Question

I’m having problems with this textView, the application stops unexpected, here is the xml file:

<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" >

<Button
    android:id="@+button/btnAnterior"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_alignParentLeft="true"
    android:layout_marginLeft="46dp"
    android:text="Anterior" />

<Button
    android:id="@+button/btnProxima"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_alignParentRight="true"
    android:layout_marginRight="45dp"
    android:text="Próxima" />

<ScrollView
    android:id="@+id/scrollView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_above="@+id/button2"
    android:layout_alignParentLeft="true"
    android:layout_alignParentRight="true"
    android:layout_alignParentTop="true" >

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent" >

        <TextView
    android:id="@+id/tvPiada"
    android:layout_width="match_parent"
    android:layout_height="400dp"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true" />

    </LinearLayout>
</ScrollView>

and here’s the .java:

// Buttons
Button btnProxima = (Button) findViewById(R.button.btnProxima);
Button btnAnterior = (Button) findViewById(R.button.btnAnterior);   
int piadaN;

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

 // Strings
    final String piada1 = ("Piada1");
    final String piada2 = ("Piada2");

    /*piada2 = ("Piada2");
    String piada3 = new String();
    piada3 = ("Piada3");
    String piada4 = new String();
    piada4 = ("Piada4");*/

    final TextView tvPiada = (TextView) findViewById(R.id.tvPiada);

    // Evento dos botões
    btnProxima.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            piadaN = piadaN + 1;

            switch (piadaN) {
            case 1:
                tvPiada.setText(piada1);
                break;
            case 2:
                tvPiada.setText(piada2);
                break;
            }               
        }
    });

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.activity_main, menu);
    return true;
}

here’s the logcat when i run the application

11-13 19:00:47.843: E/AndroidRuntime(585): FATAL EXCEPTION: main
11-13 19:00:47.843: E/AndroidRuntime(585): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.piadas/com.example.piadas.MainActivity}: java.lang.NullPointerException
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1879)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread.access$600(ActivityThread.java:122)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.os.Handler.dispatchMessage(Handler.java:99)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.os.Looper.loop(Looper.java:137)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread.main(ActivityThread.java:4340)
11-13 19:00:47.843: E/AndroidRuntime(585):  at java.lang.reflect.Method.invokeNative(Native Method)
11-13 19:00:47.843: E/AndroidRuntime(585):  at java.lang.reflect.Method.invoke(Method.java:511)
11-13 19:00:47.843: E/AndroidRuntime(585):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
11-13 19:00:47.843: E/AndroidRuntime(585):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
11-13 19:00:47.843: E/AndroidRuntime(585):  at dalvik.system.NativeStart.main(Native Method)
11-13 19:00:47.843: E/AndroidRuntime(585): Caused by: java.lang.NullPointerException
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.Activity.findViewById(Activity.java:1794)
11-13 19:00:47.843: E/AndroidRuntime(585):  at com.example.piadas.MainActivity.<init>(MainActivity.java:13)
11-13 19:00:47.843: E/AndroidRuntime(585):  at java.lang.Class.newInstanceImpl(Native Method)
11-13 19:00:47.843: E/AndroidRuntime(585):  at java.lang.Class.newInstance(Class.java:1319)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1870)
11-13 19:00:47.843: E/AndroidRuntime(585):  ... 11 more

Answer 1

It looks to me like you are getting this error because you are trying to declare your buttons before you have declared your layout. Move the button declaration to inside of your onCreate() method and place them after your setContentView(R.layout.main)

So you need to move your Button btnProxima = (Button)findViewById(R.button.btnProxima); to after you use your setContentView