Java ArrayList

A Java ArrayList is a resizable list from the java.util package. It stores values in order, lets you access them by index, and can grow or shrink while the program runs.

Use an ArrayList when you want array-like indexing but do not know the final size of the list ahead of time.

Create An ArrayList

Import java.util.ArrayList, then choose the type of objects the list will store. The type goes inside angle brackets, such as ArrayList<String>.

import java.util.ArrayList;

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

        languages.add("Java");
        languages.add("Python");
        languages.add("C++");

        System.out.println(languages);
        System.out.println("First: " + languages.get(0));
        System.out.println("Size: " + languages.size());
    }
}

Output:

[Java, Python, C++]
First: Java
Size: 3

The add() method appends items to the end of the list. The get() method reads an item by index, and size() returns the number of items. Like arrays, ArrayList indexes start at 0.

Change And Remove Items

Use set(index, value) to replace an existing item. Use remove(index) to remove the item at a position, or remove(value) to remove the first matching value.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> tasks = new ArrayList<>();
        tasks.add("Read");
        tasks.add("Practice");
        tasks.add("Review");

        tasks.set(1, "Code");
        tasks.remove("Review");

        System.out.println(tasks);
        System.out.println(tasks.contains("Code"));
    }
}

Output:

[Read, Code]
true

After set(1, "Code"), the second item changes from Practice to Code. After remove("Review"), the list has two items left. The contains() method checks whether a value is in the list.

Loop Through An ArrayList

A for-each loop is a clean way to read every item. Use a regular for loop when you need the index.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> scores = new ArrayList<>();
        scores.add(90);
        scores.add(85);
        scores.add(92);

        int total = 0;
        for (int score : scores) {
            total += score;
        }

        System.out.println("Total: " + total);
        System.out.println("Average: " + (total / scores.size()));
    }
}

Output:

Total: 267
Average: 89

This list stores Integer objects, not primitive int values directly. Java automatically boxes each added int into an Integer and unboxes it when the loop variable uses int.

ArrayList Versus Array

Feature	Array	ArrayList
Size	Fixed after creation	Can grow and shrink
Access by index	Yes	Yes
Primitive values	Can store primitives	Stores objects, such as Integer
Common length check	array.length	list.size()

Common ArrayList Methods

• add(value) adds a value to the end.
• add(index, value) inserts a value at a position.
• get(index) reads a value.
• set(index, value) replaces a value.
• remove(index) removes a value by position.
• size() returns the number of values.
• clear() removes all values.

Common Mistakes

• Forgetting import java.util.ArrayList;.
• Using length instead of size().
• Using an index less than 0 or greater than or equal to size().
• Trying to use primitive generic types such as ArrayList<int> instead of wrapper types such as ArrayList<Integer>.

Takeaway: ArrayList is the standard choice for an ordered list that needs indexed access and a size that can change.