Java ArrayList

In this page:

Introduction to ArrayList
ArrayList Operations
Iterating over an ArrayList
Common ArrayList Methods
ArrayList vs Standard Arrays

Introduction to ArrayList

ArrayList is a resizable array implementation of the List interface — unlike a plain array, it automatically grows as you add elements, so you don't need to know the final size up front.

Example: Introduction to ArrayList

java

import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		ArrayList<String> list = new ArrayList<>(); // grows automatically
		list.add("a");
		list.add("b");
		System.out.println(list);
	}
}

ArrayList Operations

Adding to the end of an ArrayList is fast (amortized O(1)), but inserting or removing from the middle is slower (O(n)) since every following element has to shift over to keep the list contiguous.

Example: ArrayList Operations

java

import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		ArrayList<String> list = new ArrayList<>();
		list.add("a");
		list.add("b");
		list.add("c");
		list.add(1, "inserted"); // slower: shifts elements after index 1
		System.out.println(list);
	}
}

Iterating over an ArrayList

You can iterate an ArrayList with a standard for-each loop, an explicit Iterator, or an index-based for loop when you need the current position — each suits different situations depending on whether you need to modify the list mid-iteration.

Example: Iterating over an ArrayList

java

import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		ArrayList<String> list = new ArrayList<>();
		list.add("a");
		list.add("b");
		for (String item : list) { // for-each
			System.out.println(item);
		}
		for (int i = 0; i < list.size(); i++) { // index-based
			System.out.println(list.get(i));
		}
	}
}

Common ArrayList Methods

Common ArrayList methods include add(), get(index), remove(), contains(), and size() — together they cover nearly everything you'll need for typical list manipulation without writing custom logic.

Example: Common ArrayList Methods

java

import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		ArrayList<String> list = new ArrayList<>();
		list.add("a");
		list.add("b");
		System.out.println(list.get(0));
		System.out.println(list.contains("b"));
		System.out.println(list.size());
		list.remove("a");
		System.out.println(list);
	}
}

ArrayList vs Standard Arrays

ArrayList trades the fixed size and slightly better raw performance of a plain array for dynamic resizing and a much richer built-in API — use a plain array only when the size is truly fixed and performance is critical.

Example: ArrayList vs Standard Arrays

java

import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		int[] fixedArray = new int[3]; // fixed size
		ArrayList<Integer> dynamicList = new ArrayList<>(); // resizes as needed
		dynamicList.add(1);
		dynamicList.add(2);
		System.out.println(fixedArray.length);
		System.out.println(dynamicList.size());
	}
}

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Java ArrayList

Introduction to ArrayList

ArrayList Operations

Iterating over an ArrayList

Common ArrayList Methods

ArrayList vs Standard Arrays

Chapter Quiz — Complete all 17 topics to unlock

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