Java tutorials for beginners: String class

Strings in Java are treated as a collection of characters. Each character has a position or an index. Now let's see the String class actions..
String class has a bunch of actions to use. The common ones are:
1 - charAt(index): Returns the character at the index. (MAKE SURE THE INDEX IS INSIDE THE BOUNDS OF THE STRING!)
2 - concat(string): Concatentate the current string with the input string. The result is returned as a new String.
3 - equalsIgnoreCase(string): Compares the current string with the input string depending if both have the same characters at the same order. Capital and small letter characters will be treated the same. The action returns a boolean as a result.
4 - replace(oldChar,newChar): Replace every occurence of oldChar with newChar. New string will be returned from this action.
5 - substring(startIndex,endIndex): Returns a substring from the current string starting from startIndex to endIndex-1. If endIndex isn't specified, then the range is startIndex to the end of the string - 1
6 - toLowerCaseLetter() + toUpperCaseLetter(): Returns a new string with all lower/upper cased letters.
7 - trim(): Returns a new string that results from the removing spaces from the start or end of the string.
8 - contains(string): Returns a boolean that specifies if the input string is contained inside the current string.

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