Create Array of Linked Lists in Java

1. Overview

2. What Is an Array of Linked Lists?

In Java, an array of linked lists is a structure where each index holds a LinkedList<T> instead of a single value. This lets us maintain indexed access while supporting dynamic insertion and deletion at each index:

LinkedList<Integer>[] listArray = new LinkedList[3];

Each slot is initialized with a LinkedList and can be accessed just like any array element:

listArray[0] = new LinkedList<>();
listArray[0].add(5);

It’s useful when we need both fast access by index and flexible storage within each group.

Now, let’s explore some of its use cases.

3. Where Can We Use an Array of Linked Lists?

This structure isn’t just a coding trick; it solves real problems. Here are a few scenarios where it shines:

• Graph representation (Adjacency List): Each node in a graph can store a list of adjacent nodes. An array of linked lists is an ideal structure for this.
• Hash table with chaining: When multiple keys hash to the same index, a linked list can store the colliding values at that index.
• Bucket sort: This sorting algorithm divides numbers into buckets based on their range and sorts each bucket individually.
• Index-based grouping: Items such as users, logs, or events can be categorized by attributes like priority, region, or age group using indexed lists.

Now that we understand why we’d use this data structure, we’ll examine several ways to implement it.

4. Using an Array of Linked Lists

To demonstrate the use of an array of linked lists, let’s consider the following problem.

Given a list of integers, let’s categorize them into three groups:

• Group 0: Numbers less than 10
• Group 1: Numbers from 10 to 19
• Group 2: Numbers 20 and above

Let’s define a few utility methods to handle grouping our numbers into the appropriate linked lists. These helpers show how we intend to use this data structure after we’ve created it, so that we can focus on the various ways to create these data structures.

We’ll provide two versions of the utility method allocateNumbers(), one for an outer List, and the other for an outer Array.

The first version works with a List of linked lists:

public static void allocateNumbers(int[] numbers, List<LinkedList<Integer>> groups) {
    for (int num : numbers) {
        int index = (num < 10) ? 0 : (num < 20 ? 1 : 2);
        groups.get(index).add(num);
    }
}

The second version uses an array of linked lists:

public static void allocateNumbers(int[] numbers, LinkedList<Integer>[] groups) {
    for (int num : numbers) {
        int index = (num < 10) ? 0 : (num < 20 ? 1 : 2);
        groups[index].add(num);
    }
}

In both examples, we use ternaries to select the index of the linked list that we want to store the number in, before adding that number to the correct list.

Now that we know how we’ll use these data structures, let’s focus on the various ways to create an array of linked lists.

5. Using a Raw Array with an Initialization Loop

Using a raw array is the most direct way to create an array of linked lists. We’ll declare an array of the desired size and then initialize each element of the array:

public static LinkedList<Integer>[] createUsingRawArray() {
    @SuppressWarnings("unchecked")
    LinkedList<Integer>[] groups = new LinkedList[3];
    for (int i = 0; i < groups.length; i++) {
        groups[i] = new LinkedList<>();
    }
    return groups;
}

In this method, we create a raw array of size three to hold three linked lists. Since Java doesn’t allow direct creation of generic arrays, we suppress the warning with @SuppressWarnings(“unchecked”). Then, we loop through the array and initialize each slot with a new LinkedList. This approach gives us fast index access and is ideal when the size is fixed.

Now, we can use this to group some numbers in our expected way:

int[] numbers = {3, 7, 12, 19, 25, 32};
LinkedList<Integer>[] groups = createUsingRawArray();
LinkedListArray.allocateNumbers(numbers, groups);

This creates the data structure and then fills each list with numbers from the given array according to our grouping rule.

The raw array approach mirrors how arrays are used in C-style languages and is perfect for fixed-size bucket-style problems.

6. Using a List of LinkedLists

Rather than using a raw Array, a safer and more flexible option is to use an ArrayList to store the LinkedList objects:

public static List<LinkedList<Integer>> createUsingList() {
    List<LinkedList<Integer>> groups = new ArrayList<>();
    for (int i = 0; i < 3; i++) {
        groups.add(new LinkedList<>());
    }
    return groups;
}

Here, we create an ArrayList and add three empty linked lists to it. This approach is fully type-safe and avoids unchecked warnings. It’s slightly more flexible than using a raw array, and it’s a common pattern in real-world Java applications.

Let’s use this approach for our problem:

int[] numbers = {3, 7, 12, 19, 25, 32};
List<LinkedList<Integer>> groups = createUsingList();
LinkedListArray.allocateNumbers(numbers, groups);

We first build the data structure and then use our utility method to group the numbers.

7. Using Java 8 Streams

If we prefer a functional style, we can use the Stream API, first introduced in Java 8. It can populate a list of linked lists concisely:

public static List<LinkedList<Integer>> createUsingStreams() {
    List<LinkedList<Integer>> groups = new ArrayList<>();
    IntStream.range(0, 3).forEach(i -> groups.add(new LinkedList<>()));
    return groups;
}

Here, we use IntStream.range() to loop from zero to two, and for each index, we add a new linked list to the main list.

We can use this data structure in the same way:

List<LinkedList<Integer>> groups = createUsingStreams();
LinkedListArray.allocateNumbers(numbers, groups);

This is useful when we prefer declarative style code or want to reduce boilerplate.

8. Using Arrays.setAll() for Java 8+ Versions

If sticking to raw arrays is important, but there’s a desire to avoid explicit loops, we can use Arrays.setAll(). This method lets us assign values to each index without writing an explicit loop:

public static LinkedList<Integer>[] createUsingSetAll() {
    @SuppressWarnings("unchecked")
    LinkedList<Integer>[] groups = new LinkedList[3];
    Arrays.setAll(groups, i -> new LinkedList<>());
    return groups;
}

We still need to suppress the warning because we’re creating a raw array, but Arrays.setAll() makes the initialization step cleaner. It sets each index in the array with a new linked list.

Let’s apply it to our problem:

LinkedList<Integer>[] groups = createUsingSetAll();
LinkedListArray.allocateNumbers(numbers, groups);

This approach is concise and expressive, especially if we’re working with Java 8 or later.

9. Conclusion

In this article, we used an array of linked lists to combine indexed access with dynamic data handling. Whether modeling graphs, implementing hash tables with chaining, or organizing elements into multiple dynamic lists, this structure is both versatile and efficient.

Java supports several approaches to creating and organizing arrays of linked lists. This includes raw arrays with manual initialization to more modern, type-safe alternatives with lists or streams. Each method has its trade-offs, and the right choice depends on the use case, performance goals, and code readability preferences.

As always, the code presented in this article is available over on GitHub.