When should you use Collections.emptyList(), and when does it make sense to create a new list instance?

Collections.emptyList() should be used when:

• You need to return an empty, immutable list.
• You want to avoid unnecessary memory allocation for a new list object. This is especially relevant in loops or methods that may frequently return empty lists.
• It is important to guarantee that the returned list will not be modified by the calling code.

Creating a new list instance (e.g., new ArrayList<>() or new LinkedList<>()) makes sense when:

• You expect the list to be modified (elements added, removed, or changed).
• The list implementation type (e.g., ArrayList for fast index access or LinkedList for quick insertions/deletions at the beginning/middle) matters for performance.
• The returned list should be mutable.

Example of using Collections.emptyList():

// Returns an empty, immutable list of users if the condition is not met.
public List<User> findActiveUsers() {
    if (!isActiveCheckEnabled) {
        return Collections.emptyList(); // Efficient and safe
    }
    // ... logic to find active users
    List<User> activeUsers = new ArrayList<>();
    // ... add users to the list
    return activeUsers;
}

Example of creating a new list instance:

// Creates a list for collecting data that will be modified.
public List<Integer> processData(List<String> rawData) {
    List<Integer> processedNumbers = new ArrayList<>(); // List will be filled
    for (String data : rawData) {
        try {
            processedNumbers.add(Integer.parseInt(data));
        } catch (NumberFormatException e) {
            // Handle error
        }
    }
    return processedNumbers;
}

Summary table:

Characteristic	Collections.emptyList()	new ArrayList<>() / new LinkedList<>()
Mutability	Immutable	Mutable
Efficiency	High (no memory allocation)	Requires memory allocation
Usage	For returning an empty, safe list	For creating a list that will be modified
Thread safety	Thread-safe (immutable)	Not thread-safe by default