Java Collections Best Practices for Developers

Java Collections Framework provides a robust architecture for storing and manipulating groups of objects. In this blog post, I will explain ten best practices for using collections in Java, each demonstrated with "Avoid" and "Better" examples to guide you in writing efficient and maintainable code.

1. Use Interface Type on Variable Declarations

Avoid: Declaring collections using concrete classes.

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

Better: Declare using interface types.

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

Explanation: Using interface types such as List, Map, etc., in variable declarations enhances code flexibility by making it easier to switch between different implementations.

2. Prefer isEmpty() Over size() for Checking Emptiness

Avoid: Checking emptiness by comparing size with zero.

if (list.size() == 0) {
    // List is empty

Better: Use isEmpty() to check if the collection is empty.

if (list.isEmpty()) {
    // List is empty

Explanation: isEmpty() is clearer and often more efficient than checking if size() == 0.

3. Initialize Collections with Capacity Where Applicable

Avoid: Not specifying initial capacity for collections that support it when size is known.

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

Better: Initialize collections with a defined capacity.

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

Explanation: Initializing collections like ArrayList or HashMap with a known capacity can reduce the need for incremental resizing and rehashing, which improves performance.

4. Use EntrySet for Map Iterations

Avoid: Iterating over keys and then calling get for each key.

for (String key : map.keySet()) {
    System.out.println(key + " -> " + map.get(key));

Better: Iterate over entrySet to access both key and value.

for (Map.Entry<String, String> entry : map.entrySet()) {
    System.out.println(entry.getKey() + " -> " + entry.getValue());

Explanation: Using entrySet() is more efficient as it avoids the additional lookup for each key.

5. Use Collections Utility Methods for Single-Element Collections

Avoid: Manually creating collections for a single element.

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

Better: Use utility methods like Collections.singletonList.

List<String> singleList = Collections.singletonList("element");

Explanation: Utility methods provide a more concise, readable, and immutable solution for single-element collections.

6. Avoid Removing Elements in a for-each Loop

Avoid: Removing elements using a for-each loop, which can cause ConcurrentModificationException.

for (String item : list) {
    if (item.equals("delete")) {

Better: Use an iterator explicitly for removal.

Iterator<String> it = list.iterator();
while (it.hasNext()) {
    if ("delete")) {

Explanation: Using an iterator's remove() method is safe and avoids concurrent modification issues.

7. Leverage Type-Specific Interfaces in Java Collections

Avoid: Using a raw typed collection.

List numbers = new ArrayList(); // raw type

Better: Use generics for type safety.

List<Integer> numbers = new ArrayList<>();
// numbers.add("two"); // This would cause a compile-time error

Explanation: Using generics enhances type safety by catching improper types at compile time.

8. Opt for Immutable Collections

Avoid: Creating modifiable collections when not necessary.

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

Better: Use immutable collections when no modification is needed.

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

Explanation: Immutable collections are safer as they prevent unintended modifications, which can be crucial for maintaining consistent state.

9. Consider Concurrent Collections for Multi-threaded Environment

Avoid: Using standard collections in multi-threaded contexts where they may be accessed concurrently.

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

Better: Use concurrent collections from java.util.concurrent package.

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

Explanation: Concurrent collections like CopyOnWriteArrayList are

designed for environments where multiple threads modify the collection, ensuring thread safety.

10. Prefer Streams for Bulk Operations on Collections

Avoid: Using explicit loops for filter/map/reduce operations.

List<String> filtered = new ArrayList<>();
for (String s : list) {
    if (s.startsWith("A")) {

Better: Use Java Stream API for more concise and readable code.

List<String> filtered =
    .filter(s -> s.startsWith("A"))

Explanation: Java Streams provide a high-level, functional-style approach to operations on collections, making the code more concise and often easier to parallelize.

These best practices can help you use Java Collections more effectively, enhancing both performance and code readability in your projects.