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ArrayList

Use a resizable list that grows and shrinks as you add and remove items.

Arrays have a fixed size. An ArrayList (from java.util) is a list that grows automatically, which makes it the most common way to store a collection of objects in Java.

Java
Open in playground
import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> cities = new ArrayList<>();
        cities.add("Delhi");
        cities.add("Mumbai");
        cities.add("Chennai");
        cities.add(1, "Jaipur");      // insert at index 1

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

        cities.set(0, "New Delhi");
        cities.remove("Chennai");
        System.out.println(cities);
        System.out.println(cities.contains("Mumbai"));
    }
}
Output
[Delhi, Jaipur, Mumbai, Chennai]
4
Mumbai
[New Delhi, Jaipur, Mumbai]
true

The type in angle brackets, <String>, is a generic type parameter. It tells Java what the list holds, so the compiler can stop you from adding the wrong kind of value.

Lists of numbers

Generics only work with objects, so use the wrapper classes Integer, Double and so on instead of int and double. Java converts between them automatically.

Java
Open in playground
import java.util.ArrayList;
import java.util.Collections;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> scores = new ArrayList<>();
        int[] raw = {72, 95, 60, 88, 95};
        for (int s : raw) scores.add(s);

        Collections.sort(scores);
        System.out.println(scores);
        System.out.println("max: " + Collections.max(scores));

        int total = 0;
        for (int s : scores) total += s;
        System.out.println("average: " + total / scores.size());
    }
}
Output
[60, 72, 88, 95, 95]
max: 95
average: 82

Removing items safely

Removing items while looping with a for-each loop throws an error. Use removeIf instead.

Java
Open in playground
import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<Integer> nums = new ArrayList<>(List.of(1, 2, 3, 4, 5, 6, 7, 8));
        nums.removeIf(n -> n % 2 == 0);
        System.out.println(nums);

        nums.forEach(n -> System.out.print(n * 10 + " "));
        System.out.println();
    }
}
Output
[1, 3, 5, 7]
10 30 50 70

n -> n % 2 == 0 is a lambda expression: a short, unnamed function you pass to a method.