Java Interview Questions and Answers (Part-III)

71. What is the difference between notify() and notifyAll() in Java?

• notify() wakes a single thread that is waiting on the object’s monitor.
  
• notifyAll() wakes all threads that are waiting on the monitor.
  

🔹 Use notify() when only one thread needs to resume, and notifyAll() when multiple waiting threads should proceed.

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Q72. What are the two types of exceptions in Java?

Java categorizes exceptions into:

1. Checked Exceptions – Handled at compile time (e.g., IOException, SQLException).
   
2. Unchecked Exceptions – Occur at runtime (e.g., NullPointerException, ArrayIndexOutOfBoundsException) and don’t require explicit handling.
   

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Q73. What does OutOfMemoryError indicate in Java?

It’s a runtime error that occurs when the Java Virtual Machine (JVM) exhausts available memory.

• Common causes include memory leaks, huge object allocation, or infinite object creation loops.
  

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Q74. Difference between == and .equals() in Java?

• == compares memory references, checking if two variables point to the same object.
  
• .equals() compares the actual content or value inside the objects.
  

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Q75. How can multiple strings be concatenated in Java?

Popular methods for joining strings include:

1. + Operator – Easy but inefficient for loops.
   
2. concat() – Used with String objects.
   
3. StringBuilder – Best for performance.
   
4. String.join() – Clean option introduced in Java 8.
   

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Q76. Array vs Collection – What’s the distinction?

• Array: Fixed-size data structure holding similar data types.
  
• Collection: Part of Java Collection Framework (e.g., ArrayList, HashSet), supports dynamic resizing and various object types.
  

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Q77. What is a BlockingQueue in Java?

A thread-safe queue that:

• Blocks when retrieving from an empty queue.
  
• Blocks when inserting into a full queue (in bounded queues).
  

🔹 Ideal for producer-consumer problems. Examples: LinkedBlockingQueue, ArrayBlockingQueue.

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Q78. Process vs Thread – What’s the difference?

• Process: Independent program with its own memory space.
  
• Thread: A lightweight sub-task of a process, sharing memory with other threads within the same process.
  

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Q79. Key benefits of multithreading in Java?

• Increases CPU efficiency.
  
• Reduces execution time.
  
• Enhances application responsiveness.
  
• Helps handle concurrent I/O operations smoothly.
  

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Q80. What is context switching in Java multithreading?

The CPU switches between threads, saving the state of one and loading another.

• It enables multitasking but incurs a performance cost due to overhead.
  

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Q81. Differences between Array and ArrayList?

• Array:
  
  • Fixed size.
    
  • Supports both primitives and objects.
    
• ArrayList:
  
  • Dynamically resizable.
    
  • Stores only objects.
    
  • Offers built-in utility methods (like add(), remove()).
    

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Q82. Why is volatile used in Java?

• Guarantees visibility of changes to variables across threads.
  
• Prevents caching and instruction reordering.
  
• Used to maintain thread safety for simple variables.
  

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Q83. What is Java NIO (New I/O)?

Java NIO provides a faster, non-blocking I/O mechanism:

• Uses Channels and Buffers instead of streams.
  
• Selectors allow one thread to manage multiple channels.
  

🔹 Excellent for high-performance I/O operations.

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Q84. Compare String, StringBuilder, and StringBuffer.

Type	Mutable	Thread-Safe	Performance
String	No	Yes	Slow
StringBuilder	Yes	No	Fast
StringBuffer	Yes	Yes	Moderate

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Q85. Difference between Runnable and Callable?

• Runnable:
  
  • No return value.
    
  • Cannot throw checked exceptions.
    
• Callable:
  
  • Returns a result via Future<T>.
    
  • Can throw checked exceptions.
    

🔹 Callable is used in ExecutorService for asynchronous programming.

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Q86. What is a checked exception in Java?

Checked exceptions are:

• Checked at compile time.
  
• Must be declared with throws or handled using try-catch.
  
• Examples: IOException, ClassNotFoundException.
  

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Q87. How do static and final keywords differ?

• static:
  
  • Belongs to the class, not instances.
    
  • Common across all objects.
    
• final:
  
  • Used to declare constants.
    
  • Prevents method overriding and class inheritance.
    

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Q88. What are unchecked exceptions?

• Not checked at compile time.
  
• Occur due to logical errors or bad programming.
  
• Examples: ArithmeticException, NullPointerException.
  

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Q89. What are Java Lambda Expressions?

Lambdas offer a functional style of programming (Java 8+):

• Allow defining anonymous functions in a concise way.
  
• Commonly used with functional interfaces.
  

java

(int a, int b) -> a + b;

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Q90. What is the use of try-catch block in Java?

• Handles runtime exceptions gracefully.
  
• Prevents abnormal program termination.
  

java

try {

   int result = 10 / 0;

} catch (ArithmeticException e) {

   System.out.println(“Cannot divide by zero.”);

}

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