ArrayList

Arrays are fast, but they have one major limitation: their size is fixed the moment they're created. If you don't know ahead of time how many elements you'll need, for example, how many game pieces get scored during a match, an array isn't flexible enough. This is where ArrayList comes in.

An ArrayList is a resizable array, provided by Java as part of the java.util package. It automatically grows and shrinks as elements are added and removed.

import java.util.ArrayList;

For more information about imports and packages, see the Object-Oriented Programming section.

Creating an ArrayList

ArrayList Formula

The general formula for creating an ArrayList is:

ArrayList<[wrapper type]> [list name] = new ArrayList<[wrapper type]>();

ArrayList<Integer> scoredPieces = new ArrayList<Integer>();

Generics and Wrapper Types

The <Integer> part is called a generic type parameter, and it tells Java what type of object the ArrayList will hold. Notice that this is Integer, not int. ArrayList (and most of Java's built-in collections) can only hold objects, not primitives, so primitive types are automatically converted to their wrapper type equivalent, a process called autoboxing.

Primitive	Wrapper Type
int	Integer
double	Double
boolean	Boolean
char	Character

Generics are covered in much more detail in the Advanced Concepts section, but for now, just know that the type inside the angle brackets tells Java what the list is allowed to hold.

Common Methods

Method	Description
add(value)	Adds a value to the end of the list
get(index)	Returns the value at the given index
set(index, value)	Replaces the value at the given index
remove(index)	Removes the value at the given index
size()	Returns the number of elements in the list
contains(value)	Returns true if the list contains the value

Show code

ArrayList<Integer> scoredPieces = new ArrayList<Integer>();

scoredPieces.add(1);
scoredPieces.add(2);
scoredPieces.add(3);

System.out.println(scoredPieces.get(0)); // Prints 1
System.out.println(scoredPieces.size()); // Prints 3

scoredPieces.remove(0);

System.out.println(scoredPieces.size()); // Prints 2

Unlike arrays, trying to get() or set() an index that's out of bounds still throws an exception, ArrayList just prevents that boundary from being fixed in place.

ArrayList vs. Array

	Array	ArrayList
Size	Fixed	Resizable
Holds	Primitives or objects	Objects only (via autoboxing)
Access syntax	arr[i]	list.get(i)

If you know exactly how many elements you need ahead of time and that number won't change, an array is simpler and slightly more efficient. If the number of elements changes while the program runs, ArrayList is almost always the better choice.