null pointer exception error that I can't figure it out

Question

I am new to programming and I have spent two days trying to figure out what is wrong with my program. I have learnt a lot trying to fix this but I am not getting anywhere.

I couldn't figure out how to copy the error in logcat in eclipse so I will type out the error:

 Caused by: java.lang.NullPointerException at com.deweyx.MainActivity.OnCreate(MainActivity.java:27)

Here is my code:

public class MainActivity extends Activity {

    int counter;
    Button add, sub;
    TextView display;

    @Override
    protected 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.xDisplay);

        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);
            }
        });

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }

}

This is the xml folder:

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



<TextView 
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/display" 
    android:layout_gravity="center"
    android:textSize="40sp"
    android:id="@+id/xDisplay"
/>


<Button
    android:layout_width="200dp"
    android:layout_height="wrap_content"
    android:text="@string/padd"
    android:layout_gravity="center"
    android:textSize="20sp"
    android:id="@+id/bAdd"
 />


<Button
    android:layout_width="200dp"
    android:layout_height="wrap_content"
    android:text="@string/psub"
    android:layout_gravity="center"
    android:textSize="20sp" 
    android:id="@+id/bSub"
/>

</LinearLayout>

logcat:

   12-19 02:11:36.423: D/AndroidRuntime(531): Shutting down VM
12-19 02:11:36.423: W/dalvikvm(531): threadid=1: thread exiting with uncaught exception (group=0x409c01f8)
12-19 02:11:36.444: E/AndroidRuntime(531): FATAL EXCEPTION: main
12-19 02:11:36.444: E/AndroidRuntime(531): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.deweyx/com.deweyx.MainActivity}: java.lang.NullPointerException
12-19 02:11:36.444: E/AndroidRuntime(531):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
12-19 02:11:36.444: E/AndroidRuntime(531):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
12-19 02:11:36.444: E/AndroidRuntime(531):  at android.app.ActivityThread.access$600(ActivityThread.java:123)
12-19 02:11:36.444: E/AndroidRuntime(531):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
12-19 02:11:36.444: E/AndroidRuntime(531):  at android.os.Handler.dispatchMessage(Handler.java:99)
12-19 02:11:36.444: E/AndroidRuntime(531):  at android.os.Looper.loop(Looper.java:137)
12-19 02:11:36.444: E/AndroidRuntime(531):  at android.app.ActivityThread.main(ActivityThread.java:4424)
12-19 02:11:36.444: E/AndroidRuntime(531):  at java.lang.reflect.Method.invokeNative(Native Method)
12-19 02:11:36.444: E/AndroidRuntime(531):  at java.lang.reflect.Method.invoke(Method.java:511)
12-19 02:11:36.444: E/AndroidRuntime(531):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
12-19 02:11:36.444: E/AndroidRuntime(531):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
12-19 02:11:36.444: E/AndroidRuntime(531):  at dalvik.system.NativeStart.main(Native Method)
12-19 02:11:36.444: E/AndroidRuntime(531): Caused by: java.lang.NullPointerException
12-19 02:11:36.444: E/AndroidRuntime(531):  at com.deweyx.MainActivity.onCreate(MainActivity.java:27)
12-19 02:11:36.444: E/AndroidRuntime(531):  at android.app.Activity.performCreate(Activity.java:4465)
12-19 02:11:36.444: E/AndroidRuntime(531):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
12-19 02:11:36.444: E/AndroidRuntime(531):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
12-19 02:11:36.444: E/AndroidRuntime(531):  ... 11 more
12-19 02:11:36.653: I/dalvikvm(531): threadid=3: reacting to signal 3
12-19 02:11:36.673: I/dalvikvm(531): Wrote stack traces to '/data/anr/traces.txt'
12-19 02:11:36.983: I/dalvikvm(531): threadid=3: reacting to signal 3
12-19 02:11:36.993: I/dalvikvm(531): Wrote stack traces to '/data/anr/traces.txt'
12-19 02:11:52.452: I/dalvikvm(551): threadid=3: reacting to signal 3
12-19 02:11:52.502: I/dalvikvm(551): Wrote stack traces to '/data/anr/traces.txt'
12-19 02:11:52.783: D/AndroidRuntime(551): Shutting down VM
12-19 02:11:52.783: W/dalvikvm(551): threadid=1: thread exiting with uncaught exception (group=0x409c01f8)
12-19 02:11:52.793: E/AndroidRuntime(551): FATAL EXCEPTION: main
12-19 02:11:52.793: E/AndroidRuntime(551): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.deweyx/com.deweyx.MainActivity}: java.lang.NullPointerException
12-19 02:11:52.793: E/AndroidRuntime(551):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
12-19 02:11:52.793: E/AndroidRuntime(551):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
12-19 02:11:52.793: E/AndroidRuntime(551):  at android.app.ActivityThread.access$600(ActivityThread.java:123)
12-19 02:11:52.793: E/AndroidRuntime(551):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
12-19 02:11:52.793: E/AndroidRuntime(551):  at android.os.Handler.dispatchMessage(Handler.java:99)
12-19 02:11:52.793: E/AndroidRuntime(551):  at android.os.Looper.loop(Looper.java:137)
12-19 02:11:52.793: E/AndroidRuntime(551):  at android.app.ActivityThread.main(ActivityThread.java:4424)
12-19 02:11:52.793: E/AndroidRuntime(551):  at java.lang.reflect.Method.invokeNative(Native Method)
12-19 02:11:52.793: E/AndroidRuntime(551):  at java.lang.reflect.Method.invoke(Method.java:511)
12-19 02:11:52.793: E/AndroidRuntime(551):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
12-19 02:11:52.793: E/AndroidRuntime(551):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
12-19 02:11:52.793: E/AndroidRuntime(551):  at dalvik.system.NativeStart.main(Native Method)
12-19 02:11:52.793: E/AndroidRuntime(551): Caused by: java.lang.NullPointerException
12-19 02:11:52.793: E/AndroidRuntime(551):  at com.deweyx.MainActivity.onCreate(MainActivity.java:27)
12-19 02:11:52.793: E/AndroidRuntime(551):  at android.app.Activity.performCreate(Activity.java:4465)
12-19 02:11:52.793: E/AndroidRuntime(551):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
12-19 02:11:52.793: E/AndroidRuntime(551):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
12-19 02:11:52.793: E/AndroidRuntime(551):  ... 11 more
12-19 02:11:52.963: I/dalvikvm(551): threadid=3: reacting to signal 3
12-19 02:11:52.963: I/dalvikvm(551): Wrote stack traces to '/data/anr/traces.txt'
12-19 02:11:53.343: I/dalvikvm(551): threadid=3: reacting to signal 3
12-19 02:11:53.353: I/dalvikvm(551): Wrote stack traces to '/data/anr/traces.txt'

Answer 1

Because it crashes on the findViewById when looking for the id bAdd, this is probably because that id is not defined yet. Are you sure you are using the correct xml? What I mean is, are you sure you're using activity_main.xml as the XML layout? Because maybe you are looking in the wrong XML file, it has happened to me :-)

Answer 2

Everything is correct just do Project->Clean

If it didn't help you then you need to restart your IDE!

Answer 3

Do you want to learn a shortcut way to define button click?

Here is shortcut way to define click listener for any button.

Step 1: include android:onClick="btnAddButtonClick" inside your add button xml:

<Button
    android:layout_width="200dp"
    android:layout_height="wrap_content"
    android:text="@string/padd"
    android:layout_gravity="center"
    android:textSize="20sp"
    android:onClick="btnAddButtonClick"     <<----- android:onClick
    android:id="@+id/bAdd"
 />

Step 2: Create a method inside your activity, with name the same value you have given inside onClick attribute:

public void btnAddButtonClick(View v) 
{
     counter++;
     display.setText("Your total is " + counter);   
}