Java Coding Interview Questions and Answers for 5 years experience

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100 Java Interview Questions & Answers (5 Years Experience)

  1. What is the difference between `==` and `.equals()` in Java?

    • Answer: `==` compares memory addresses for primitive data types and object references. `.equals()` compares the content of objects. For Strings and other objects, you should always use `.equals()` for content comparison. Overriding `.equals()` is crucial for proper object comparison.

  2. Explain the concept of garbage collection in Java.

    • Answer: Garbage collection is the automatic memory management process in Java. It reclaims memory occupied by objects that are no longer reachable by the program. This prevents memory leaks. The JVM has a garbage collector that runs periodically, identifying and freeing unused memory.

  3. What are the different types of garbage collectors in Java?

    • Answer: Different JVMs offer various garbage collectors like Serial, Parallel, CMS (Concurrent Mark Sweep), G1GC (Garbage-First Garbage Collector), ZGC (Z Garbage Collector), and Shenandoah. Each has its strengths and weaknesses regarding throughput, latency, and memory footprint. The choice depends on the application's requirements.

  4. What is the difference between `HashMap` and `TreeMap`?

    • Answer: `HashMap` provides constant time complexity O(1) for basic operations (get, put, remove) on average, but the order of elements is not guaranteed. `TreeMap` is based on a tree structure (Red-Black tree), providing O(log n) complexity for these operations but keeps elements sorted according to their keys (natural ordering or a custom Comparator).

  5. What is the difference between `ArrayList` and `LinkedList`?

    • Answer: `ArrayList` uses a dynamic array, providing fast random access (O(1)) but slower insertion/deletion in the middle (O(n)). `LinkedList` uses a doubly linked list, allowing fast insertion/deletion (O(1)) anywhere, but slower random access (O(n)).

  6. Explain the concept of multithreading in Java.

    • Answer: Multithreading allows multiple threads to execute concurrently within a single program. This improves performance, especially in I/O-bound or CPU-bound applications. Threads share the same memory space, requiring careful synchronization to avoid race conditions.

  7. What are the ways to achieve thread synchronization in Java?

    • Answer: Synchronization can be achieved using `synchronized` blocks or methods, `ReentrantLock`, `Semaphore`, `CountDownLatch`, and other concurrency utilities. These mechanisms control access to shared resources, preventing race conditions and ensuring data consistency.

  8. What is a deadlock in Java? How can you prevent it?

    • Answer: A deadlock occurs when two or more threads are blocked indefinitely, waiting for each other to release the resources that they need. Prevention strategies include avoiding circular dependencies in resource locking, acquiring locks in a consistent order, using timeouts for acquiring locks, and employing deadlock detection and recovery mechanisms.

  9. Explain the difference between `throws` and `throw` keywords in Java.

    • Answer: `throws` is used in a method signature to declare that the method might throw certain checked exceptions. The caller of the method is then responsible for handling these exceptions. `throw` is used to explicitly throw an exception from within a method.

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