应用程序在 android 模拟器中停止

Question

我对 java 完全陌生，所以我开始制作一个应用程序来通过按下加减按钮来查找 TOTAL。我在FRAGMENT_MAIN.XML 中定义了按钮和文本视图，并在 main activity.JAVA 中定义了它们的功能。在主 activity.java 中定义按钮的功能后，我的程序中出现 logcat 错误，并且该应用程序不幸在 android 模拟器中停止。我使用的是 android sdk 版本 22.6.4，java 开发者的 eclipse ide 版本是 2.0.2.20140224-0000，eclipse 平台版本是 4.3.2.v2.140221-1852。

我的 MainActivity.java

package com.counterehr.abhi;

public class MainActivity extends ActionBarActivity {

    int counter;
    Button add, sub;
    TextView display;   

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        counter = 0;
        add = (Button) findViewById(R.id.bAdd);
        sub = (Button) findViewById(R.id.bSub);
        display = (TextView) findViewById(R.id.tvDisplay);
        add.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                counter++;
                display.setText("Your total is " + counter);
            }
        });
  sub.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                counter--;
                display.setText("Your total is " + counter);
            }
        });

  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.
        int id = item.getItemId();
        if (id == 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;
        }
    }

}

而FRAGMENT_MAIN.XML 是

FRAGMENT_MAIN.XML

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="fill_parent"
    android:layout_height="fill_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="com.counterehr.abhi.MainActivity$PlaceholderFragment" >

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Your total is 0" 
        android:textSize="45sp"
        android:layout_gravity="center"
        android:gravity="center" 
        android:id="@+id/tvDisplay"/>

        <Button
            android:id="@+id/bAdd"
            android:layout_width="250dp"
            android:layout_height="wrap_content"
            android:layout_below="@+id/tvDisplay"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="46dp"
            android:text="Add one"
            android:textSize="20sp" />

        <Button
            android:id="@+id/bSub"
            android:layout_width="250dp"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/bAdd"
            android:layout_centerVertical="true"
            android:text="Subtract one"
            android:textSize="20sp" />

</RelativeLayout>

在运行应用程序时，我收到 logcat 错误。

logcat 错误

    07-01 18:16:17.963: E/Trace(833): error opening trace file: No such file or directory (2)
    07-01 18:16:19.675: D/dalvikvm(833): GC_FOR_ALLOC freed 80K, 9% free 2419K/2644K, paused 356ms, total 356ms
    07-01 18:16:19.704: I/dalvikvm-heap(833): Grow heap (frag case) to 3.415MB for 960016-byte allocation
    07-01 18:16:19.802: D/dalvikvm(833): GC_FOR_ALLOC freed 1K, 7% free 3355K/3584K, paused 97ms, total 97ms
    07-01 18:16:20.002: D/dalvikvm(833): GC_CONCURRENT freed <1K, 7% free 3355K/3584K, paused 9ms+33ms, total 201ms
    07-01 18:16:20.382: D/dalvikvm(833): GC_CONCURRENT freed <1K, 6% free 3779K/4004K, paused 5ms+4ms, total 52ms
    07-01 18:16:20.543: D/AndroidRuntime(833): Shutting down VM
    07-01 18:16:20.543: W/dalvikvm(833): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
    07-01 18:16:20.583: E/AndroidRuntime(833): FATAL EXCEPTION: main
    07-01 18:16:20.583: E/AndroidRuntime(833): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.counterehr.abhi/com.counterehr.abhi.MainActivity}: java.lang.NullPointerException
    07-01 18:16:20.583: E/AndroidRuntime(833):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at android.app.ActivityThread.access$600(ActivityThread.java:141)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at android.os.Handler.dispatchMessage(Handler.java:99)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at android.os.Looper.loop(Looper.java:137)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at android.app.ActivityThread.main(ActivityThread.java:5041)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at java.lang.reflect.Method.invokeNative(Native Method)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at java.lang.reflect.Method.invoke(Method.java:511)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at dalvik.system.NativeStart.main(Native Method)
    07-01 18:16:20.583: E/AndroidRuntime(833): Caused by: java.lang.NullPointerException
    07-01 18:16:20.583: E/AndroidRuntime(833):  at com.counterehr.abhi.MainActivity.onCreate(MainActivity.java:33)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at android.app.Activity.performCreate(Activity.java:5104)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
    07-01 18:16:20.583: E/AndroidRuntime(833):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
    07-01 18:16:20.583: E/AndroidRuntime(833):  ... 11 more
    07-01 18:16:23.873: I/Process(833): Sending signal. PID: 833 SIG: 9

请！有人帮我解决这个问题，我尝试了所有可能的方法，但没有得到解决方案。

Answer 1

问题是您混淆了 Fragment 和 Activity，因此在 Activity 中处理事件，即使这些 View 显示在 Fragment 中。基本上，您应该在 Activity 中有一个 FrameLayout，仅此而已 - 逻辑在 Fragment 中处理。

一个例子如下：

public class ContainerActivity extends ActionBarActivity
{
    @Override
    public void onCreate(Bundle saveInstanceState)
    {
        super.onCreate(saveInstanceState);
        this.setContentView(R.layout.activity_container);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        if (saveInstanceState == null)
        {               
             getSupportFragmentManager().beginTransaction()
                .add(R.id.activity_container_container, new ExampleFragment())
                .addToBackStack(null)
             .commit();
        }
        getSupportFragmentManager().addOnBackStackChangedListener(new OnBackStackChangedListener()
        {
            public void onBackStackChanged()
            {
                int backCount = getSupportFragmentManager().getBackStackEntryCount();
                if (backCount == 0)
                {
                    finish();
                }
            }
        });
    }
}

activity_container.xml：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <FrameLayout
        android:id="@+id/activity_container_container"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</RelativeLayout>

示例片段：

public class ExampleFragment extends Fragment implements View.OnClickListener
{
    private Button btnOne;
    private Button btnTwo;
    private Button btnThree;

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

        btnOne = (Button) rootView.findViewById(R.id.example_button_one);
        btnTwo = (Button) rootView.findViewById(R.id.example_button_two);
        btnThree = (Button) rootView.findViewById(R.id.example_button_three);

        btnOne.setOnClickListener(this);
        btnTwo.setOnClickListener(this);
        btnThree.setOnClickListener(this);
        return rootView;
    }

    @Override
    public void onClick(View v)
    {
        if (btnOne == v)
        {
            Toast.makeText(getActivity(), "One.", Toast.LENGTH_LONG).show();
        }
        else if (btnTwo == v)
        {
            Toast.makeText(getActivity(), "Two.", Toast.LENGTH_LONG).show();
        }
        else if (btnThree == v)
        {
            Toast.makeText(getActivity(), "Three.", Toast.LENGTH_LONG).show();
        }
    }
}

fragment_example.xml：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent" >

        <Button
            android:id="@+id/example_button_one"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_alignParentTop="true"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="30dp"
            android:text="@string/hello" 
            android:layout_marginLeft="20dp"
            android:layout_marginRight="20dp"/>

        <Button
            android:id="@+id/example_button_two"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/example_button_one"
            android:layout_alignRight="@+id/example_button_one"
            android:layout_below="@+id/example_button_one"
            android:layout_marginTop="30dp"
            android:text="@string/hello" />

        <Button
            android:id="@+id/example_button_three"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/example_button_two"
            android:layout_alignRight="@+id/example_button_two"
            android:layout_below="@+id/example_button_two"
            android:layout_marginTop="30dp"
            android:text="@string/hello" />

</RelativeLayout>

Android-Manifest.xml：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="19" />
    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.ContainerActivity"
            android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

这应该是一个有效的例子，它展示了如何使用 Activity 来显示 Fragment，并处理该 Fragment 中的事件。它目前没有显示 Fragment 如何与 Activity 通信以更改为另一个 Fragment，这是通过接口完成的，您可以将其作为变量并存储在 onAttach() 回调方法中。

但我确实在NullPointerException accessing views in onCreate() 上的“温和提炼”版中也包含了这一点