Understanding the Thread Safety of Collection Classes

When writing concurrent programs in Java, it is crucial to ensure the safety of shared data to avoid race conditions and other concurrency issues. Java provides several collection classes that can be utilized in multithreaded environments, but not all of them are inherently thread-safe. In this article, we will delve into the concept of thread safety and discuss the thread safety characteristics of commonly used collection classes.

Thread Safety in Java

Thread safety refers to the property of an object or a block of code that guarantees safe execution in a multithreaded environment. A thread-safe object or class ensures that its methods behave correctly when accessed by multiple threads concurrently. In other words, the internal state of a thread-safe object remains consistent and unaffected by concurrent access.

Thread Safety Levels

In Java, the thread safety of collection classes can be categorized into three levels:

1. Non-thread-safe

Non-thread-safe classes are not designed to handle concurrent access and can lead to inconsistent behavior when used in multithreaded programs. Concurrent modifications, such as adding or removing elements concurrently, may result in unexpected exceptions or data corruption.

2. Thread-compatible

Thread-compatible classes generally support concurrent read operations, but modifications while reading can lead to unexpected behavior. These classes may require external synchronization to ensure consistency.

3. Thread-safe

Thread-safe classes are specifically designed to allow safe access from multiple threads. They handle concurrent operations internally and guarantee correctness even without external synchronization. These classes are typically slower compared to non-thread-safe or thread-compatible classes due to the added synchronization overhead.

Thread Safety of Collection Classes

The following are some commonly used collection classes in Java and their thread safety characteristics:

1. ArrayList

ArrayList is not thread-safe. Concurrent modifications can lead to unexpected behavior, such as ConcurrentModificationException. Synchronization must be externally applied if ArrayList needs to be safely accessed from multiple threads.

2. LinkedList

Similar to ArrayList, LinkedList is also not thread-safe. It requires explicit synchronization to ensure consistent behavior in concurrent environments.

3. HashSet

HashSet is not thread-safe. Concurrent modifications may lead to inconsistencies. External synchronization is necessary for safe concurrent access.

4. HashMap

Like HashSet, HashMap is not thread-safe. Concurrent modifications can result in undefined behavior. External synchronization is required for safe concurrent access.

5. ConcurrentHashMap

ConcurrentHashMap is a thread-safe alternative to HashMap. It allows concurrent modifications and read operations without explicit external synchronization. It achieves thread safety through internal locking mechanisms, providing good performance in concurrent scenarios.

6. CopyOnWriteArrayList

CopyOnWriteArrayList is a thread-safe variant of ArrayList that provides safe concurrent access without external synchronization. It achieves this by making a new copy of the underlying array whenever a modification occurs. While efficient for read-heavy scenarios, it can be costly in terms of memory usage and may not be suitable for high-frequency modifications.

7. CopyOnWriteArraySet

CopyOnWriteArraySet is a thread-safe variant of HashSet. It shares similar characteristics with CopyOnWriteArrayList and provides safe concurrent access without external synchronization.


Understanding the thread safety characteristics of collection classes in Java is crucial for developing correct and efficient concurrent programs. By utilizing the appropriate collection classes that match the requirements of a program, developers can ensure safe access and manipulation of shared data in a multithreaded environment.

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