String s1 = "abc";
String s2 = "ab";
String s3 = "c";
s1==(s2+s3)结果是false
换一下顺序：
String s1 = "ab";
String s2 = s1 + "c";
String s3 = "abc";
s3 == s2结果为true；
求解

String s1 = "abc";
String s2 = "ab";
String s3 = "c";
s1==(s2+s3)结果是false
换一下顺序：
String s1 = "ab";
String s2 = s1 + "c";
String s3 = "abc";
s3 == s2结果为true；
求解