ArrayList

An ArrayList in Java is a dynamic array that can hold elements of any data type.

Unlike a static array in Java, an ArrayList can be resized dynamically. This means that we can add and remove values from an ArrayList.

An ArrayList is a class in the Java Collection framework that is part of the java.util package

Java ArrayList Requirements

1. Import ArrayList from java.util

import java.util.ArrayList; // import the ArrayList class

2. Create an ArrayList object

// Some code

import java.util.ArrayList; // import the ArrayList class

class Main {
    public static void main(String[] args) {
        ArrayList<String> cars = new ArrayList<String>();
        // Initializing the new arraylist about cars
    }
}

The data type of the ArrayList's items must be OBJECTS. Moreover, they should be Generic/Wrapper Class not primitive data types.

Primitive Data Type 👎	Wrapper Class Equivalent 👍
int	Integer
double	Double
boolean	Boolean
does not exit	String

In Java, whenever we need data to be “objects” we have a Wrapper class to represent built-in data to be “objects”.

Common ArrayList Methods

1. Add an item to an ArrayList

// Add an item to an ArrayList

ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(3); nums.add(1); nums.add(4);
// nums is now composed of 3, 1, 4

We can use → .add() to add a value to an ArrayList to the END OF THE ARRAYLIST.

What happens if you want to add a value at a certain index?

// Add an item to an ArrayList at a certain index
ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(3); nums.add(1); nums.add(4);

// nums is currrently: [3, 1, 4]
nums.add(3,1); // Adds 1 to the end of the list even though index is out of bounds; 
// The provided index can only be from [0, current length of arraylist] 
       
nums.add(0, 0); // Adds 0 to the front of the list, shifts everything back 
nums.add(2, 99); // Adds 99 at index 2; shifts everything from 2 onwards back by one.
        
//nums.add(100, 5); // Creates an error since the index is greater than the length.
System.out.println(nums); // Outputs: [0, 3, 99, 1, 4, 1]

.add(int index, value) can be also be used to add values to your ArrayList at certain locations following the rules/concepts below:

Rule A: Valid Index Range

The index provided must be within the range of 0 to the size of the ArrayList (inclusive). If the index is outside this range, it will throw an IndexOutOfBoundsException.

Rule B:. Shift Existing Elements

When you add an element at a specific index, all elements at that index or greater are shifted one position to the right.

Rule C. Capacity Check and Resize

If adding an element exceeds the current capacity of the ArrayList, it will automatically increase its capacity to accommodate the new element. This involves creating a new array with a larger capacity, copying the existing elements into the new array, and then adding the new element.

More reading on ArrayList Sizing

2. Grab a value from an ArrayList

// Grabbing a Value

ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(3); nums.add(1); nums.add(4);

System.out.println(nums.get(0)); // prints 3
System.out.println(nums.get(2)); // prints 4

We can use → .get(INDEX) to grab value at a positive integer index value. (Index starts at zero always)

You can get an error if the index does not exist.

3. Change a value at an index

// Change a value at a index

ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(3); nums.add(1); nums.add(4);

nums.set(0, 9);
// index of 0 has a value of 9 now

We can use → .set(INDEX, NEWVALUE) to change a value at a certain location.

4. Remove a value at an index

// Remove a Value
ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(3); nums.add(1); nums.add(4);

nums.remove(2);
// nums ArrayLists now is only [3,1]

We can use → .remove(INDEX) to remove a value at a certain location.

5. Get the size of the ArrayList

// Get Size of the ArrayList
ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(3); nums.add(1); nums.add(4);

System.out.println(nums.size()); // outputs: 3

We can use → .size() to determine the number of elements in an ArrayList.

6. Check if a value exists in an ArrayList

// Does a value exist in an arraylist?
ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(3); nums.add(1); nums.add(4);

System.out.println(nums.contains(1)); // outputs: true; 1 exists in nums
System.out.println(nums.contains(10)); // outputs: false; 10 does not exist in nums

We can use → .contains() to determine if a value exists in an ArrayList.

7. Determine the index of a value in an ArrayList

// Does a value exist in an arraylist?
ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(3); nums.add(1); nums.add(4); nums.add(1); nums.add(5); nums.add(9);

System.out.println(nums.indexOf(4)); // Returns 2 (4 is at index 2)
System.out.println(nums.indexOf(1)); // Returns 1 (The first location of 1)
System.out.println(nums.indexOf(10)); // Returns -1 (10 DNE)
System.out.println(nums.lastIndexOf(1)); // Returns 3 (Last occurrence of 1)

We can use → .indexOf() to determine an index of a value in an ArrayList. Returns -1 if not found.

We can use → .lastIndexOf() to determine an index of a value in an ArrayList from the right. Returns -1 if not found.

ArrayLists are printable!

// Example

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

cars.add("Honda");
cars.add("Toyota");
cars.add("Mercedes");
cars.add("Ford");

System.out.println("Our car brands: " + cars);

Output:

Our car brands: [Honda, Toyota, Mercedes, Ford]

As long as the ArrayList contains primitive equivalent wrapper class items, we can simply print the ArrayList within System.out.println().