MainAcitivity中的java.lang.NullPointerException setOnClickListener

Question

When I run my application, I keep getting a NullPointerException on line 56 as shown in the debug log below. I am relatively new to programming android applications, so I can't figure out what's going on here. Can someone please help me out with this? My MainActivity.java:

package com.termite.pockettube;

import java.util.Locale;

import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.app.NavUtils;
import android.support.v4.view.ViewPager;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends FragmentActivity implements View.OnClickListener{

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a
     * {@link android.support.v4.app.FragmentPagerAdapter} derivative, which
     * will keep every loaded fragment in memory. If this becomes too memory
     * intensive, it may be best to switch to a
     * {@link android.support.v4.app.FragmentStatePagerAdapter}.
     */
    SectionsPagerAdapter mSectionsPagerAdapter;

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    ViewPager mViewPager;

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

        // Create the adapter that will return a fragment for each of the three
        // primary sections of the app.
        mSectionsPagerAdapter = new SectionsPagerAdapter(
                getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        button = (Button) findViewById(R.id.button);
        button.setOnClickListener(this); //this is line 56
    }
    public void countryOnClick() {
        startActivity(new Intent("com.termite.pockettube.COUNTRY"));
    }
    @Override
public void onClick(View v) {
    switch (v.getId()){
    case R.id.button:
        countryOnClick();
        break;
    }
}
    @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;
    }

    /**
     * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            // getItem is called to instantiate the fragment for the given page.
            // Return a DummySectionFragment (defined as a static inner class
            // below) with the page number as its lone argument.
            switch (position) {
            case 0:
                Fragment1 fragment1 = new Fragment1();
                return fragment1;
            case 1:
                Fragment2 fragment2 = new Fragment2();
                return fragment2;
            case 2:
                Fragment3 fragment3 = new Fragment3();
                return fragment3;
            case 3:
                Fragment4 fragment4 = new Fragment4();
                return fragment4;
            case 4:
                Fragment5 fragment5 = new Fragment5();
                return fragment5;
            }
            return null;
        }

        @Override
        public int getCount() {
            // Show 5 total pages.
            return 5;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            Locale l = Locale.getDefault();
            switch (position) {
            case 0:
                return getString(R.string.title_section1).toUpperCase(l);
            case 1:
                return getString(R.string.title_section2).toUpperCase(l);
            case 2:
                return getString(R.string.title_section3).toUpperCase(l);
            case 3:
                return getString(R.string.title_section4).toUpperCase(l);
            case 4:
                return getString(R.string.title_section5).toUpperCase(l);
            }
            return null;
        }
    }

}

This is my debug log when attempting to run the application:

12-20 01:13:26.898: D/AndroidRuntime(6539): Shutting down VM
12-20 01:13:26.906: W/dalvikvm(6539): threadid=1: thread exiting with uncaught exception (group=0x40af21f8)
12-20 01:13:26.906: E/AndroidRuntime(6539): FATAL EXCEPTION: main
12-20 01:13:26.906: E/AndroidRuntime(6539): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.termite.pockettube/com.termite.pockettube.MainActivity}: java.lang.NullPointerException
12-20 01:13:26.906: E/AndroidRuntime(6539):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2035)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2060)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at android.app.ActivityThread.access$600(ActivityThread.java:127)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1181)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at android.os.Handler.dispatchMessage(Handler.java:99)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at android.os.Looper.loop(Looper.java:137)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at android.app.ActivityThread.main(ActivityThread.java:4558)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at java.lang.reflect.Method.invokeNative(Native Method)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at java.lang.reflect.Method.invoke(Method.java:511)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at dalvik.system.NativeStart.main(Native Method)
12-20 01:13:26.906: E/AndroidRuntime(6539): Caused by: java.lang.NullPointerException
12-20 01:13:26.906: E/AndroidRuntime(6539):     at com.termite.pockettube.MainActivity.onCreate(MainActivity.java:56)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at android.app.Activity.performCreate(Activity.java:4635)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
12-20 01:13:26.906: E/AndroidRuntime(6539):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1999)
12-20 01:13:26.906: E/AndroidRuntime(6539):     ... 11 more

Manifest xml code:

    <uses-sdk
        android:minSdkVersion="11"
        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.termite.pockettube.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name="com.termite.pockettube.CountryActivity"
            android:label="@string/title_activity_country" >
            <intent-filter>
                <action android:name="com.termite.pockettube.COUNTRY" />

                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity>
    </application>

</manifest>

activity_main.xml:

<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity" >

    <android.support.v4.view.PagerTabStrip
        android:id="@+id/pager_title_strip"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:layout_gravity="top"
        android:background="#33b5e5"
        android:paddingBottom="4dp"
        android:paddingTop="4dp"
        android:textColor="#fff" 
         >

    </android.support.v4.view.PagerTabStrip>



</android.support.v4.view.ViewPager>

fragment1.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: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=".MainActivity$DummySectionFragment" >

    <Button
        android:id="@+id/button"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:layout_alignParentTop="true"
        android:text="@string/button" />

</RelativeLayout>

Answer 1

button = (Button) findViewById(R.id.button);
button.setOnClickListener(this); //this is line 56

button is null. Initialization fails

Need button in activity_main.xml and also id should be button

findViewById looks for a view with the id in the current inflated layout. So if you have button in activity_main.xml then your initialization works else you get NPE`

There is no button in activity_main.xml . S0 button = (Button) findViewById(R.id.button); fails

Button is in fragment1. So you will need to inflate the layout fragment1.xml and then initialize the button in onCreateView not initialize button in Activity.

Answer 2

You need to button in activity_main.xml as you had added that button in fragment_1.xml this is is the cause of NPE.

add button in activity_main.xml will solve your problem

**activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="@dimen/padding_Small"
    android:shrinkColumns="*"
    android:stretchColumns="*" >

    <android.support.v4.view.ViewPager
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity" >

        <android.support.v4.view.PagerTabStrip
            android:id="@+id/pager_title_strip"
            android:layout_width="match_parent"
            android:layout_height="50dp"
            android:layout_gravity="top"
            android:background="#33b5e5"
            android:paddingBottom="4dp"
            android:paddingTop="4dp"
            android:textColor="#fff" />
    </android.support.v4.view.ViewPager>

    <Button
        android:id="@+id/button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:text="ButtonText" />

</RelativeLayout>

Answer 3

Thank you all for your input, and it all worked to end the NPE, yet it didn't perform the task I wanted it to complete. I added the button to ,y Fragment1.java file and then it worked.

package com.termite.pockettube;

import com.termite.pockettube.MainActivity.SectionsPagerAdapter;

import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.view.ViewPager;
import android.view.LayoutInflater;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;

public class Fragment1 extends Fragment implements View.OnClickListener {
 Button button;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        // Get the view from fragmenttab1.xml
        View view = inflater.inflate(R.layout.fragment1, container, false);
        button = (Button)view.findViewById(R.id.button);
        button.setOnClickListener(this);
        return view;
    }
   /* @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);


    }*/
    public void countryOnClick() {
        startActivity(new Intent("com.termite.pockettube.COUNTRY"));
    }
    @Override
public void onClick(View v) {
    switch (v.getId()){
    case R.id.button:
        countryOnClick();
        break;
    }
}
}

Answer 4

Check R.id.button , button is it in R.layout.activity_main layout..

Post your activity_main.xml layout.

Do below thing.

Add your Button in activity_main.xml or change code

button = (Button) mViewPager.findViewById(R.id.button);
button.setOnClickListener(this);

Answer 5

Try

Button button = (Button) findViewById(R.id.button);
                button.setOnClickListener(buttonListener);

in onCreate

Then try

private OnClickListener buttonListener = new OnClickListener() {
    public void onClick(View v) {
        //Do Work Here
    }
};

Answer 6

Your error is in the getItem(poistion) method. even your adapter size is 5, it give you positions grater than 5. at such moments you return null from there.

you can check this by adding a Log statement to print the position value at the beginning of the getItem method

Answer 7

Button button=(Button)findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {   Toast.makeText(MainActivity.this,"RunAPP",Toast.LENGTH_LONG).show();
        }
    });