JAVA- some string manipulation methods

The String class defines a lot of functions. Here are some that you might find useful. Assume that s1 and s2 refer to values of type String:
s1.equals(s2) is a function that returns a boolean value. It returns true if s1 consists of exactly the same sequence of characters as s2, and returns false otherwise.
s1.equalsIgnoreCase(s2) is another boolean-valued function that checks whether s1 is the same string as s2, but this function considers upper and lower case letters to be equivalent. Thus, if s1 is "cat", then s1.equals("Cat") is false, while s1.equalsIgnoreCase("Cat") is true.
s1.length(), as mentioned above, is an integer-valued function that gives the number of characters in s1.
s1.charAt(N), where N is an integer, returns a value of type char. It returns the N-th character in the string. Positions are numbered starting with 0, so s1.charAt(0) is actually the first character, s1.charAt(1) is the second, and so on. The final position is s1.length() - 1. For example, the value of "cat".charAt(1) is 'a'. An error occurs if the value of the parameter is less than zero or greater than s1.length() - 1.
s1.substring(N,M), where N and M are integers, returns a value of type String. The returned value consists of the characters in s1 in positions N, N+1,..., M-1. Note that the character in position M is not included. The returned value is called a substring of s1.
s1.indexOf(s2) returns an integer. If s2 occurs as a substring of s1, then the returned value is the starting position of that substring. Otherwise, the returned value is -1. You can also use s1.indexOf(ch) to search for a particular character, ch, in s1. To find the first occurrence of x at or after position N, you can use s1.indexOf(x,N).
s1.compareTo(s2) is an integer-valued function that compares the two strings. If the strings are equal, the value returned is zero. If s1 is less than s2, the value returned is a number less than zero, and if s1 is greater than s2, the value returned is some number greater than zero. (If both of the strings consist entirely of lower case letters, then "less than" and "greater than" refer to alphabetical order. Otherwise, the ordering is more complicated.)
s1.toUpperCase() is a String-valued function that returns a new string that is equal to s1, except that any lower case letters in s1 have been converted to upper case. For example, "Cat".toUpperCase() is the string "CAT". There is also a function s1.toLowerCase().
s1.trim() is a String-valued function that returns a new string that is equal to s1 except that any non-printing characters such as spaces and tabs have been trimmed from the beginning and from the end of the string. Thus, if s1 has the value "fred ", then s1.trim() is the string "fred".