这个空指针异常怎么处理？

Question

package b;

public class MyDate {
private int year;
private int month;
private LeapJduge lj;

public MyDate(int year, int month, LeapJduge lj) {
    super();
    this.year = year;
    this.month = month;
    this.lj = lj;
}

int lastDayMonth() {
    int i = 0;
    switch (month) {
    case 1:
    case 3:
    case 5:
    case 7:
    case 8:
    case 10:
    case 12:
        i = 31;
        break;
    case 4:
    case 6:
    case 9:
    case 11:
        i = 30;
        break;
    case 2:
        i = lj.isLeap(year) ? 29 : 28;
    default:
        throw new IllegalArgumentException("month is out of range");
    }
    return i;
}

}

package b;

public interface LeapJduge {
boolean isLeap(int year);
}

package b;

import org.junit.Assert;
import org.junit.Test;

public class 测试 {
@Test
public void Tset1() {
MyDate a=new MyDate(2000, 1, null);
Assert.assertEquals(31, a.lastDayMonth());
}
@Test
public void Tset2() {
MyDate a=new MyDate(1999, 2, null);
Assert.assertEquals(28, a.lastDayMonth());
}
@Test
public void Tset3() {
MyDate a=new MyDate(2000, 2, null);
Assert.assertEquals(28, a.lastDayMonth());
}

Test2 和Test3 都抛出了了空指针异常，但是Test1没有
求大佬指教！！

Answer 1

出现在2月份没有实例化对象LeapJduge，new MyDate(1999, 2, null);不能直接传null，要改成如new MyDate(1999, 2, new LeapJduge());