Standard arrays are useful tools in the Java programmer's toolbox. There are times when we need something a little more dynamic, an array that we can add and/or subtract from, or even grow or shrink as the need arises. Thankfully, Java provides the ArrayList, which is a class for us to use when we need to do that.
Before we can use ArrayList we have to import the Java utility, as you can see below.
import java.util.ArrayList;
In order to create a new array list, you create an instance of the ArrayList class, telling Java the type of data in the array and the size of the array. The array can be String, Integer, Double, etc.
Below is the code to declare an instance of the ArrayList class and call it employees. It's a String array with five items, as you can see:
ArrayList <String> employees = new ArrayList <String>(5);
Now we can start using the ArrayList class and its add method to insert items into the array. At first, we're going to create the list using the add method. Later, we will actually tell Java what to insert into the array, and at what position.
The code to populate our array is below, as you can see, with names of famous fictional characters:
employees.add("Jane Eyre");
employees.add("Sherlock Holmes");
employees.add("Edmond Dantes");
employees.add("Jean Valjean");
employees.add("Sam Spade");
The next lines of code use the size method of ArrayList to display the number of items. Next we loop through the array and display each item. You'll notice that this for loop is a little different than others. Because we have an ArrayList, we can take a shortcut on our loop. We only need to specify the data type of the ArrayList (String), give a counter variable name (counter), and the name of the ArrayList (employees).
The add method of ArrayList can take up to two parameters. The first is the index where you are inserting. The second is the value of the item being inserted. When you add an item to the ArrayList it shifts everything over.
This highlights the beauty of the ArrayList versus a standard array. Not only can we insert a value into a specific point of the array, it actually increases the size of the array.
//show arraylist size
System.out.println("Size = " + employees.size());
//loop through list and display
for (String counter : employees) {
System.out.println("Employee = " + counter);
}
When run, the output is as follows, in which our famous characters are listed as employees:
If you don't specify an index, Java simply appends the item at the end of the list. It then increases the size by 1.
