Below are the few necessary methods I often look for:

Sub Array

Arrays.copyOfRange(int[] original, int from, int to)

from must be within the original.length. to can be greater than or euqal to from.

Substring

string.subtring(int beginIndex, int endIndex)

beginIndex is inclusive of the substring and endIndex is exclusive of the substring.

SubList

list.subList(int beginIndex, int endIndex); endIndex exclusive.

Some usefull Collections method

Collections.sort(list, customComparator); Collections.reverse(list) it will reverse the list

Implement a custom comparator

    Comparator<String> comp = new Comparator<String>(){
        @Override
        public int compare(String a, String b){
            return b.compareTo(a);
        }
    };

Things to remember while implementing a custom comparator

Given two data x and y => 0: if (x==y) => -1: if (x < y) => 1: if (x > y)

Is letter?

Character.isLetter(‘a’)

string.split() function can take second parameter

String[] word = "function can take second parameter".split(" ",2); word will have ["function","can take second parameter"]

string.trim() will remove empty space from corner