Java Coding Cheatsheet

Formula to make life easier while coding leetcode-styled problems in java.

Convert an Array to a List

Map<Integer, List<Character>> numToChar = new HashMap<>();

1. Using List.of

numToChar.put(1, List.of('a', 'b', 'c')) 

• creates an immutable read only array
• can’t be modified
• can’t be extended
• doesn’t create an hidden arrays, hence efficient
• the exception to the above statement is, it create a hidden array only when the number of elements that are passed as params is greater than 10 as static <E> List<E> of(E e1, E e2, E e3, E e4) { ... } supports till 10 params

2. Using Arrays.asList

numToChar.put(1, Arrays.asList('a', 'b', 'c'))

• creates an fixed sized mutable array
• can’t be extended
• creates a hidden array

3. Using new ArrayList

numToChar.put(1, new ArrayList<>(List.of('a', 'b', 'c'))); 

• creates fully mutable and dynamic list
• it creates a whole new structure and copies the data from it.
• can wrap Arrays.asList() as well.

Create a Array of HashSet

1. Using arrays

Set<String>[] array = (Set<String>[]) new Set[size];
for (int i = 0; i < size; i++) {
    array[i] = new HashSet<String>();
}

• this is fine but there is a need to manually cast it.

2. Using List

List<Set<String>> listOfSets = new ArrayList<>();
for (int i = 0; i < size; i++) {
    listOfSets.add(new HashSet<>());
}

• much more flexible and much cleaner