CODEWITHSUNDEEP
javaandroidlayouttabsnullpointerexception
Maciej Socha
Maciej Socha

Reputation: 390

NullPointerException on implementation of Tab Layout

I've implemented Tab Layout and my app is not starting. I get NullPointerException on this line:

actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

I think that everything is done properly. I can't find my mistake.

MainActivity:

import android.app.ActionBar;
import android.app.ActionBar.Tab;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;

import com.msh.numeral_system_converter_adapter.TabsPagerAdapter;

public class MainActivity extends FragmentActivity implements
    ActionBar.TabListener {

private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;

// Tab titles
private String[] tabs = { "Converter", "Calculator"};

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

    // Initilization
    viewPager = (ViewPager) findViewById(R.id.pager);
    actionBar = getActionBar();
    mAdapter = new TabsPagerAdapter(getSupportFragmentManager());

    viewPager.setAdapter(mAdapter);
    //actionBar.setHomeButtonEnabled(false);
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    // Adding Tabs
    for (String tab_name : tabs) {
        actionBar.addTab(actionBar.newTab().setText(tab_name)
                .setTabListener(this));
    }

    /**
     * on swiping the viewpager make respective tab selected
     * */
    viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

        @Override
        public void onPageSelected(int position) {
            // on changing the page
            // make respected tab selected
            actionBar.setSelectedNavigationItem(position);
        }

        @Override
        public void onPageScrolled(int arg0, float arg1, int arg2) {
        }

        @Override
        public void onPageScrollStateChanged(int arg0) {
        }
    });
}

@Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
}

@Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
    // on tab selected
    // show respected fragment view
    viewPager.setCurrentItem(tab.getPosition());
}

@Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
}

}

Log file:

04-11 17:44:02.553    1545-1545/com.msh.numeral_system_converter E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.msh.numeral_system_converter, PID: 1545
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.msh.numeral_system_converter/com.msh.numeral_system_converter.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.msh.numeral_system_converter.MainActivity.onCreate(MainActivity.java:34)
            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)

Upvotes: 2

Views: 7321

Answers (4)

Parag Kadam
Parag Kadam

Reputation: 3850

This exception arises when you have selected a theme with no ActionBar. Try using a theme like,

<style name="AppTheme" parent="@android:style/Theme.Holo.Light">
    <!-- Customize your theme here. -->
</style>

Upvotes: 0

kumar kundan
kumar kundan

Reputation: 2057

Using above fix you may get another error :-

requestFeature() must be called before adding content

to solve that problem use this fix :-

Don't call setContentView() before requestFeature().

Upvotes: 0

S.Thiongane
S.Thiongane

Reputation: 6905

getActionBar() can retrun null if your window don't have an ActionBar. Like if you chose the Theme.NoTitleBar style as an Activity/Application theme.

Use this code instead (picked from there):

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

// The Action Bar is a window feature. The feature must be requested
// before setting a content view. Normally this is set automatically
// by your Activity's theme in your manifest. The provided system
// theme Theme.WithActionBar enables this for you. Use it as you would
// use Theme.NoTitleBar. You can add an Action Bar to your own themes
// by adding the element <item name="android:windowActionBar">true</item>
// to your style definition.
    getWindow().requestFeature(Window.FEATURE_ACTION_BAR);
    setContentView(R.layout.main);

// experiment with the ActionBar 
    ActionBar actionBar = getActionBar();
}

This link also may help.

Upvotes: 3

FrobberOfBits
FrobberOfBits

Reputation: 18002

This code:

actionBar = getActionBar();
mAdapter = new TabsPagerAdapter(getSupportFragmentManager());

viewPager.setAdapter(mAdapter);
//actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

You assign actionBar from the getActionBar() method, but you didn't provide that code. My guess is that getActionBar() is returning null. The NullPointerException is coming from trying to call setNavigationMode on a null object, probably not from referencing the field on ActionBar.

Upvotes: 1

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