Pre-wrote android code throws nullPointerException
===

I dont know why this code throws me nullPointerException. I dont wrote this part and im pretty new with this staff. I was searching everywhere but dont found an answer.
    
    public class ProgramExamActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);        
        setContentView(R.layout.main);
        setTitle("Screen #1");
    }
    public void onClick(View v) {    	
    	switch(v.getId()) {    	
    		case R.id.btn2:
    				setContentView(R.layout.screen2);
    				setTitle("Screen #2");
    				break;
    	}
    }

Sorry for my English:)