Q:

Android NullPointerException when findViewById

when i run my app i get this error in the log-cat:
Caused by: java.lang.NullPointerException
at com.myfirstapp.myfirstapp.MainActivity.onCreate(MainActivity.java:52)
at android.app.Activity.performCreate(Activity.java:5312)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1111)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2552)
... 11 more

Here's line 52:
String message = editTextInput.getText().toString();

So far i've worked out that the NPE must be when I define the EditText (or any view for that matter):
EditText editText = (EditText) findViewById(R.id.input_text);

However though, I don't get an NPE when I define a EditText without: findViewById() like this View:
TextView desc = new TextView(this);

Here is the entire onCreate() method:
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }

    EditText editText = (EditText) findViewById(R.id.input_text);
    Button submit = (Button) findViewById(R.id.submit);
    LinearLayout layout = (LinearLayout) findViewById(R.id.layout_main);
    Intent intent = new Intent(this, testActivity.class);
    final TextView desc = new TextView(this);
    final TextView title = new TextView(this);

    String message = editText.getText().toString();

    submit.setVisibility(View.VISIBLE);
    submit.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            editText.setVisibility(View.GONE);}});

    desc.setTextSize(20);
    desc.setText(message);
    desc.setGravity(Gravity.LEFT | Gravity.TOP);
    desc.setPadding(5, 5, 5, 5);
    desc.setTextColor(getResources().getColor(R.color.black));

    title.setTextSize(10);
    title.setText(R.string.title_activity_dictionary);
    title.setGravity(Gravity.LEFT | Gravity.TOP);
    title.setPadding(5, 5, 5, 5);
    title.setTextColor(getResources().getColor(R.color.black));

    layout.addView(desc);
    layout.addView(title);
}

What I dont get is why is there a NPE when I clearly define the View?

A:

This is because findViewById() searches in the activity_main layout, while the button is located in the fragment's layout fragment_main.
Move that piece of code in the onCreateView() method of the fragment:
//...

View rootView = inflater.inflate(R.layout.fragment_main, container, false);
Button buttonClick = (Button)rootView.findViewById(R.id.button);
buttonClick.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        onButtonClick((Button) view);
    }
});

Notice that now you access it through rootView view:
Button buttonClick = (Button)rootView.findViewById(R.id.button);

otherwise you would get again NullPointerException.

