String类题目总结

用到的Class和methods：

1. 类String

String s;

s.charAt(i);

2. 类Character: http://docs.oracle.com/javase/7/docs/api/java/lang/Character.html

isLetterOrDigit(char ch)

Determines if the specified character is a letter or digit.

Return static boolean

toLowerCase(char ch)

Converts the character argument to lowercase using case mapping information from the UnicodeData file.

Return static char

3. 类StringBuilder：http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html

append(char c)

Appends the string representation of the char argument to this sequence.

return StringBuilder

toString()

Returns a string representing the data in this sequence.

return String