Java

TOPIC 47 – ARRAYLISTS

LESSON NOTE

JAVA ARRAYS = STATIC SIZE

Arrays in java have a static size. That means that they cannot be resized.

If you ever had to remove an element from an array, it would require you to create an entirely new array and copy the content of one array to the other. Similarly, adding an element to an array would also require that you create a new array and copy the content from the original array to the new one. Doing this is quite tedious and can be very inefficient.

ARRAYLISTS

Luckily, in Java we can use ArrayLists. They are essentially arrays that can be resized. It is important to note that an ArrayList object is really built on top of a normal static array but that the work required to change its size is hidden away from us.

For now, we will not worry about the efficiency of using ArrayLists. Such details will be discussed in grade 12.

We will simply focus on how we can use them.

CREATING AN ARRAYLIST

Before creating an ArrayList, we need the import statement:

import java.util.ArrayList;

Creating an ArrayList requires that we specify the type of data that we want to store in the structure. This is done using generics – something we will learn about next year.

For generics, instead of using int, we use Integer and instead of using double, we use Double.

Here is how we create an integer ArrayList:

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

Here is how we create a double ArrayList:

ArrayList<Double> ald = new ArrayList<Double>();

Here is how we create a Point ArrayList:

ArrayList<Point> points = new ArrayList<Point>();

INITIAL SIZE

When an ArrayList is created, it contains no elements. Therefore, its initial size is zero.

ADDING ELEMENTS

We can add elements by using the add method.

EXAMPLE

Create an integer ArrayList called numbers and insert the numbers 3, 5 and 8 in it.

SOLUTION

ArrayList<Integer> numbers = new ArrayList<Integer>();
numbers.add(3);

numbers.add(5);

numbers.add(8);

OUTPUTTING THE CONTENT TO SCREEN

We can simply output the content of an ArrayList by including it in a sysout statement. This makes use of the ArrayList’s toString() method.

EXAMPLE

Create an integer ArrayList, add a couple of values in it and output it to screen.

SOLUTION

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

al.add(4);

al.add(6);

System.out.println(al);

The above code will output the following:

[4, 6]

ADDING AN ELEMENT AT A SPECIFIC LOCATION

We can add an element at a specific location using the add method and specifying the index number as the first argument.

EXAMPLE

Demonstrate how one can insert an element at a specific location in an ArrayList.

SOLUTION

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

al.add(5);

al.add(8);

al.add(1, 3); //add the value 3 at index 1

System.out.println(al);

ARRAYLIST’S SIZE

We can get the size of an ArrayList using the size method.

EXAMPLE

Create an integer ArrayList called n, add a few elements and then output the size to screen.

SOLUTION

ArrayList<Integer> n = new ArrayList<Integer>();
n.add(3);
n.add(12);
n.add(13);
n.add(45);
n.add(-3);
System.out.println(n.size());

REMOVING AN ELEMENT

We can remove an element based on its index number by using the remove method.

EXAMPLE CODE

The following code will create an ArrayList, add some elements and then remove element 1 from the list.

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

al.add(5);

al.add(8);

al.add(3);

al.add(2);

System.out.println(al);

al.remove(1);

System.out.println(al);

The code above will output the following:

[5, 8, 3, 2]

[5, 3, 2]

GETTING THE VALUE OF AN ELEMENT

We can get the value of an element with the get method.

EXAMPLE CODE

The following code will display the content of element 0 onto the screen.

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

al.add(2);

al.add(4);

System.out.println(al.get(0));

CLEARING THE ARRAYLIST

Given an ArrayList name al, we can remove all the elements from al by using:

al.clear();