JAVA问题

创建一个程序，它将MM/DD/YYYY格式 (例如4/23/2000) 这样的生日转换为3个单独的字符串。
class Birthday {
public static void main(String[] arguments) {
String birthday = "08/23/2002";
String month = birthday.substring(0, 2);
String day = birthday.substring(3, 5);
String year = birthday.substring(6, 10);
System.out.println("Birthday: " + birthday);
System.out.println("Month: " + month);
System.out.println("Day: " + day);
System.out.println("Year: " + year); }
}
为什么这个要这样写 birthday.substring(0, 2);
birthday.substring(3, 5);
birthday.substring(6, 10);

请具体解释一下 最好在具例说明！ 非常谢谢！

[code="java"] /**
* 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"

     * 

*
* @param beginIndex the beginning index, inclusive.
* @param endIndex the ending index, exclusive.
* @return the specified substring.
* @exception IndexOutOfBoundsException if the
* beginIndex is negative, or
* endIndex is larger than the length of
* this String object, or
* beginIndex is larger than
* endIndex.
*/
public String substring(int beginIndex, int endIndex) {[/code]