Overview

Concurrency issues in Java arise when multiple threads attempt to modify the same resources leading to errors like race conditions, deadlocks, and thread interference. Properly handling concurrency is crucial for creating efficient, error-free Java applications, especially in a multi-threaded environment.

Key Concepts

1. Synchronization: Ensures that only one thread can access a resource at a time.
2. Locks: Explicit locking mechanisms like ReentrantLock to control access to resources.
3. Concurrent Collections: Thread-safe variants of standard collections like ConcurrentHashMap.

Common Interview Questions

Basic Level

1. What is a race condition and how can it be prevented in Java?
2. How do you use the synchronized keyword in Java?

Intermediate Level

3. How does the ConcurrentHashMap differ from Hashtable in Java?

Advanced Level

4. Explain the concept of lock stripping and how it can be used to improve concurrency.

Detailed Answers

1. What is a race condition and how can it be prevented in Java?

Answer: A race condition occurs when two or more threads access shared data and try to change it at the same time. It can lead to unpredictable results because the threads schedule can vary, causing different outcomes on different executions. In Java, race conditions can be prevented using synchronization techniques, ensuring that only one thread can access the shared resource at a time.

Key Points:
- Race conditions lead to inconsistent state of shared data.
- synchronized blocks or methods prevent race conditions by allowing only one thread to access a resource at a time.
- Atomic variables in Java, such as AtomicInteger, can also help prevent race conditions without using synchronized.

Example:

public class Counter {
    private int count = 0;

    public synchronized void increment() {
        count++; // Only one thread can execute this at a time
    }
}

2. How do you use the synchronized keyword in Java?

Answer: The synchronized keyword in Java is used to ensure that a method or block can only be executed by one thread at a time. It can be applied to instance methods, static methods, and code blocks within methods. When a thread enters a synchronized method or block, it acquires the intrinsic lock for the object (or class, for static methods) on which it is synchronizing, and releases it when it exits the method or block.

Key Points:
- Synchronization can be applied to methods and blocks.
- Acquires the intrinsic lock of the object or class.
- Helps in preventing race conditions.

Example:

public class SyncExample {
    public synchronized void syncMethod() {
        // Only one thread at a time can execute this method for any given instance
    }

    public void syncBlock() {
        synchronized(this) { // Synchronizing on the current instance
            // Code here is synchronized
        }
    }
}

3. How does the ConcurrentHashMap differ from Hashtable in Java?

Answer: ConcurrentHashMap and Hashtable are both thread-safe collections that can be used in concurrent Java applications. However, ConcurrentHashMap offers better concurrency compared to Hashtable. While Hashtable locks the entire map for a single thread access, ConcurrentHashMap uses a segment of locks, allowing multiple threads to read and write concurrently with less contention, leading to better performance.

Key Points:
- Hashtable locks the entire map, causing high contention.
- ConcurrentHashMap allows concurrent reads and segmented locks for writes.
- Offers better performance and scalability in multi-threaded environments.

Example:

ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("key", 1); // Multiple threads can safely update different segments concurrently

4. Explain the concept of lock stripping and how it can be used to improve concurrency.

Answer: Lock stripping is a technique used to improve concurrency by dividing the lock on a data structure into several finer-grained locks on its constituent parts. Instead of locking the entire structure, threads can lock only parts of it they are accessing or modifying. This approach reduces contention and increases the potential for concurrency, as multiple threads can work on different parts of the data structure simultaneously.

Key Points:
- Reduces lock contention by using finer-grained locks.
- Increases concurrency and performance in multi-threaded environments.
- Often used in implementations of concurrent collections.

Example:

// Hypothetical example illustrating the concept
public class LockStrippingExample {
    private final Object[] locks = new Object[16]; // Array of locks for different segments
    private final int[] data = new int[16]; // Shared data structure

    public LockStrippingExample() {
        for (int i = 0; i < locks.length; i++) {
            locks[i] = new Object();
        }
    }

    public void update(int index, int value) {
        synchronized(locks[index % locks.length]) { // Locking only the relevant segment
            data[index] = value;
        }
    }
}