public String substring(int beginIndex, int endIndex)
第一个int为开始的索引，对应String数字中的开始位置，
第二个是截止的索引位置，对应String中的结束位置
1、取得的字符串长度为：endIndex - beginIndex;
2、从beginIndex开始取，到endIndex结束，从0开始数，其中不包括endIndex位置的字符
如：
"androidstudio".substring(4, 8) returns "oids"
"remerber".substring(1, 6) returns "emerb"
取长度大于等于5的字符串a的后三个子字符串，只需a.subString(a.length()-5, a.length());
手册中具体说明如下：
substring
public String substring(int beginIndex, int endIndex)
Returns a new string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex - 1. Thus the length of the substring is endIndex-beginIndex.
Examples:
"hamburger".substring(4, 8) returns "urge"
"smiles".substring(1, 5) returns "mile"
Parameters:
beginIndex - the beginning index, inclusive.
endIndex - the ending index, exclusive.
Returns:
the specified substring.
Throws:
IndexOutOfBoundsException - if the beginIndex is negative, or endIndex is larger than the length of this String object, or beginIndex is larger than endIndex.
