String类compareTo比较日期
int java.lang.String.compareTo(String anotherString) Compares two strings lexicographically（字典序; 按字典顺序;）.
The comparison is based on the Unicode value of each character in the strings.
The character sequence represented by this String object is compared lexicographically to the character sequence represented by the argument string.
The result is a negative integer if this String object lexicographically precedes （在…之前发生）the argument string.
The result is a positive integer if this String object lexicographically follows the argument string.
The result is zero if the strings are equal; compareTo returns 0 exactly when the equals(Object) method would return true.
This is the definition of lexicographic ordering.
If two strings are different, then either they have different characters at some index that is a valid (有效的) index for both strings, or their lengths are different, or both.
(1) 第一种情况
If they have different characters at one or more index positions, let k be the smallest such index;
then the string whose character at position k has the smaller value, as determined by using the < operator, lexicographically precedes the other string.
In this case, compareTo returns the difference of the two character values at position k in the two string -- that is, the value:
this.charAt(k)-anotherString.charAt(k)
（2）第二种情况
If there is no index position at which they differ, then the shorter string lexicographically precedes the longer string.
In this case, compareTo returns the difference of the lengths of the strings -- that is, the value:
this.length()-anotherString.length()
参数： anotherString the String to be compared.
Returns:
the value 0 if the argument string is equal to this string;
a value less than 0 if this string is lexicographically less than the string argument;
and a value greater than 0 if this string is lexicographically greater than the string argument.
数字 转 Unicode编码对照：
0123456789
0123456789
例子：
"2019-05-02".compareTo("2019-05-28") 的返回值为 -2
