java string null + ""

字符串拼接操作问题，还是上代码吧。

package string;public class DemoString{public static void main(String[] args){String s = null;  System.out.println(String.valueOf(s));//null//s += "abc";s = s + "abc";System.out.println(s);//nullabcSystem.out.println(String.valueOf(s));//nullabc}}