In this example we are going to learn how to obtain the size of an ArrayList object. As you might already know, a java.util.ArrayList is a class that we can use to create a dynamic sized array. We can add and remove elements from the ArrayList dynamically.

Because its elements can be added or removed at anytime, we might want to know the number of elements currently stored in this list. To obtain the size we can use the size() method. This method returns an int value that tells us the number of elements stored in the ArrayList object.

When the list contains more than Integer.MAX_VALUE elements this method returns Integer.MAX_VALUE.

package org.kodejava.util;

import java.util.ArrayList;
import java.util.List;

public class ArrayListSize {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("Item 1");
        list.add("Item 2");
        list.add("Item 3");

        int size = list.size();
        System.out.println("List size = " + size);
    }
}

In the code snippet above we start creating an instance of ArrayList that can holds String values. As a good practice we should always use the interface as the type of the declared object, in this case we use the List interface. The <> is a diamond operator, started from JDK 7 you can use this operator so that you don’t need to repeat the generic type twice between the declaration and instantiation.

After we create the ArrayList object and add string elements to the list object, we get the size of the list by calling the size() method. We store the result in the size variable and print out its value. If you compile and run the code above you will get the following output:

List size = 3

In this example we will learn how to use the java.util.ArrayList class. An ArrayList is part of the Java Collection Framework. By using this class we can create a dynamic size array of data, which means we can add or remove elements from the array dynamically.

In the code below we will see the demonstration on how to create an instance of ArrayList, add some elements, remove elements and iterate through the entire ArrayList elements either using a for-loop or using the for-each syntax.

We can also see how to convert the ArrayList into an array object using the toArray() method. And we use the Arrays.toString() utility method when we want to print the content of an array.

When creating a class instance it is a good practice to use the interface as the type of variable instead of the concrete type directly. This can make us easily update our code if we don’t want to use ArrayList in the future.

And here is the code snippet:

package org.kodejava.util;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class ArrayListExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();

        // Add items into ArrayList
        list.add("Item 1");
        list.add("Item 2");
        list.add("Item 3");

        // Remove the third item from ArrayList, first index = 0
        list.remove(2);

        // Iterate ArrayList item using for loop statement
        for (int i = 0; i < list.size(); i++) {
            String item = list.get(i);
            System.out.println("Item = " + item);
        }

        // Iterate ArrayList item using enhanced for statement
        for (String item : list) {
            System.out.println("Item = " + item);
        }

        // Iterate ArrayList item using forEach
        list.stream().map(item -> "Item = " + item).forEach(System.out::println);

        // Convert ArrayList into array of object
        String[] array = list.toArray(new String[0]);
        System.out.println("Items = " + Arrays.toString(array));
    }
}

Executing the program will give us the following output printed in our console.

Item = Item 1
Item = Item 2
Item = Item 1
Item = Item 2
Item = Item 1
Item = Item 2
Items = [Item 1, Item 2]