ArrayList

The ArrayList in Java is a linear collection of same-typed elements. It handles allocations on its own, so we do not need to resize it to add more elements.

For strings, the ArrayList can be used with straightforward style. But for ints, some complexity is introduced. To use ArrayList, we first initialize it with data.

Initial example

When we create an ArrayList, we can omit the element type, and use var to reference the ArrayList. Sometimes it is better to specify the types however.

import java.util.ArrayList;

public class Program {
    public static void main(String[] args) {

        var colors = new ArrayList<>();
        colors.add("?");
        colors.add("red");
        colors.add("bird");
        colors.add("blue");

        // Part 2: remove first element.
        colors.remove(0);
        System.out.println(colors);

        // Part 3: remove element with value of bird.
        colors.remove("bird");
        System.out.println(colors);
    }
}
[red, bird, blue]
[red, blue]

Set element

To assign to a position, we use set. The index must be valid. The ArrayList must already contain a reference at the index. When set() succeeds, the new element is stored.

import java.util.ArrayList;

public class Program {
    public static void main(String[] args) {

        // Create an ArrayList.
        ArrayList<String> list = new ArrayList<>();
        list.add("Venus"); // [0]
        list.add("Mars");  // [1]
        list.add("Earth"); // [2]

        // Set index 0 to a new String.
        list.set(0, "Saturn");

        for (String value : list) {
            System.out.println(value);
        }
    }
}
Saturn
Mars
Earth

Method argument

An ArrayList can be passed as an argument to another method. This modifies an ArrayList in many places without copying. We can share one ArrayList among many methods.

import java.util.ArrayList;

public class Program {

    static void addCats(ArrayList<String> list) {
        list.add("Fluffy");
        list.add("Max");
    }

    public static void main(String[] args) {

        ArrayList<String> list = new ArrayList<>();

        // Call method and pass ArrayList as argument.
        addCats(list);

        for (String value : list) {
            System.out.println(value);
        }
    }
}
Fluffy
Max

Custom class elements

An ArrayList can use built-in types like Strings or Integers. But we can also place user-defined classes in one. Here I create a Philosopher class and use it.

import java.util.ArrayList;

class Philosopher {
    public int value;
    public String name;

    public Philosopher(int value, String name) {
        this.value = value;
        this.name = name;
    }

    public String toString() {
        return "value = " + this.value + ", name = " + this.name;
    }
}

public class Program {
    public static void main(String[] args) {

        // Create an ArrayList of objects.
        ArrayList<Philosopher> list = new ArrayList<>();
        list.add(new Philosopher(1, "Socrates"));
        list.add(new Philosopher(2, "Plato"));

        // Display our objects.
        for (Philosopher p : list) {
            System.out.println(p);
        }
    }
}
value = 1, name = Socrates
value = 2, name = Plato

Collections.sort

We can sort an ArrayList with this method. We first import java.util.Collections into our program. This method sorts in ascending (low to high) order.

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

public class Program {
    public static void main(String[] args) {

        ArrayList<String> list = new ArrayList<>();
        list.add("cat");
        list.add("bird");
        list.add("ant");
        list.add("dog");

        // Sort the elements alphabetically.
        Collections.sort(list);

        for (String value : list) {
            System.out.println(value);
        }
    }
}
ant
bird
cat
dog

RetainAll

Suppose you have an ArrayList, and want to keep (or retain) only certain elements in it. With retainAll we can specify a list of elements to keep—all the rest are removed.

import java.util.ArrayList;

public class Program {

    public static void main(String[] args) {
        ArrayList<String> values = new ArrayList<>();
        values.add("bird");
        values.add("bird");
        values.add("frog");
        values.add("fish");
        values.add("elephant");
        values.add("elephant");
        System.out.println(":::VALUES::: " + values);

        // Add elements we want to keep here.
        ArrayList<String> retains = new ArrayList<>();
        retains.add("frog");
        retains.add("elephant");
        System.out.println(":::RETAINS::: " + retains);

        // Remove all elements not in the "retains" collection.
        values.retainAll(retains);
        System.out.println(":::AFTER RETAINALL::: " + values);
    }
}
:::VALUES::: [bird, bird, frog, fish, elephant, elephant]
:::RETAINS::: [frog, elephant]
:::AFTER RETAINALL::: [frog, elephant, elephant]

Linear collections are commonly needed in Java programs. Programs often store elements in ArrayList instances. And this type provides diverse, powerful methods.