A Java string (from now on just called string) holds a sequence of 0 or more characters. If there are n, then the chars in the string are indexed (numbered) from 0 to n-1 inclusive.

A string instance has various methods, including the following:

• indexOf: that returns the index of a given char in the string, or the starting index of a give substring in the string.

• lastIndexOf: similar but returning the last index rather than the first index in the string.

• compareTo: allows strings to be compared using lexicographic (alphabetic) order. If compareTo yields 0 the strings consist of the same chars.

• equals: the same as compareTo returning 0. Not at all the same as ==, since this asks if two strings are exactly the same object (have the same address as their reference). There is also equalsIgnoreCase.

• concat: the same as the string operator +, so that s + t is the same as s.concat(t).

• toUpperCase, toLowerCase: returns a new string converted to the designated case (non-letters are left alone).

• charAt: return the char at the given index.

• trim: returns a new string with leading and trailing whitespace removed.

• valueOf: converts any given primitive type to a string. For any type at all, it is easier to write anytype + "" to convert to a string.

• For any primitive type except boolean, Java provides Type.parseType to convert a string to type.

• substring: Here if s is a string, and startPos and endPos are indexes, then s.substring(startPos, endPos + 1) returns the string that goes from index values startPos to index value endPos inclusive. substring(startPos) returns a string from startPos on to the end.

  Here is a Java function setCharAt with three parameters: a string s, an index i, and a char ch, that replace the char at index iwith the parameter ch, so that setCharAt("abcdef", 1, 'X') returns the string aXcdef. setCharAt method. This static method is inefficient and ought to use the StringBuffer class, but we won't deal with that here.