ArrayList to Array Java

Convert ArrayList to Array Java

When you’re working with an ArrayList, you may encounter a situation where you want to convert it into an array, or vice versa. For instance, you may have a list of stocks in your portfolio stored in an array and want to convert it into an ArrayList.

In Java, the toArray() method is used to convert an ArrayList to an array, and the asList() method is used to convert a list to an ArrayList. This tutorial will discuss, with reference to examples, how to convert an ArrayList to an array and vice versa in Java.

Java Array and ArrayList

In Java, arrays are used to store a sequence of zero or more values. For instance, an array may store a list of candies sold at a candy store, or a list of orders that have been placed at a local cafe. Arrays can store any data type such as floats, booleans, and strings.

Here’s an example of an array in Java:

String[] names = new String[5];

In this example, we have initialized an array called names which stores five string values.

Before you can use an array in Java, you need to declare its size. In addition, once you have declared the size of an array, it can be difficult to change it.

That’s where the ArrayList class comes in. ArrayList is an implementation of the Java List interface and allows developers to create resizable arrays. When you add or remove an object from an ArrayList, the capacity of the list is automatically changed to reflect the number of values that are stored in the list. In this tutorial, we will simply refer to the ArrayList class as a list.

Here’s an example of an ArrayList in Java:

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

In the above example, we have declared an ArrayList called names which stores any number of string values.

Now that we know the basics of Java arrays and ArrayLists, we can explore how to convert data using these data types.

Convert List to Array

In Java, the list.toArray() method is used to convert a list to an array.

Suppose we have a list of stocks in our retirement portfolio stored in an ArrayList. We have decided that we want this to be stored in an array because we do not intend on adding new stocks to our retirement portfolio.

We could convert the list with our list of stocks to a Java array using this code:

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

class Main {
	public static void main(String[] args) {
		List<String> portfolio = new ArrayList<String>();
		portfolio.add("BAM");
		portfolio.add("GOOGL");
		portfolio.add("SBUX");

		String[] newPortfolio = new String[portfolio.size()];
		portfolio.toArray(newPortfolio);

		System.out.println("Portfolio: " + Arrays.toString(newPortfolio));
	}
}

Our code returns:

Portfolio: [BAM, GOOGL, SBUX]

Let’s break down our code. First, we import three libraries into our program, which are: List, Arrays, and ArrayList. The List library allows us to work with the Java List interface, the ArrayList library allows us to work with ArrayLists, and Arrays allows us to use the toString() method to print our array to the console.

Then we declare an ArrayList called portfolio which stores string values. We then use the add() method to add three values to our portfolio: BAM, GOOGL, and SBUX.

On the next line of our code, we create an array called newPortfolio and initialize the array and the size of the array. We set the capacity of the newPortfolio string array — the number of values it can hold — to be equal to the length of the portfolio array. So, in this case, the newPortfolio array will be capable of holding three values.

Next, we use the toArray() method to convert our portfolio ArrayList to an array and assign the newPortfolio array the values stored within the variable portfolio. We then use Arrays.toString() to convert the newPortfolio variable to a readable string of values. Finally, we print out the message Portfolio: , followed by the value returned by Arrays.toString(), to the console. 

Convert Array to List

The asList() method is used to convert an array to an ArrayList in Java.

Let’s return to the retirement portfolio example from earlier. Suppose we had an array of stocks in our retirement portfolio which we want to convert into an ArrayList. We could do so using this code:

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

class Main {
	public static void main(String[] args) {
		String[] portfolio = {"BAM", "GOOGL", "SBUX"};
		List<String> newPortfolio = Arrays.asList(portfolio);

		System.out.println("New portfolio: " + newPortfolio);
	}
}

Our code returns:

New portfolio: [BAM, GOOGL, SBUX]

In this example, we have used asList() to convert our retirement portfolio array called portfolio to a list. Let’s break down our code.

First, we import the Arrays and List packages, which contain the methods we need to convert our array to a list. Then we declare a variable called portfolio which contains an array of the stocks in our retirement portfolio.

On the next line, we use the Arrays.asList() method to convert the contents of the portfolio array to a list. We then assign the new list to the variable newPortfolio. Finally, we print out the message New portfolio: , followed by the contents of the newPortfolio array

Conclusion

The Java toArray() method is used to convert a list to an array, and the asList() method is used to convert an array to a list.

This tutorial discussed, with reference to examples, how to convert an ArrayList to array and an array to an ArrayList in Java. In addition, this tutorial discussed the main difference between the ArrayList class and the array data type. You’re now ready to start converting ArrayLists to arrays, and vice versa, like a professional Java coder!