Java ArrayList

An array has a fixed length: once created, it cannot grow. Most programs, though, do not know in advance how many items they will store: the products in a cart, the names typed by a user, the lines of a file. An ArrayList is a list that grows and shrinks as you add and remove items. It is the collection Java programmers use most.

Creating a list and adding items

ArrayList is in java.util. In angle brackets you write the type of the items; this is called a generic type, explained in Generics. The type must be a class, so for numbers you write Integer, the wrapper class:

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> cities = new ArrayList<>();
        cities.add("Chennai");
        cities.add("Jaipur");
        cities.add("Surat");
        System.out.println(cities);
        System.out.println(cities.size());
        System.out.println(cities.get(1));
    }
}

[Chennai, Jaipur, Surat]
3
Jaipur

add puts an item at the end, size() gives the number of items, and get(i) reads the item at index i, starting at 0 like an array. Printing a list shows all its items, with no need for Arrays.toString.

Changing and removing items

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> tasks = new ArrayList<>();
        tasks.add("shop");
        tasks.add("cook");
        tasks.add("study");
        tasks.add(1, "clean");
        System.out.println(tasks);
        tasks.set(0, "buy milk");
        tasks.remove("cook");
        tasks.remove(0);
        System.out.println(tasks);
        System.out.println(tasks.contains("study"));
        System.out.println(tasks.indexOf("study"));
        tasks.clear();
        System.out.println(tasks.isEmpty());
    }
}

[shop, clean, cook, study]
[clean, study]
true
1
true

add(index, item) inserts at a position and moves the rest along. remove works with an item or with an index. Note that for an ArrayList<Integer>, remove(1) removes the element at index 1; to remove the number 1 write remove(Integer.valueOf(1)).

Looping over a list

The for-each loop works exactly as with arrays:

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> marks = new ArrayList<>();
        marks.add(72);
        marks.add(88);
        marks.add(65);
        int total = 0;
        for (int m : marks) {
            total += m;
        }
        System.out.println("Average: " + (double) total / marks.size());
    }
}

Average: 75.0

Do not remove items inside a for-each loop over the same list: Java stops with a ConcurrentModificationException. The Iterator topic shows the safe way, and marks.removeIf(m -> m < 70) does it in one line using a lambda.

Sorting and quick lists

Collections.sort sorts a list, and List.of creates a small fixed list in one line. A fixed list cannot be changed, but you can copy it into an ArrayList:

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

public class Main {
    public static void main(String[] args) {
        List<String> fixed = List.of("pear", "apple", "fig");
        ArrayList<String> fruits = new ArrayList<>(fixed);
        fruits.add("banana");
        Collections.sort(fruits);
        System.out.println(fruits);
        Collections.reverse(fruits);
        System.out.println(fruits);
        System.out.println(Collections.max(fruits));
    }
}

[apple, banana, fig, pear]
[pear, fig, banana, apple]
pear

The variable type is often written as the interface List (List<String> list = new ArrayList<>();): the code then works with any kind of list, such as the LinkedList of the next topic.

Practice exercises

1. Read five names from the user into an ArrayList and print them in alphabetical order.
2. Remove all names shorter than four letters from a list with removeIf.
3. Store some prices in an ArrayList<Double> and print the most expensive one with Collections.max.