indexOf()和charAt()

indexOf(string a)：查找字符串出现的第一个位置。假如字符串里有多个该字符，只显示第一个位置。没出现则返回-1.
int java.lang.String.indexOf(String str)
Returns the index within this string of the first occurrence of the specified substring. The integer returned is the smallest value k such that:

this.startsWith(str, k)

is true.
Parameters:
str any string.
Returns:
if the string argument occurs as a substring within this object, then the index of the first character of the first such substring is returned; if it does not occur as a substring, -1 is returned.



charAt(index)：查找该索引指向的字符，返回值是char。
Returns the char value at the specified index. An index ranges from 0 to length() - 1. The first char value of the sequence is at index 0, the next at index 1, and so on, as for array indexing.

If the char value specified by the index is a surrogate, the surrogate value is returned.

Specified by: charAt(...) in CharSequence
Parameters:
index the index of the char value.
Returns:
the char value at the specified index of this string. The first char value is at index 0.
Throws:
IndexOutOfBoundsException - if the index argument is negative or not less than the length of this string.