How to Initialize an ArrayList in One Line in Java

There are several ways to initialize an ArrayList in one line in Java. Here are two possible approaches:

Approach 1: Using the Arrays.asList() method

You can use the Arrays.asList() method to initialize an ArrayList in one line. This method takes a variable number of arguments and returns a fixed-size List backed by the specified array. Here’s an example:

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

public class Main {
    public static void main(String[] args) {
        // Initialize ArrayList in one line
        List fruits = new ArrayList(Arrays.asList("Apple", "Banana", "Orange"));

        // <a href="https://www.squash.io/how-to-print-arraylist-in-java/">Print the ArrayList</a>
        System.out.println(fruits);
    }
}

This will output:

[Apple, Banana, Orange]

In the above example, we create a new ArrayList and pass the result of Arrays.asList() as an argument to the ArrayList constructor. This initializes the ArrayList with the elements “Apple”, “Banana”, and “Orange” in one line.

Approach 2: Using Java 9’s List.of() method

If you are using Java 9 or later, you can also use the List.of() method to initialize an ArrayList in one line. This method is a convenient way to create an immutable List with a fixed number of elements. Here’s an example:

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

public class Main {
    public static void main(String[] args) {
        // Initialize ArrayList in one line
        List fruits = new ArrayList(List.of("Apple", "Banana", "Orange"));

        // Print the ArrayList
        System.out.println(fruits);
    }
}

This will output:

[Apple, Banana, Orange]

In the above example, we create a new ArrayList and pass the result of List.of() as an argument to the ArrayList constructor. This initializes the ArrayList with the elements “Apple”, “Banana”, and “Orange” in one line.

Alternative ideas and suggestions

– If you don’t want to use the ArrayList constructor, you can also initialize an ArrayList using the add() method. Here’s an example:

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

public class Main {
    public static void main(String[] args) {
        // Initialize ArrayList in one line
        List fruits = new ArrayList();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");

        // Print the ArrayList
        System.out.println(fruits);
    }
}

This will output:

[Apple, Banana, Orange]

– If you know the size of the ArrayList in advance, you can also specify the initial capacity when creating the ArrayList. This can help improve performance if you know the approximate number of elements the ArrayList will hold. Here’s an example:

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

public class Main {
    public static void main(String[] args) {
        // Initialize ArrayList with initial capacity
        List fruits = new ArrayList(3);
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");

        // Print the ArrayList
        System.out.println(fruits);
    }
}

This will output:

[Apple, Banana, Orange]

Best practices

– When initializing an ArrayList in one line, consider using the Arrays.asList() method if you need a mutable ArrayList or the List.of() method if you need an immutable ArrayList.
– If you are using an older version of Java that does not support List.of(), consider upgrading to a newer version to take advantage of the convenience it offers.
– If you know the size of the ArrayList in advance and performance is a concern, consider specifying the initial capacity when creating the ArrayList.