Built-in methods

Sort

/***** Primitive *****/
import java.util.Arrays;

int[] nums
Arrays.sort(nums); // Ascending - No buit-in


/***** List *****/
import java.util.Collections;
import java.util.Comparator;

List<Integer> nums;
Collections.sort(nums); // Ascending
Collections.sort(nums, Comparator.reverse()) // Descending

Flatmap

Flattens out the inner list of properties of a list of object. For example if we have a list of User and every user has a list of Address, we can flatten out the addresses:

    static class Address {
        public String addy;

        public Address(String addy) {
            this.addy = addy;
        }
    }

    static class User {
        public String name;
        public List<Address> addresses;
        public User(String name, List<Address> addresses ) {
            this.name = name;
            this.addresses = addresses;
        }
    }
    public static void main(String[] args) {
        User john = new User("john", Arrays.asList(new Address("Arctic"), new Address("Atlantic")));
        User bob = new User("bob", Arrays.asList(new Address("Indian"), new Address("Pacific")));

        List<User> users = Arrays.asList(john, bob);

        List<Address> allAddys = users.stream().flatMap(user -> user.addresses.stream()).toList();
        allAddys.stream().map(a -> a.addy).forEach(System.out::println);
    }

    // Output
    Arctic
    Atlantic
    Indian
    Pacific

lowercase, alphanumeric

    String s = "Hello";
    String lower = s.toLowerCase();
    String upper = s.toUpperCase();


    Character c = 'c';
    Character lower = Character.toLowerCase(c);
    Character upper = Character.toUpperCase(c);
    Character.isLetter(c);
    Character.isDigit(c);
    Character.isLetterOrDigit(c);

Stack, Queue

• Stack: push peak top
• Queue: offer peak poll

    Stack<Integer> s = new Stack<>();
    s.push(1); s.push(2);

    // Peek top
    int top = s.peek();

    // Remove top
    top = s.pop();

    Queue<Integer> q = new LinkedList<>();
    q.offer(1); q.offer(2);

    // peak head
    int head = q.peek();

    // remove head
    head = q.poll();