Question

我正在尝试Android Studio。我创建了一个带有onClickLister()按钮的简单应用程序，以显示Toast消息。我收到消息“应用程序已停止”。

- 相同的代码适用于Eclipse - 我创建了更多项目 - 在虚拟和真实设备上 - 重新安装Android Studio

有什么想法吗？

XML：

<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:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context=".MainActivity$PlaceholderFragment">

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

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="New Button"
        android:id="@+id/button"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true" />
</RelativeLayout>

我的活动课程在这里：

public class MainActivity extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Button mibtn = (Button)findViewById(R.id.button);

    mibtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Toast.makeText(getApplicationContext(),"Apretado", Toast.LENGTH_LONG).show();
        }
    });

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }
}


@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.
    switch (item.getItemId()) {
        case R.id.action_settings:
            return true;
    }
    return super.onOptionsItemSelected(item);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        return rootView;
    }
}

}

logcat的：

01-14 12:43:11.759    5651-5651/? W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x41538ba8)
01-14 12:43:11.759    5651-5651/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.omaxlive.myapplicationtest, PID: 5651
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.omaxlive.myapplicationtest/com.omaxlive.myapplicationtest.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.omaxlive.myapplicationtest.MainActivity.onCreate(MainActivity.java:25)
            at android.app.Activity.performCreate(Activity.java:5231)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

此行显示您创建的按钮位于fragment_main.xml

 tools:context=".MainActivity$PlaceholderFragment"

但您在activity_main.xml

中找到了按钮

将代码从onCreate()移至onCreateView的PlaceholderFragment，如

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_main, container, false);
    Button mibtn = (Button)rootView.findViewById(R.id.button);

    mibtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
         Toast.makeText(getApplicationContext(),"Apretado", Toast.LENGTH_LONG).show();
        }
});

    return rootView;
}

Answer 2

您正在寻找您的布局：

setContentView (R.layout.activity_main);

但您使用的布局是fragment_main.xml

所以将setContentView(R.layout.activity_main);更改为setContentView(R.layout.fragment_main);

然后从.java文件中删除此代码

if (savedInstanceState == null) {
       getSupportFragmentManager().beginTransaction()
            .add(R.id.container, new PlaceholderFragment())
            .commit();
}

Android Studio - 应用程序已停止

2 个答案: