String charAt() Method
Java String charAt() returns the character located at the specified index in String. The string indexes start from zero and ranges from 0 to length() - 1.
It is important to note that, it will thrown an IndexOutOfBoundsException if the index is less than zero or greater than equal to the length of the string (index<0 index>=length()).
String Change to Uppercase
Java String toUpperCase() converting the string to uppercase without needing to assign the result to a new variable.
String Change to Lowecase
Java String toLowerCase() converting the string to lowercase without needing to assign the result to a new variable.