Difference between Long and long?

I am going through the 2nd exercise of the android example the notepad app, I have this a question about the difference between Long and long that was used to define the mRowId.

The exercise is here: http://developer.android.com/resources/tutorials/notepad/notepad-ex2.html

And below is the code piece that I am having problem with:

public class NoteEdit extends Activity {

private Long mRowId; 
private Ed开发者_如何学CitText mTitleText;
private EditText mBodyText; 

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);

    setContentView(R.layout.note_edit);
    setTitle(R.string.edit_note);

    mTitleText = (EditText) findViewById(R.id.title);
    mBodyText = (EditText) findViewById(R.id.body);
    Button confirmButton = (Button) findViewById(R.id.confirm);

    mRowId = null;

When I declared mRowId with long, I got an error when I tried to set mRowId to null, the error is "type mismatch". But if I use Long, the error goes away. Why doesn't long work?

Long is a wrapper class around the primitive long. Therefore Long is an object; objects can be null, primitives can not.

See the Long class documentation.

long is primitive type and Long is boxed type of long. After auto-boxing feature is released in java the primitive long can be automatically converted to Long, which is an object.

But sometmimes this creates issue also. For example the below code is terribly slow:

public static void main(String[] args)
{
    Long sum = 0L;
    for(long i=0; i < Integer.MAX_VAL; i++){
        sum+=i;
    }
}

This is because the program unintentionally creating 2^31 objects unnecessarily because of capital L in sum declaration.