Ошибка attempt to invoke virtual method

I have a code module which implements viewpager with navigation drawer, however, when I run the code I get the following error

01-26 09:20:02.958: D/AndroidRuntime(18779): Shutting down VM
01-26 09:20:02.959: E/AndroidRuntime(18779): FATAL EXCEPTION: main
01-26 09:20:02.959: E/AndroidRuntime(18779): Process: com.example.tabwithslidingdrawer, PID: 18779
01-26 09:20:02.959: E/AndroidRuntime(18779): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.tabwithslidingdrawer/com.example.tabwithslidingdrawer.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2314)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2388)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread.access$800(ActivityThread.java:148)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1292)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.os.Handler.dispatchMessage(Handler.java:102)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.os.Looper.loop(Looper.java:135)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread.main(ActivityThread.java:5312)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at java.lang.reflect.Method.invoke(Native Method)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at java.lang.reflect.Method.invoke(Method.java:372)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)
01-26 09:20:02.959: E/AndroidRuntime(18779): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
01-26 09:20:02.959: E/AndroidRuntime(18779):    at com.example.tabwithslidingdrawer.MainActivity.onCreate(MainActivity.java:95)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.Activity.performCreate(Activity.java:5953)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1128)
01-26 09:20:02.959: E/AndroidRuntime(18779):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2267)
01-26 09:20:02.959: E/AndroidRuntime(18779):    ... 10 more

09:20:02.959: E/AndroidRuntime(18779): Caused by:
java.lang.NullPointerException: Attempt to invoke virtual method 'void
android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null
object reference 01-26 09:20:02.959: E/AndroidRuntime(18779):     at
com.example.tabwithslidingdrawer.MainActivity.onCreate(MainActivity.java:95)

points to this line

// enabling action bar app icon and behaving it as a toggle button
        getActionBar().setDisplayHomeAsUpEnabled(true);
        getActionBar().setHomeButtonEnabled(true);

entire code
http://pastebin.com/u1K72fr7

My manifest

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.tabwithslidingdrawer"
    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/Theme.Tabwithpager" >
        <activity
            android:name="com.example.tabwithslidingdrawer.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>
    </application>

</manifest>

android:theme=»@style/Theme.Tabwithpager»

code
http://pastebin.com/EFQLzRej

==================================================================
EDIT/UPDATE:

What I learnt from this

Whenever such an error occurs

1. Check what kind of Activity is being used, is it a simple android.app Activity or an AppCompatActivity or an ActionBarActivity and so on.

2. Check if your activity type which is extended falls under the compat category

example android.app based Activity/Fragment are non appCompat types,
whereas android.support.v4.app.Fragment or android.support.v4.app.ActivityCompat
are appCompat based

if it falls under appCompat we use getSupportActionBar()
else for android.app types we can use getActionBar()

3. Check the theme applied to the activity in question in the manifest file

example:
In the manifest file if theme applied is say
android:theme=»@android:style/Theme.Holo.Dialog»
getActionBar() will work

but if theme applied for the activity in the manifest is as follows
android:theme=»@style/Theme.AppCompat.Light»
then you have to use
getSupportActionBar()

This tutorial demonstrates how to solve the Attempt to invoke virtual method on a null object reference error in Java.

the Attempt to invoke virtual method on a null object reference Error in Java

While working on an Android application, the error Attempt to invoke virtual method on a null object reference can occur, a type of NullPointerException. Whenever we try to invoke a method using a null object, it will throw this error.

Here is a system based on the Android platform, which throws the NullPointerException with the attempt to invoke virtual method on a null object reference description.

This example has three different Java classes.

Main_Player.java:

package delftstack;

import javax.naming.Context;

public class Main_Player {

    private Context AppContext;
    private SharedPreferences SharedPreferencesSettingsU;
    private SharedPreferences.Editor PreferencesEditorU;
    private static final int PREFERENCE_MODE = 0;
    private static final String UNIQUE_PREFERENCES_FILE = "DemoApp";

    public Player(Context AppContext, String Player_Name) {
        this.AppContext = AppContext;
        Save_Name(Player_Name);
    }

    public void Save_Name(String nValue) {
    	SharedPreferencesSettingsU = AppContext.getSharedPreferences(UNIQUE_PREFERENCES_FILE, PREFERENCE_MODE);
    	PreferencesEditorU = SharedPreferencesSettingsU.edit();
    	PreferencesEditorU.putString("keyName", nValue);
    	PreferencesEditorU.commit();
    }

    public String Get_Name(Context DemoContext) {
    	SharedPreferencesSettingsU = DemoContext.getSharedPreferences(UNIQUE_PREFERENCES_FILE, PREFERENCE_MODE);
        String PlayerName = SharedPreferencesSettingsU.getString("keyName", "ANONYMOUS");
        return PlayerName;
    }
}

Player_Name.java:

package delftstack;

import javax.naming.Context;

public class Player_Name extends Activity {

private EditText Player_Name;
private Player M_Player;
public static Context AppContext;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.player_name);
    AppContext = this;
    Player_Name = (EditText) findViewById (R.id.etName);

}

public void onC_Confirm(View btnclick) {
	M_Player = new Player(AppContext, String.valueOf(Player_Name.getText()));
    //M_Player.saveName();
    Intent intent = new Intent(Player_Name.this, Game_Play.class);
    startActivity(intent);
}

public void onC_testShPref(View btnclick) {
    Intent intent = new Intent(Player_Name.this, Game_Play.class);
    startActivity(intent);
}

Game_Play.java:

public class Game_Play extends Activity {

    private TextView Welcome_Player;
    private ListView Games_Created;
    private Player M_Player;

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

        Welcome_Player = (TextView) findViewById (R.id.tvPlayer_Name);
        Welcome_Player.setText("Welcome Back Player, " + String.valueOf(M_Player.getName(this)) + " !");

        Games_Created = (ListView) findViewById (R.id.listCreatedGames);
        Games_Created.setEmptyView(findViewById (R.id.tvNoGames));

    }

}

We run the above system on an Android platform, which will throw an error caused by the java.lang.NullPointerException: Attempt to invoke virtual method.

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String plp.cs4b.thesis.drawitapp.Main_Player.getName()' on a null object reference
    at plp.cs4b.thesis.drawitapp.Game_Play.onCreate(Game_Play.java:20)
    at android.app.Activity.performCreate(Activity.java:5933)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
    ... 10 more

The error occurs in the below code line in the class Game_Play.java.

Welcome_Player.setText("Welcome Back Player, " + String.valueOf(M_Player.getName(this)) + " !");

This error occurs because the M_Player comes as Null. After all, it is not initialized. By initializing, we can solve that issue.

See the solution:

Game_Play.java:

package delftstack;

public class Game_Play extends Activity {

    private TextView Welcome_Player;
    private ListView Games_Created;
	//initialize the M_Player
    private Player M_Player = new Main_Player(AppContext,"");

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

        Welcome_Player = (TextView) findViewById (R.id.tvPlayer_Name);
        Welcome_Player.setText("Welcome Back Player, " + String.valueOf(M_Player.getName(this)) + " !");

        Games_Created = (ListView) findViewById (R.id.listCreatedGames);
        Games_Created.setEmptyView(findViewById (R.id.tvNoGames));

    }

}

So whenever working on a system like this, you must ensure that a method is not invoking any null value; otherwise, it will throw this exception.

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