Java String+String

    public static void main(String[] args) throws Exception {

        String s1 = "YES";
        String s2 = "";
        String s3 = null;

        String all = s1 + s2 + s3;
        System.out.println(all);

        //String sss = s1.concat(s2).concat(s3);//报异常:java.lang.NullPointerException
        //System.out.println(sss);
    }

YESnull