为什么为字符串声明final返回TRUE

Question

   final String s1 = "a";
   final String s2 = "b";
   String s3 = "ab";
   System.out.println(s1 + s2 == s3);

输出：是

  String s1 = "a";
  String s2 = "b";
  String s3 = "ab";
  System.out.println(s1 + s2 == s3);

输出：错误

为什么要声明最终返回结果为真