String Methods

1)charAt 2)str.toCharArray() 3) s.indexOf(c) 4)s.lastIndexOf(c)

Ascii

Ascii has 255 Characters

1) Any Ascii character has its integer value from 0-255

Java Arrays

Type[] name = new Type[Size];

Example : boolean[] charSet = new boolean[256];

Note : 

1)Default value of Boolean array element is False

2) Default value of Int array element is 0

Java Random numbers

Use
Import java.util.Random;


1)To fetch a random number from 0 to that number

Random().nextInt(int bound) generates a random integer from 0 (inclusive) to bound (exclusive).

2)To fetch a random number in a Range

private static int getRandomNumberInRange(int min, int max) {
 Random r = new Random();
 return r.nextInt((max - min) + 1) + min;
}

3)To fetch Random objects from an array
String []guess={"Red","Blue","Green"};
char[] randomguess=guess[random.nextInt(guess.length)].toCharArray();


Source : https://www.mkyong.com/java/java-generate-random-integers-in-a-range/