Java ArrayList

An ArrayList is a list which can grow and shrink, unlike an array which has a fixed size. It is part of the Collections framework in java.util.

Create and use

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();   // the type goes in <>

        names.add("Riya");
        names.add("Aman");
        names.add("Meera");
        names.add(1, "Karan");          // add at a position

        System.out.println(names);
        System.out.println(names.get(2));
        System.out.println(names.size());

        names.set(0, "Riya S");         // change an item
        names.remove("Aman");           // remove by value
        names.remove(0);                // remove by index
        System.out.println(names);
        System.out.println(names.contains("Meera"));
        System.out.println(names.indexOf("Meera"));
        System.out.println(names.isEmpty());
    }
}

Loop through a list

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

public class Loop {
    public static void main(String[] args) {
        List<Integer> marks = new ArrayList<>();
        marks.add(70);
        marks.add(85);
        marks.add(90);

        for (int m : marks) {
            System.out.print(m + " ");
        }
        System.out.println();

        for (int i = 0; i < marks.size(); i++) {
            System.out.println(i + " -> " + marks.get(i));
        }

        marks.forEach(m -> System.out.print(m * 2 + " "));
        System.out.println();
    }
}

We wrote List<Integer> marks = new ArrayList<>(). It is a good habit to use the interface (List) as the variable type.

Only objects, not primitives

A list can not hold int, only Integer. Java converts between them automatically (autoboxing).

Sort and other helpers

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class Sort {
    public static void main(String[] args) {
        List<String> fruits = new ArrayList<>(List.of("mango", "apple", "banana", "cherry"));

        Collections.sort(fruits);
        System.out.println(fruits);
        Collections.reverse(fruits);
        System.out.println(fruits);

        fruits.sort(Comparator.comparing(String::length).thenComparing(Comparator.naturalOrder()));
        System.out.println(fruits);

        System.out.println(Collections.max(fruits));
        System.out.println(fruits.subList(1, 3));
    }
}

A common beginner error is to use Comparator without the import. Java tells exactly what is missing:

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

public class Sort2 {
    public static void main(String[] args) {
        List<String> fruits = new ArrayList<>(List.of("b", "a"));
        fruits.sort(Comparator.naturalOrder());
    }
}

Remove while looping

Removing items inside a for each loop throws a ConcurrentModificationException. Use removeIf:

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

public class Remove {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>(List.of(1, 2, 3, 4, 5, 6));
        numbers.removeIf(n -> n % 2 == 0);
        System.out.println(numbers);

        List<Integer> copy = new ArrayList<>(List.of(1, 2, 3, 4));
        try {
            for (Integer n : copy) {
                if (n == 1) copy.remove(n);        // removing inside the loop
            }
        } catch (java.util.ConcurrentModificationException e) {
            System.out.println("ConcurrentModificationException");
        }
    }
}

Immutable lists

import java.util.List;

public class Immutable {
    public static void main(String[] args) {
        List<String> fixed = List.of("a", "b", "c");
        System.out.println(fixed);
        fixed.add("d");
    }
}

ArrayList or array?

Point	Array	ArrayList
Size	Fixed	Grows and shrinks
Types	Primitives and objects	Objects only
Speed	A little faster	Very fast, with more helper methods

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