Java Interview Questions and Answers (Part-IV)

 Q91. What is the difference between Collection and Collections in Java?

Answer:

• Collection is a core interface in the Java Collection Framework that defines common behaviors for data structures like List, Set, and Queue.
  
• Collections is a utility class in java.util that offers static methods for operations such as sorting, reversing, shuffling, and synchronizing collections.
  

Example: Collections.sort(myList);

Keywords: Java Collection vs Collections, Java utility classes

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Q92. What is the use of the finalize() method in Java?

Answer:

• The finalize() method is called by the Garbage Collector just before an object is removed from memory.
  
• Used to release system resources like file handles or database connections.
  
• Deprecated in Java 9, as it’s unpredictable and can delay garbage collection.
  

It’s recommended to use try-with-resources or explicit resource management instead.

Keywords: Java finalize method, garbage collection cleanup

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Q93. What is Java 8 Optional and how is it used?

Answer:

• Optional<T> is a container object that may or may not hold a non-null value.
  
• Prevents NullPointerException by explicitly checking for presence.
  
• Common methods: of(), ofNullable(), isPresent(), orElse(), map().
  

java

Optional<String> name = Optional.ofNullable(user.getName());

System.out.println(name.orElse(“Anonymous”));

Keywords: Java Optional example, avoid null checks in Java

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Q94. What is the Java Collection Framework?

Answer:

• A comprehensive architecture for storing and manipulating groups of objects.
  
• Includes interfaces like List, Set, Map, and classes like ArrayList, HashSet, HashMap.
  
• Supports sorting, searching, iteration, and thread-safe collections via Concurrent APIs.
  

Keywords: Java Collection Framework overview, Java data structures

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Q95. What is CompletableFuture in Java 8?

Answer:

• CompletableFuture is part of Java’s asynchronous programming model.
  
• Enables non-blocking execution of tasks with methods like thenApply(), thenAccept(), and thenCompose().
  
• Internally uses ForkJoinPool for efficient thread management.
  

java

CompletableFuture.supplyAsync(() -> “Data”)

                 .thenApply(data -> data + ” processed”)

                 .thenAccept(System.out::println);

Keywords: Java CompletableFuture example, asynchronous programming Java

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Q96. What is multithreading in Java?

Answer:

• Multithreading allows the parallel execution of tasks, improving performance and responsiveness.
  
• Threads can be created using:
  
  • Thread class
    
  • Runnable interface
    
  • ExecutorService or ForkJoinPool
    
• Java provides synchronization tools to handle shared resources safely.
  

Keywords: Java multithreading tutorial, creating threads in Java

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Q97. What is the purpose of the strictfp keyword in Java?

Answer:

• Ensures platform-independent floating-point calculations by enforcing IEEE 754 standards.
  
• Can be applied to classes or methods but not variables.
  
• Guarantees consistent results across different systems.
  

java

strictfp class Calculator { /* consistent float logic */ }

Keywords: strictfp keyword in Java, float consistency Java

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Q98. What is the difference between sleep() and wait()?

Answer:

Feature	sleep()	wait()
Lock Handling	Retains the lock	Releases the lock
Belongs to	Thread class	Object class
Wake Mechanism	Auto after timeout	Must be notified using notify()

sleep() is used to pause execution, while wait() is used for thread communication.

Keywords: Java wait vs sleep, thread synchronization

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Q99. Difference between LinkedHashSet and TreeSet?

Answer:

• LinkedHashSet maintains insertion order using a linked list.
  
• TreeSet maintains sorted order (natural or via Comparator).
  
• Performance:
  
  • LinkedHashSet: O(1) for basic operations.
    
  • TreeSet: O(log n) due to Red-Black Tree.
    
• Nulls:
  
  • LinkedHashSet: allows null.
    
  • TreeSet: does not allow null.
    

Keywords: LinkedHashSet vs TreeSet, Java Set types comparison

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Q100. What is synchronization in Java?

Answer:
Synchronization is a concurrency control mechanism that ensures thread safety by restricting access to shared resources.

• Prevents race conditions.
  
• Achieved using the synchronized keyword:
  
  • Synchronized blocks
    
  • Synchronized methods
    
• Alternatives include:
  
  • ReentrantLock
    
  • volatile variables
    
  • AtomicInteger, ConcurrentHashMap
    

java

synchronized void update() {

  // only one thread can enter

}

Keywords: Java synchronization, thread safety techniques