方法如下：
public String substring(int beginIndex, int endIndex)
第一個int爲開始的索引，對應String數字中的開始位置，
第二個是截止的索引位置，對應String中的結束位置
1、取得的字符串長度爲：endIndex - beginIndex;
2、從beginIndex開始取，到endIndex結束，從0開始數，其中不包括endIndex位置的字符
如：
"hamburger".substring(4, 8) returns "urge"
"smiles".substring(1, 5) returns "mile"
取長度大於等於3的字符串a的後三個子字符串，只需a.subString(a.length()-3, a.length());
手冊中的具體說明如下：
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.