Collections: ArrayList and HashMap

Generics, the List and Map interfaces, and the complexity behind each operation.

12 min read · Lesson 5 of 6 · Pro

Arrays are fixed, collections are not

A Java array has a length fixed at creation. int[] a = new int[5]; is five ints forever. Collections grow.

Java

import java.util.ArrayList;
import java.util.List;

public class Basics {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();

        names.add("Adnan");
        names.add("Priya");
        names.add("Rahul");

        System.out.println(names.size());          // 3
        System.out.println(names.get(1));          // Priya
        System.out.println(names.contains("Rahul"));  // true

        names.remove("Priya");
        names.add(0, "Meera");                     // insert at front

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