyEvent { action=ACTION_DOWN, keyCode=KEYCODE_MENU, scanCode=139, metaState=0, flags=0x48, repeatCount=0, eventTime=60889321, downTime=60889321, deviceId=7, source=0x101 } to com.android.internal.policy.impl.PhoneWindow$DecorView{4286a2b8 V.E..... R....... 0,0-1080,1920}
11-29 16:55:04.293: I/ViewRootImpl(2740): ViewRoot's KeyEvent { action=ACTION_UP, keyCode=KEYCODE_MENU, scanCode=139, metaState=0, flags=0x48, repeatCount=0, eventTime=60889391, downTime=60889321, deviceId=7, source=0x101 } to com.android.internal.policy.impl.PhoneWindow$DecorView{4286a2b8 V.E..... R....... 0,0-1080,1920}
11-29 16:55:04.873: I/ViewRootImpl(2740): ViewRoot's Touch Event : Touch Down
11-29 16:55:04.933: I/ViewRootImpl(2740): ViewRoot's Touch Event : Touch UP
11-29 16:55:05.003: W/I AM HERE ?!(2740): YES02
11-29 16:55:05.023: W/I AM HERE ?!(2740): Intent { cmp=com.example.pingtest/.InformationActivity }
11-29 16:55:05.023: I/ActivityManager(2740): Timeline: Activity_launch_request id:com.example.pingtest time:60890123
11-29 16:55:05.093: D/AndroidRuntime(2740): Shutting down VM
11-29 16:55:05.093: W/dalvikvm(2740): threadid=1: thread exiting with uncaught exception (group=0x41822e48)
11-29 16:55:05.103: E/AndroidRuntime(2740): FATAL EXCEPTION: main
11-29 16:55:05.103: E/AndroidRuntime(2740): Process: com.example.pingtest, PID: 2740
11-29 16:55:05.103: E/AndroidRuntime(2740): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.pingtest/com.example.pingtest.InformationActivity}: java.lang.NullPointerException
11-29 16:55:05.103: E/AndroidRuntime(2740):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2202)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2252)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at android.app.ActivityThread.access$800(ActivityThread.java:139)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1200)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at android.os.Handler.dispatchMessage(Handler.java:102)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at android.os.Looper.loop(Looper.java:136)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at android.app.ActivityThread.main(ActivityThread.java:5103)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at java.lang.reflect.Method.invokeNative(Native Method)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at java.lang.reflect.Method.invoke(Method.java:515)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:606)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at dalvik.system.NativeStart.main(Native Method)
11-29 16:55:05.103: E/AndroidRuntime(2740): Caused by: java.lang.NullPointerException
11-29 16:55:05.103: E/AndroidRuntime(2740):     at com.example.pingtest.InformationActivity.onCreate(InformationActivity.java:24)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at android.app.Activity.performCreate(Activity.java:5275)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
11-29 16:55:05.103: E/AndroidRuntime(2740):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2166)
11-29 16:55:05.103: E/AndroidRuntime(2740):     ... 11 more

这是InformationActivity.java文件。我在这里看不到任何错误。

public class InformationActivity extends Activity {
    LinearLayout layout2;
    TextView label2;
    @Override
    protected void onCreate(Bundle savedInstanceState) 
    {
        super.onCreate(savedInstanceState);
        //readTask = new ReadTask();
        Typeface type = Typeface.createFromAsset(getAssets(),"fonts/41370_WINDSORCONDENSED.TTF");
        layout2=new LinearLayout(this);
        label2.setText("Text here\n");
        label2.setTextColor(Color.rgb(0,0,0));
        label2.setBackgroundColor(Color.rgb(255,194,75));
        label2.setTypeface(type);
        setContentView(R.layout.activity_information);
    }

Answer 1

在获得对label2.setText的引用之前，会调用

label2。首先，通过调用setContentView(R.layout.activity_information);来设置活动布局。然后使用findViewById获取对相应TextView的引用。

编辑：完整修复：

@Override
protected void onCreate(Bundle savedInstanceState) 
{
    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_information);

    //readTask = new ReadTask();
    Typeface type = Typeface.createFromAsset(getAssets(),"fonts/41370_WINDSORCONDENSED.TTF");

    label2 = (TextView) findViewById(R.id.label_2_id_here);
    label2.setText("Text here\n");
    label2.setTextColor(Color.rgb(0,0,0));
    label2.setBackgroundColor(Color.rgb(255,194,75));
    label2.setTypeface(type);
}

我的应用程序崩溃了NullPointerException。为什么？

1 个答案: