for example,PhoneNumber@163b91 class name+@+unsigned hexadecimal representation of hashcode.
define: a concise but informative representation that is easy for a person to read
providing a good toString implementation makes your class much more pleasant to use.
when practical,the toString method should return all of the interesting information contained in the object
whether or not you decide to specify the format,you should clearly document your intentions.
for example：
provide programmatic access to all of the information contained in the value returned by toString.
总体感觉：需要利用toString 来描述对象时候才重载，感觉应用机会比较少