Collections

ArrayList, HashMap and HashSet: the classes you will use most.

Collections grow as needed, unlike arrays. They hold objects, so use Integer rather than int in the angle brackets.

Java

import java.util.*;

public class Main {
  public static void main(String[] args) {
    List<String> skills = new ArrayList<>();
    skills.add("Java");
    skills.add("SQL");
    skills.remove("SQL");
    System.out.println(skills + " " + skills.size());

    Map<String, Integer> counts = new TreeMap<>();
    for (char c : "banana".toCharArray()) {
      counts.merge(String.valueOf(c), 1, Integer::sum);
    }
    System.out.println(counts);

    Set<Integer> unique = new TreeSet<>(List.of(3, 1, 3, 2));
    System.out.println(unique);
  }
}

Output

[Java] 1
{a=3, b=1, n=2}
[1, 2, 3]

Class	Use it when
`ArrayList`	You want an ordered list with fast access by index
`HashMap`	You look values up by key. Order is not guaranteed
`HashSet`	You need unique items and fast contains()
`TreeMap / TreeSet`	You want them kept sorted
`ArrayDeque`	You need a stack or queue

Streams

Java

import java.util.List;

public class Main {
  public static void main(String[] args) {
    List<Integer> nums = List.of(1, 2, 3, 4, 5, 6);
    int total = nums.stream().filter(n -> n % 2 == 0).mapToInt(n -> n * n).sum();
    System.out.println(total);
  }
}

Output

← Exceptions Java quick reference →