java String 空指针异常

 1 import java.util.*;
 2 import java.io.*;
 3 
 4 public class test {
 5     public static void main(String[] args) {
 6         
 7         String a = null;
 8         if (a.equals(""))
 9             System.out.println(1);
10         if (a.isEmpty())
11             System.out.println(2);
12         if (a==null)
13             System.out.println(3);
14     }
15 }