Java - String - null

public static String s;
public static void main(String[] args) {
    if (s.length() == 0) {
        System.out.println("11111");
    } else {
        System.out.println("afafa");
    }
}

控制台窗口报错如下：

Exception in thread "main" java.lang.NullPointerException  
    at com.merck.app.coronavirus.application.schedule.FastaStatusJob.main(FastaStatusJob.java:262)

解决方案：将nul更改为""

public static String s=””;  
public static void main(String[] args) {  
    if (s.length() == 0) {  
        System.out.println("11111");  
    } else {  
        System.out.println("afafa");  
    }  
}