Java List Initialization Guide: Best Practices, Examples, and Performance Tips

1. Introduction

2. Basics of Lists and the Importance of Initialization

• Order is preserved: Elements maintain the order in which they were inserted.
• Duplicates are allowed: Multiple identical values can be stored.
• Dynamic size: No need to specify the size beforehand; elements can be added or removed freely.

3. Five Ways to Initialize a List

3.1 Creating an Empty List

List<String> list = new ArrayList<>();

• Use case: When adding elements later.
• Key point: Initially empty, but elements can be freely added using add(). You may also choose other implementations, such as LinkedList, depending on your needs.

3.2 Using Arrays.asList

List<String> list = Arrays.asList("A", "B", "C");

• Use case: When creating a List from multiple fixed values.
• Note: Lists created with this method have a fixed size, so adding or removing elements using add() or remove() is not allowed. However, set() can modify existing values.
• If modification is required: Create a new ArrayList as follows:

List<String> list = new ArrayList<>(Arrays.asList("A", "B", "C"));

3.3 Using List.of (Java 9+)

List<String> list = List.of("A", "B", "C");

• Use case: When the content is fixed and does not need to change.
• Characteristics: Lists created using this method are fully immutable. No modifications—including add(), remove(), or even set()—are allowed. This is ideal for constants and safety-critical data.

3.4 Using an Instance Initializer

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

• Use case: When many elements or complex initialization logic is needed.
• Caution: Because it creates an anonymous class, it is not recommended for large projects or performance-critical environments due to memory and maintainability concerns.

3.5 Creating an ArrayList with an Initial Capacity

List<String> list = new ArrayList<>(100);

• Use case: When you expect to insert many elements and already know the approximate size.
• Benefit: Reduces internal resizing operations and improves performance.

4. Comparing Each Initialization Method

• If you need to add or remove elements later ⇒ new ArrayList<>() or new ArrayList<>(Arrays.asList(...))
• If you want a List from fixed values without adding/removing ⇒ Arrays.asList(...)
• If you need a completely immutable List (safety-critical) ⇒ List.of(...) (Java 9+)
• If you need multi-line or complex initialization logic ⇒ Instance initializer
• If you expect a large number of elements and want better performance ⇒ new ArrayList<>(initial capacity)

• Lists created using Arrays.asList have fixed size—adding or removing elements results in an UnsupportedOperationException.
• List.of supports zero or multiple elements, but it is immutable—add, remove, and set cannot be used.
• Instance initializers are powerful but create anonymous classes, which can hurt readability and performance.

5. Practical Usage Examples

5.1 Creating an Empty List and Adding Values Later

List<String> names = new ArrayList<>();
names.add("Satou");
names.add("Suzuki");
names.add("Takahashi");
System.out.println(names); // Output: [Satou, Suzuki, Takahashi]

5.2 Creating a Fixed-Size List with Arrays.asList

List<String> fruits = Arrays.asList("Apple", "Banana", "Mikan");
System.out.println(fruits); // Output: [Apple, Banana, Mikan]
// fruits.add("Grape"); // ← This will cause an error
fruits.set(0, "Pineapple"); // This is allowed
System.out.println(fruits); // Output: [Pineapple, Banana, Mikan]

5.3 Creating an Immutable List with List.of (Java 9+)

List<String> colors = List.of("Red", "Blue", "Green");
System.out.println(colors); // Output: [Red, Blue, Green]
// colors.add("Yellow"); // ← Will throw an exception

5.4 Setting Complex Initial Values with an Instance Initializer

List<Integer> numbers = new ArrayList<>() {{
    for (int i = 1; i <= 5; i++) {
        add(i * i); // 1, 4, 9, 16, 25
    }
}};
System.out.println(numbers); // Output: [1, 4, 9, 16, 25]

5.5 Adding Large Amounts of Data with Initial Capacity

List<Integer> bigList = new ArrayList<>(1000);
for (int i = 0; i < 1000; i++) {
    bigList.add(i);
}
System.out.println(bigList.size()); // Output: 1000

6. Summary

• Lists are ordered, allow duplicates, and support dynamic resizing, making them more flexible than arrays.
• Java provides various initialization methods: empty Lists, Lists with initial values, immutable Lists, Lists with specified initial capacity, and more.
• Choosing the right method depends on whether you need to add/remove elements, handle fixed data, ensure immutability, or manage large datasets efficiently.
• Arrays.asList and List.of have specific limitations (fixed size or full immutability), so understanding their behavior is essential.

7. Frequently Asked Questions (FAQ)

• List.of (Java 9+) creates a fully immutable List. No modifications are allowed—not even set().
• Arrays.asList creates a fixed-size List. You cannot add or remove elements, but overwriting values with set() is allowed.

• Increased memory usage
• Reduced maintainability
• Potential issues during serialization

• new LinkedList<>(existingList)
• Using Arrays.asList or List.of combined with conversion