Initialise List and ArrayList in One Line

1. Introduction

When it comes to collections in Java, List and ArrayList, continue to reign supreme. Thanks to their quick retrieval times – perfect for apps where fetching data is a priority – they’ve become the go-to choice. But did you know these bad boys can be initialized with just one swift line? This article will show how to initialise List and ArrayList in one line.

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2. Initialising List and ArrayList

In a List and ArrayList, you have total control over where the elements are inserted. They allow duplicates, and the elements can be accessed using their index, starting from zero. It’s impossible to create an instance of an interface, but a couple of implementations will allow List initialisation.

Initializing a List or an ArrayList in one line can be helpful to simplify the code, especially when adding multiple elements during declaration.

We can use the Arrays.asList() function to fill a List with values. However, this only returns a fixed-size List, and any attempts to modify it will result in an UnsupportedOperationException. Here’s an illustration:

List<String> names = Arrays.asList("Alice", "Bob", "Cathy");

For an ArrayList, we can create a new instance and use the Arrays.asList() method, along with the addAll() method, as shown here:

ArrayList<String> names = new ArrayList<String>();
names.addAll(Arrays.asList("Alice", "Bob", "Cathy"));

Alternatively, we can use double brace initialization, which adds items to an instance initializer block and generates an anonymous subclass. Due to probable memory leaks, performance concerns, and poor maintainability, this approach is not advised. Here is an illustration of this strategy:

ArrayList<String> names = new ArrayList<String>() {{
    add("Alice");
    add("Bob");
    add("Cathy");
}};

Since Java 8, we can also use Stream.of() to initialize an ArrayList more elegantly:

ArrayList<String> names = 
    Stream.of("Alice", "Bob", "Cathy")
        .collect(Collectors.toCollection(ArrayList::new));

In conclusion, various ways exist to create and initialize Lists and ArrayLists. For fixed-size Lists, using Arrays.asList() is a practical choice; for ArrayLists, using addAll() in conjunction with Arrays.asList() or Stream.of() allows greater flexibility. To create compact and legible code, the right approach must be chosen for the particular use case.

3. One-Line Initialization Syntax Using Collections and Constructors

We can initialize a List or an ArrayList in a single line using constructors and the Collections utility in Java. First, let’s explore using the size and the constructor to create a List with a predefined size. This method is proper when we know the number of elements our List instance will hold. Here’s an example:

List<String> myList = new ArrayList<>(5);

With this code, a myList ArrayList with a maximum capacity of 5 String entries is initialised. You’ll see that we employed a constructor that accepts an integer argument for the List’s size. The List will be created using this method with the specified size, but the elements won’t be filled in yet.

3.1. Creating an Immutable List

A list that cannot be changed once it has been generated is said to be immutable. The List.of() function, introduced in Java 9, can generate an immutable List in one line. A succinct, immutable List instance can be created in a single using the List.of() method. Here’s an illustration:

List<String> immutableList = List.of("apple", "banana", "cherry");

In this example, three String components were produced as part of an immutable List. We won’t be able to add, remove, or edit List elements after it has been formed.

3.2. Creating a Mutable List

The package’s Arrays.asList() method enables us to construct a mutable List to add, remove, and update entries. A fixed-size List is created by this method. Here’s an illustration:

List<String> mutableList = 
    new ArrayList<>(Arrays.asList("one", "two", "three"));

In this example, three String components were added to a changeable List. We can add, remove, and update elements because the List is mutable.

3.3. Utilising Stream API and Collectors

Let’s begin by demonstrating a straightforward way to use the Collectors.toList() method to create and initialize a List in one line. Here is a sample code:

List<String> myList = 
    Stream.of("one", "two", "three").collect(Collectors.toList());

In this example, we create a List called myList with the components “one,” “two,” and “three.” To do this, we use the Stream.of() function and the collect() method, which gathers the Stream items into a List by using Collectors.toList() as a parameter.

3.4. Arrays

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

public class InitializeArrayList {
    public static void main(String[] args) {
        List<String> myArrayList = 
            new ArrayList<>(Arrays.asList("one", "two", "three"));
        System.out.println(myArrayList);
    }
}

The numbers “one”, “two”, and “three” are provided in one line using the new ArrayList>() and Arrays.asList() functions in an array we build called myArrayList. A List is obtained using the Arrays.asList() function and then transformed into an ArrayList.

4. Additional Initialization Methods

One strategy is using the anonymous inner class, called double brace initialization. We may create and initialize a List or ArrayList using this technique with just one line of code. It is important to remember that this method produces an anonymous subclass with an instance initializer, which could impact performance. Here is an ArrayList example:

List<String> list = new ArrayList<String>() {{
    add("A");
    add("B");
    add("C");
}};

Remember that there are other ways to create and initialize a List or ArrayList besides these two techniques. However, they ought to give a clear grasp of how to implement a straightforward, concise, and compelling initialization procedure.

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5. Summary

This article has described numerous methods for initialising List and ArrayList in one line of code. By examining the many options, we have shown how flexible and adaptable the Java code is. Depending on the Java Development Kit (JDK) version and your need for immutability, you can use various techniques to produce lists tailored to your specific requirements. The developer’s decisions and the particular use case will ultimately decide the startup strategy. You may select your project’s best course of action by comprehending the numerous possibilities available.