GregorianCalendar validates the field values before resolving the calendar fields set by an application. As a result, it performs lenient operations in non-lenient mode. The following is a sample program.

--
import java.util.*;


public class Feb29 {
    public static void main(String[] args) {
        GregorianCalendar gcal = new GregorianCalendar();
        gcal.clear();
        gcal.setLenient(false);
        //gcal.set(gcal.YEAR, 1970);
        gcal.set(gcal.MONTH, gcal.FEBRUARY);
        gcal.set(gcal.DAY_OF_MONTH, 29);
        Date date = gcal.getTime();
        System.out.println(date);
    }
}

---
The output:

% java Feb29
Sun Mar 01 00:00:00 JST 1970

If the comment line is enabled, it throws an exception.

% java Feb29
Exception in thread "main" java.lang.IllegalArgumentException
        at java.util.GregorianCalendar.computeTime(GregorianCalendar.java:1519)
        at java.util.Calendar.updateTime(Calendar.java:1552)
        at java.util.Calendar.getTimeInMillis(Calendar.java:912)
        at java.util.Calendar.getTime(Calendar.java:888)
        at Feb29.main(Feb29.java:12)