[GUIDE] Debugging apps

06-15 12:45:02.205      805-805/?                              E/AndroidRuntime: FATAL EXCEPTION: main
        java.lang.RuntimeException: Unable to start activity ComponentInfo{de.nikwen.myapplication/de.nikwen.myapplication.MainActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
        at android.app.ActivityThread.access$600(ActivityThread.java:141)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:5041)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
        at dalvik.system.NativeStart.main(Native Method)
        Caused by: java.lang.NullPointerException
        at de.nikwen.myapplication.MainActivity.doSomething(MainActivity.java:21)
        at de.nikwen.myapplication.MainActivity.onCreate(MainActivity.java:17)
        at android.app.Activity.performCreate(Activity.java:5104)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
        ... 11 more

package de.nikwen.myapplication;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.widget.Button;

public class MainActivity extends Activity {

    Button myButton;
    
    @[B][/B]Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        doSomething();
    }

    private void doSomething() {
        myButton.setText("Crashing here");
    }
    
}

java.lang.RuntimeException: Unable to start activity ComponentInfo{de.nikwen.myapplication/de.nikwen.myapplication.MainActivity}: java.lang.[COLOR="Red"]NullPointerException[/COLOR]

java.lang.RuntimeException: [COLOR="Red"]Unable to start activity[/COLOR] ComponentInfo{de.nikwen.myapplication/de.nikwen.myapplication.MainActivity}: java.lang.NullPointerException

Caused by: java.lang.NullPointerException

[COLOR="Red"]at de.nikwen.myapplication.MainActivity.doSomething(MainActivity.java:21)
at de.nikwen.myapplication.MainActivity.onCreate(MainActivity.java:17)[/COLOR]
at android.app.Activity.performCreate(Activity.java:5104)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)

at de.nikwen.myapplication.MainActivity.onCreate[COLOR="Red"](MainActivity.java:17)[/COLOR]

at de.nikwen.myapplication.MainActivity.doSomething[COLOR="Red"](MainActivity.java:21)[/COLOR]

myButton.setText("Crashing here");

@[B][/B]Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
        
    myButton = (Button) findViewById(R.id.my_button);

    doSomething();
}

Log.d("myTag", "myMessage");

Log.e("myTag", "myErrorMessage");

Log.d("start", "hey");

String[] myStringArray = new String[] {"Hello world", "Debugging is fun (normally not ;))", "XDA is great!!!"};

String all = "Words: ";

for (String s: myStringArray) {
    Log.d("String s", s);
    all = all + s + ", ";
    Log.d("new all", all);
}
        
Log.d("status", "done");

06-15 13:15:03.014    1347-1347/de.nikwen.myapplication        D/start: hey
06-15 13:15:03.014    1347-1347/de.nikwen.myapplication        D/String s: Hello world
06-15 13:15:03.014    1347-1347/de.nikwen.myapplication        D/new all: Words: Hello world,
06-15 13:15:03.014    1347-1347/de.nikwen.myapplication        D/String s: Debugging is fun (normally not ;))
06-15 13:15:03.014    1347-1347/de.nikwen.myapplication        D/new all: Words: Hello world, Debugging is fun (normally not ;)),
06-15 13:15:03.014    1347-1347/de.nikwen.myapplication        D/String s: XDA is great!!!
06-15 13:15:03.014    1347-1347/de.nikwen.myapplication        D/new all: Words: Hello world, Debugging is fun (normally not ;)), XDA is great!!!,
06-15 13:15:03.014    1347-1347/de.nikwen.myapplication        D/status: done

for (int i = 0; i < 20; i--) {
    //do something
    
    Log.d("i", String.valueOf(i));
}

Log.d("status", "1");

myButton.setText("New text");

Log.d("status", "2");

Log.d("status", "done");

try {
    Thread.sleep(5000);
} catch(Exception e) {
    e.printStackTrace();
    
    //do something here
}