The previous section discussed the Map interface with implementation classes, including internal mechanisms and use cases. If you missed it, please find it here.
This final section will discuss concurrent collections, collections utilities (algorithms), best practices for using the Java Collection Framework, and how to avoid common pitfalls.
Concurrent Collections (java.util.concurrent)
This package is designed to provide thread-safe versions of collections that support concurrent access and modifications without needing external synchronization for building high-performance applications.
Why are concurrent collections preferable over their synchronized standard counterparts?
The collections in the concurrent package are not just thread-safe, they are designed to be used safely by multiple threads without additional synchronization. They utilize advanced techniques like lock-free algorithms and concurrent data structures to minimize contention and maximize throughput, ensuring efficient performance in multi-threaded environments.
Let’s explain the internal mechanism of read-write operations of CopyOnWriteArrayList.
Read Operations: These operations access the array’s current state directly. Since the array is not modified during reads, multiple threads can read concurrently without synchronization.
Write Operations: When a write operation occurs:
- A new copy of the underlying array is created.
- The modification is applied to the new array.
- The reference to the array is updated atomically to point to the new array.
Example Code:
public void add(E element) {
// Create a new copy of the array with the new element
Object[] oldArray;
Object[] newArray;
do {
oldArray = array.get(); // Get current array
newArray = Arrays.copyOf(oldArray, oldArray.length + 1);
newArray[oldArray.length] = element; // Add new element
} while (!array.compareAndSet(oldArray, newArray)); // Atomically update the reference
}]
// Get an element
public E get(int index) {
return (E) array.get()[index]; // Safe to access directly
}
Using concurrent collections can significantly improve performance in multi-threaded environments by reducing the need for locking and by providing non-blocking algorithms.
Key Classes in the Concurrent Package
- ConcurrentHashMap: A highly efficient thread-safe implementation of Map, suitable for high-concurrency scenarios. It divides the map into segments, allowing multiple threads to read and write without locking the entire map (a specific segment might be closed for modification, but other threads can still access different segments concurrently).
- CopyOnWriteArrayList: A thread-safe variant of ArrayList where all mutative operations (add, set, etc.) are implemented by making a fresh copy of the underlying array. This is ideal for scenarios where reads are frequent, and writes are infrequent, such as a list of subscribers in a multi-threaded messaging system.
- BlockingQueue: This interface represents a queue that supports operations that wait for the queue to become non-empty when retrieving elements and for space to become available when storing elements. Implementations like ArrayBlockingQueue or LinkedBlockingQueue are designed for concurrent producer-consumer scenarios, where one or more threads are producing data and one or more threads are consuming that data.
Collections Utilities (Algorithms)
Java provides utility classes such as Collections and Arrays that offer a range of static methods to manipulate collections. These utilities, with their simple and consistent API, can sort, search, reverse, and create thread-safe wrappers around collections, making everyday tasks easier and improving code readability.
- Sorting and Searching: Collections.sort() sorts any List based on the natural ordering of its elements or a provided Comparator. Collections.binarySearch() allows for fast searching in sorted lists.
Example: Sorting with a Custom Comparator
List<String> names = Arrays.asList("Alice", "Charlie", "Bob");
Collections.sort(names, new Comparator<String>() {
@Override
public int compare(String o1, String o2) {
return o1.length() - o2.length(); // Sort by string length
}
});
names.forEach(System.out::println); // Prints names sorted by length
Example: Searching in a collection
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
int index = Collections.binarySearch(numbers, 3);
System.out.println(index); // Output: 2
- Reversing, Shuffling, and Swapping: Methods like Collections.reverse(), Collections.shuffle(), and Collections.swap() offer convenient ways to manipulate list elements.
Example: The shuffle() method randomizes the order of elements in a list.
Collections.shuffle(names);
System.out.println(names); // Output: Random order of names
Example: The reverse() method reverses the order of elements in a list.
Collections.reverse(names);
System.out.println(names); // Output: Names in reverse order
- Creating Unmodifiable Collections: To make a collection read-only, use Collections.unmodifiableCollection(), unmodifiableList(), unmodifiableSet(), unmodifiableMap(), etc. This is particularly useful for returning collections from methods without exposing them to modification.
List<String>unmodifiableList = Collections.unmodifiableList(names);
// unmodifiableList.add("New"); // Throws UnsupportedOperationException
- Creating Synchronized Collections: The synchronizedList(), synchronizedMap(), and synchronizedSet() methods return synchronized (thread-safe) versions of collections.
List<String> synchronizedList = Collections.synchronizedList(new ArrayList<>());
Internal Mechanism of Collections.sort()
The Collections.sort() method internally utilizes a modified version of the Merge Sort algorithm known as Timsort. Timsort is a hybrid sorting algorithm derived from merge sort and insertion sort. It is optimized for real-world data and is designed to perform well on many kinds of data, including partially sorted arrays.
Here’s a brief overview of how Timsort works:
- Divide: It divides the array into small chunks or “runs,” sorted using insertion sort.
- Merge: It then merges these runs using a technique similar to merge sort but with efficiency optimizations, such as minimizing the number of comparisons and merging runs that exploit the already sorted parts.
This combination helps Timsort achieve efficient sorting with a worst-case time complexity of O(n log n) and performs exceptionally well with real-world datasets.
Benefits of Using Collections Utility Class
- Ease of Use: Provides a simple and consistent API for everyday operations on collections.
- Enhanced Performance: Many methods are optimized for performance and efficiency.
- Thread Safety: Simplifies the creation of synchronized collections, making it easier to manage concurrent access.
- Immutability: Helps in creating unmodifiable collections, ensuring data integrity.
Following best practices for using Java Collections is crucial for secure and efficient coding. These practices, such as preferring interfaces to implementations and considering initial capacity, can enhance the performance and readability of your code.
- Prefer Interfaces to Implementations: When declaring collection variables, prefer using interface types over concrete implementation types. This promotes flexibility by decoupling the code from a specific implementation, making it easier to switch implementations if necessary.
- Consider Initial Capacity: For classes like ArrayList and HashSet, consider the initial capacity to minimize the number of rehashing or resizing, which can be costly for performance.
- Use Collections Utilities: Java provides utility classes like Collections and Arrays, which offer various methods to manipulate collections, such as sorting, searching, and converting arrays to lists. These utilities can simplify everyday tasks and improve code readability.
- Consider thread-safety: Collections from the standard Java.util package are not thread-safe. For concurrent access, consider using collections from the Java.util.concurrent package or wrapping your collection using methods like Collections.synchronizedList().
- Use Diamond Operator: When initializing a list with an ArrayList in Java, it is not strictly required to mention the type if you are using Java 7 or later due to type inference with the diamond operator (<>). However, it is generally good practice to specify the type for clarity and type safety.
- Specify the type of the elements when initializing a collection: Generics were introduced in Java 5 to provide compile-time type checking and eliminate the need for casting, which can lead to ClassCastException at runtime. Therefore, always specify the type of elements they will hold using generics. This also makes your code easier to read and maintain.
For instance, instead of using a raw type like List, specify the type of elements the list will contain, like List<String> or List<Integer>. This way, the compiler will ensure that only aspects of the specified type are added to the collection, preventing runtime errors.
List<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
// names.add(123); // This line would cause a compile-time error
for (String name : names) {
System.out.println(name);
}
The Java Collections Framework is a powerful toolset that, when used effectively, can solve a wide array of programming problems. By understanding the core interfaces, choosing the correct implementations, and following best practices, developers can write more efficient, cleaner, and more maintainable Java code.
Avoiding Common Problems
While collections are robust, there are common pitfalls that can lead to bugs or inefficient code:
- Modifying a collection while iterating can cause a ConcurrentModificationException. If you need to modify a collection during iteration, use an Iterator’s remove() method or iterate over a copy of the collection.
- Using mutable objects as keys in Maps or elements in Sets: If a mutable object’s state changes in a way that affects its equals or hashCode method, it can “get lost” in the collection, leading to unpredictable behavior. Prefer immutable objects for map keys and set elements.
- Initializing collection with primitive types: Collections are designed to work with objects. This is because collections are based on the concept of generics, which requires using reference types (objects).
Therefore, the following statement won’t work because of the primitive type:
List<int> numbers = new ArrayList<>();
By understanding and applying these practices, developers can use Java Collections more effectively, leading to cleaner, safer, and more efficient code.
Conclusion
In this blog, we have discussed quite a few topics. Through this discussion, we have ensured that readers understand how to study the Collection Framework to derive the maximum benefit and avoid common mistakes. Additionally, we aimed to provide a general understanding of Java’s design philosophies and algorithms in creating the Collection Framework so that we can apply these design philosophies and algorithms in our everyday coding. This will improve the quality of our coding. We hope that one day, we can elevate our coding standards to the level of Java. At that point, we can claim that our coding level is similar to Java’s. We may eventually learn to think like those masterminds who created Java.
Happy Coding!
