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Concept

ArrayLists

Using ArrayList as a resizable alternative to arrays, common methods, and looping with forEach.

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We know we can use an array to store a bunch of values, but perhaps the biggest issue with arrays is that we cannot resize them, unless we create a copy of the array and change the length of that copy. Because of this, a type called ArrayList exists — it works like a dynamic array.

To use an ArrayList, import it from java.util.ArrayList. To declare one, use the ArrayList keyword followed by a pair of angle brackets containing the type of data to be stored (typed in full, since it must be a reference type rather than a primitive), then the name of the list, then assign it to a new object of the ArrayList class.

import java.util.ArrayList;
import java.util.Comparator; // we use this for the sorting


class Main {
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<Integer>();


        numbers.add(1); // adding a value to an arraylist
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);


        numbers.remove(4); // removes the element at index 4
        numbers.remove(Integer.valueOf(4)); // removes the element with value 4 from the arraylist


        numbers.clear(); // clears everything, making it an empty arraylist


        numbers.indexOf(1); // gets the index of a value in the arraylist


        numbers.set(1, 30); // updates the element at index one with 30


        numbers.sort(Comparator.naturalOrder()); // sorts the arraylist in ascending order
        numbers.sort(Comparator.reverseOrder()); // sorts the arraylist in descending order


        System.out.println(numbers.size()); // prints the size of the arraylist


        System.out.println(numbers.contains(1)); // true if the arraylist contains the integer 1


        System.out.println(numbers.isEmpty()); // true if the arraylist is empty


        System.out.println(numbers.toString()); // convert it to string to print it out
        System.out.println(numbers.get(2)); // prints the element at index 2
    }
}

ArrayLists are dynamic arrays, so you can add as many elements as you want.

For-each loop

class Main {
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<Integer>();


        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);


        numbers.forEach(number -> {
            System.out.println(number * 2);
        });
        System.out.println(numbers.toString());
    }
}

For each number in the numbers arraylist, we double it and print it out.