How do I avoid thread safety issues using concurrent collections?

When working with concurrent collections in Java, thread safety issues can be minimized or completely avoided by using the right tools and patterns provided by the Java concurrency library. Here are some guidelines for avoiding thread safety issues using concurrent collections:


1. Use Concurrent Collections

Java provides a range of thread-safe collections in the java.util.concurrent package. These collections provide built-in locking or non-blocking mechanisms to handle concurrent access safely.

Some commonly used concurrent collections include:

  • ConcurrentHashMap: A thread-safe alternative to HashMap. It minimizes contention by using segment-level locks (or CAS-based approaches in newer implementations).
  • ConcurrentLinkedQueue: A thread-safe non-blocking queue implementation.
  • CopyOnWriteArrayList: A thread-safe alternative to ArrayList. Suitable for scenarios with frequent reads and infrequent writes.
  • CopyOnWriteArraySet: A thread-safe variant of HashSet.
  • LinkedBlockingQueue: A bounded or unbounded thread-safe blocking queue.
  • PriorityBlockingQueue: A thread-safe alternative to PriorityQueue.

Example: ConcurrentHashMap

package org.kodejava.util.concurrent;

import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentCollectionExample {
    public static void main(String[] args) {
        ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>();
        map.put(1, "One");
        map.put(2, "Two");

        map.forEach((key, value) -> System.out.println(key + ": " + value));
    }
}

2. Understand the Collection’s Guarantees

Each concurrent collection has different thread safety guarantees:

  • Non-blocking vs blocking: Non-blocking collections like ConcurrentHashMap allow concurrent reads and writes without locking, while blocking collections like LinkedBlockingQueue block threads under certain conditions.
  • Consistency during iteration: Iterating over a ConcurrentHashMap may reflect updates made during the iteration, whereas CopyOnWriteArrayList provides a snapshot of the collection at the time of iteration.

Pick the appropriate collection based on your requirements.


3. Avoid External Synchronization

Avoid wrapping concurrent collections with synchronized blocks or manually synchronizing around them. Their thread-safety mechanisms are carefully designed, and external synchronization can lead to:

  • Performance bottlenecks.
  • Deadlocks.

Instead, rely on provided atomic operations like putIfAbsent, replace, compute, or merge.

Example: Avoid manual locking

// Bad practice: External synchronization
Map<Integer, String> map = new ConcurrentHashMap<>();
synchronized (map) {
   map.put(1, "One");
}

// Better: Let ConcurrentHashMap handle thread safety
map.put(1, "One");

4. Use Atomic Methods for Compound Actions

Use atomic methods on concurrent collections for compound actions to avoid race conditions. These operations combine checks and updates into a single atomic operation.

Example: putIfAbsent

ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>();
map.putIfAbsent(1, "One");

Example: compute and merge

// Using compute
map.compute(1, (key, value) -> (value == null) ? "One" : value + "-Updated");

// Using merge
map.merge(1, "Value", (oldValue, newValue) -> oldValue + "," + newValue);

5. Minimize Lock Contention

  • Collections like ConcurrentHashMap use techniques such as striped locks or non-blocking CAS operations to minimize lock contention.
  • For extremely high-concurrency cases, you may use LongAdder or LongAccumulator to handle summations without contention, as these are designed for heavy-write scenarios.

6. Choose the Right Collection for Blocking Scenarios

When you need blocking behavior in concurrent programming, prefer blocking queues or deque implementations such as ArrayBlockingQueue, LinkedBlockingQueue, or LinkedBlockingDeque.

Example: Producer-Consumer using LinkedBlockingQueue

package org.kodejava.util.concurrent;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;

public class ProducerConsumerExample {
    public static void main(String[] args) {
        BlockingQueue<Integer> queue = new LinkedBlockingQueue<>();

        Thread producer = new Thread(() -> {
            try {
                for (int i = 0; i < 10; i++) {
                    queue.put(i); // Blocks if the queue is full.
                    System.out.println("Produced: " + i);
                }
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        });

        Thread consumer = new Thread(() -> {
            try {
                while (true) {
                    int value = queue.take(); // Blocks if the queue is empty.
                    System.out.println("Consumed: " + value);
                }
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        });

        producer.start();
        consumer.start();
    }
}

7. Avoid Using Non-Thread-Safe Collections in Multi-Threaded Scenarios

Avoid using standard collections like HashMap or ArrayList in multithreaded environments unless explicitly synchronized. Instead, use the concurrent alternatives.


8. Consider Higher-Level Constructs

For more complex concurrent programming, Java provides higher-level frameworks and tools:

  • Executor framework: Manages thread pools for efficient task execution.
  • ForkJoinPool: Efficient parallel task execution.
  • java.util.concurrent.locks: Fine-grained lock management.

Combining concurrent collections with these tools can help avoid thread safety issues altogether.


By following these practices and using the right tools provided by the java.util.concurrent package, you can safely work with collections in multithreaded environments while minimizing performance overhead.

How do I use Collection.removeIf() method?

The Collection.removeIf() method was introduced in Java 8, and it allows for the removal of items from a collection using a condition defined in a lambda expression.

The primary purpose of the Collection.removeIf() method in Java is to filter out elements from a collection based on a certain condition or predicate. It’s a more efficient and concise way of performing this type of operation than traditional for or iterator-based loops.

The method iterates over each element in the collection and checks whether it satisfies the condition described by the given Predicate. If the Predicate returns true for a particular element, removeIf() removes that element from the collection.

Here’s a simple example:

package org.kodejava.util;

import java.util.ArrayList;
import java.util.List;

public class CollectionRemoveIfExample {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);

        // Use removeIf method to remove all numbers greater than 2
        numbers.removeIf(n -> n > 2);

        System.out.println(numbers); // Outputs: [1, 2]
    }
}

In this example, n -> n > 2 is a lambda expression that defines a Predicate, which returns true for all numbers greater than 2. The removeIf() method uses this Predicate to determine which elements to remove.

Please be aware that not all Collection implementations support the removeIf() method. For example, if you try to use it with an unmodifiable collection (like the ones returned by Collections.unmodifiableList()), it will throw an UnsupportedOperationException.

As removeIf() is a default method, it’s provided with a default implementation, and it’s available for use with any classes that implement the Collection interface (like ArrayList, HashSet, etc.) without requiring those classes to provide their own implementation.

However, classes can still override this method with their own optimized version if necessary. Here’s another example of removeIf() method:

package org.kodejava.util;

import java.util.ArrayList;
import java.util.List;

public class CollectionRemoveIfSecond {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");
        names.add("David");
        names.add("Rosa");

        // Remove names that start with 'B'
        names.removeIf(name -> name.startsWith("B"));

        System.out.println(names); // Outputs: [Alice, Charlie, David, Rosa]
    }
}

Remember, it’s a bulk operation that can lead to a ConcurrentModificationException if the collection is modified while the operation is running (for example, removing an element from a collection while iterating over it with removeIf()), except if the collection is a Concurrent Collection.

In conclusion, the Collection.removeIf() default method provides a unified, efficient, and convenient way to remove items from a collection based on certain conditions.

How do I use Map.of() factory method to create a map object?

In Java, the Map.of() factory method can be used to create an unmodifiable map of specified key-value pairs. This method is available in Java 9 and later versions.

Creating a map is a bit more complicated than creating lists or sets. Because we need to provide keys and values when creating a map. When using the Map.of() factory method we set the content of the map by alternating between the keys and values of the map.

Consider the following example:

package org.kodejava.util;

import java.util.Map;

public class MapOfExample {
    public static void main(String[] args) {
        Map<String, Integer> map = Map.of("John", 25, "Mary", 30, "Alice", 27, "Rosa", 22);

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

Output:

Rosa : 22
Mary : 30
John : 25
Alice : 27

In the example above, the Map.of("John", 25, "Mary", 30, "Alice", 27, "Rosa", 22) statement creates an unmodifiable map with three key-value pairs. After the map is created, any attempt to modify the map (add, update or remove elements) will throw an UnsupportedOperationException.

Note that Map.of() doesn’t accept null keys or values. If a null key or value is provided, then a NullPointerException is thrown. Besides, if duplicate keys are provided, an IllegalArgumentException is thrown.

The Map.of() method is overloaded to accept up to 10 key-value pairs. If there are more than 10 pairs, you can use Map.ofEntries() factory method to create a map. This is how we use it:

Map<String, Integer> map = Map.ofEntries(
    Map.entry("John", 25),
    Map.entry("Mary", 30),
    Map.entry("Alice", 27),
    Map.entry("Bob", 32),
    // ...
);

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

The Map.entry() is another factory method provided to create Map.Entry object.

How do I use Set.of() factory method to create a set object?

As with List.of(), in Java 9, the Set.of() factory method can be used to create an unmodifiable set of specified elements.

Here is a simple example:

package org.kodejava.util;

import java.util.Set;

public class SetOfExample {
    public static void main(String[] args) {
        Set<String> names = Set.of("Rosa", "John", "Mary", "Alice");

        for (String name : names) {
            System.out.println(name);
        }
    }
}

Output:

John
Rosa
Alice
Mary

In this example, the Set.of("Rosa", "John", "Mary", "Alice") statement creates an unmodifiable set of strings containing “Rosa”, “John”, “Mary”, and “Alice”. The resulting set is unmodifiable, so attempting to add, update, or remove elements from it will throw an UnsupportedOperationException.

If you try to create a Set by providing a duplicate elements, an IllegalArgumentException will be thrown. A Set is a type of collection container that cannot have duplicate values in it.

Note that the Set.of() method doesn’t accept null values. If you try to insert a null value, it will throw a NullPointerException. If you add a null value using the add() method UnsupportedOperationException will be thrown.

Set.of() is overloaded similarly to List.of(), allowing you to create a set with varying numbers of elements. The below examples demonstrate the use of Set.of() with different numbers of arguments:

Set<String> a = Set.of(); // An empty set
Set<String> b = Set.of("One"); // A set with one element
Set<String> c = Set.of("One", "Two"); // A set with two elements
// ...
Set<String> j = Set.of("One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten"); // A set with ten elements

If you need to create a set with more than 10 elements, Set.of() offers an overloaded version that accepts an array or varargs:

Set<String> set = Set.of("One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven");

Remember that sets created with Set.of() are unmodifiable. Attempting to add, remove or change an element in these sets after their creation causes an UnsupportedOperationException.

Also, Set.of() doesn’t allow duplicate or null elements. If you pass duplicate or null values, it will throw IllegalArgumentException and NullPointerException respectively.

How do I use Collectors.toCollection() method?

The Collectors.toCollection() method is a static method in the java.util.stream.Collectors class of Java 8. This method is used with streams when you want to convert a list to another collection type.

Here’s a simple example of how to use the method:

package org.kodejava.stream;

import java.util.*;
import java.util.stream.Collectors;

public class CollectorsToCollection {
    public static void main(String[] args) {
        List<String> list = 
                Arrays.asList("Java", "Kotlin", "Python", "Scala", "Kotlin");

        // Convert List to TreeSet
        TreeSet<String> treeSet = list.stream()
                .collect(Collectors.toCollection(TreeSet::new));

        System.out.println(treeSet);
    }
}

Output:

[Java, Kotlin, Python, Scala]

In this code:

  • We have a List of Strings.
  • We convert this list into a TreeSet.
  • Collectors.toCollection(TreeSet::new) is the collector that collects the data from the stream into a new TreeSet.
  • The method referenced by TreeSet::new is a constructor reference that creates a new empty TreeSet.

The output of the program will be the TreeSet containing the elements of the list.

Keep in mind that a TreeSet automatically orders its elements (in this case, alphabetically since the elements are Strings) and does not allow duplicates. So, if the list had duplicate values, and you wanted to maintain them in your new collection, you would need to choose a different type of Set or use a List.