android 中 findViewById() 上的 NullPointerException

In the following code i get a NullPointerException on lines 9/10 with findViewById().
In my main class I just instantiated an object from this class, to use .getFrom()

public class UserInteraction extends Activity {
EditText etFrom;
int from;
EditText etTill;
int till;

public UserInteraction(){
    etFrom = (EditText)findViewById(R.id.et_from);
    etTill = (EditText)findViewById(R.id.et_till);
}

public int getFrom() {
    String s = etFrom.getText().toString();
    int i = Integer.parseInt(s);
    return i;
}

public int getTill() {
    String s = etTill.getText().toString();
    int i = Integer.parseInt(s);
    return i;
}

是不是我的主类中设置了 contentView ..?可能是什么原因?