应用程序在android模拟器中停止

Question

I'm completely new to java, so i have started making an app to find the TOTAL by presing add and subtract buttons. I have defined buttons and the text view in FRAGMENT_MAIN.XML , and their functions in main activity.JAVA . After defining the functions of buttons in main activity.java , I'm getting logcat errors in my program and the app is getting unfortunately stoped in android emulator. i am using android sdk version 22.6.4, eclipse ide for java develpers version is 2.0.2.20140224-0000 and eclipse platform version is 4.3.2.v2.140221-1852.

my 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;
        }
    }

}

and FRAGMENT_MAIN.XM L is

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>

on running the application i am getting logcat errors as.

logcat errors

    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

please! someone help me out of this, i tried every possible thing but not getting the solution.

Answer 1

The problem is that you are mixing up Fragments and Activities, and as such, handling events in the Activity, even though these Views are displayed in the Fragment. Basically you are meant to have a FrameLayout in the Activity, and that's it - the logic is handled in the Fragment.

An example is the following:

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>

ExampleFragment:

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>

And that should be a valid example, it shows how you can use an Activity to display a Fragment, and handle events in that Fragment. It currently doesn't show how the Fragment communicates with the Activity to change to another Fragment, which is by the way done by an interface, which you would make a variable to and store in the onAttach() callback method.

But I did include that as well in the "mildly refined" edition on NullPointerException accessing views in onCreate()