Core Java Interview Questions & Answers – Part 2 (Advanced Basics with Clear Use Cases)

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21. What is the final keyword and where is it used?

Answer: final is used to restrict modification.

Where applied:

• final variable: Value cannot change
• final method: Cannot be overridden
• final class: Cannot be inherited

Why used:

• Security
• Prevent misuse
• Maintain consistent behavior

Use case: Constants like tax rate, application version.

final double TAX_RATE = 0.15;

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22. What is the difference between == and equals()?

Answer:

• == compares memory reference
• equals() compares actual value

Why important: Avoid logical bugs when comparing objects.

Use case: User input comparison, database record checks.

String a = new String("Java");
String b = new String("Java");
a.equals(b); // true
a == b;      // false

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23. Why is String immutable in Java?

Answer: Once created, a String cannot be changed.

Why Java designed it this way:

• Security (passwords, URLs)
• Thread safety
• Performance optimization (String pool)

Use case: Login credentials, configuration keys.

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24. Difference between String, StringBuilder, and StringBuffer

Answer:

• String: Immutable
• StringBuilder: Mutable, fast, not thread-safe
• StringBuffer: Mutable, thread-safe, slower

Use case:

• String → Fixed text
• StringBuilder → Loop concatenation
• StringBuffer → Multithreaded environment

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25. What is the Java Memory Model?

Answer: Java memory is divided into areas to manage data efficiently.

Main parts:

• Heap → Objects
• Stack → Method calls, local variables
• Method Area → Class metadata

Why important: Helps understand performance and memory leaks.

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26. What is a Wrapper Class?

Answer: Wrapper classes convert primitive data types into objects.

Why needed: Collections work only with objects.

Use case: Storing numbers in ArrayList.

Integer num = 10;

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27. What is Autoboxing and Unboxing?

Answer:

• Autoboxing: Primitive → Object
• Unboxing: Object → Primitive

Why useful: Simplifies code and improves readability.

Integer a = 5;  // Autoboxing
int b = a;      // Unboxing

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28. What is this keyword and why is it needed?

Answer: this refers to the current object.

Why used:

• Avoid variable confusion
• Call current class methods or constructors

Use case: Constructor parameter naming.

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29. What is super keyword?

Answer: super refers to the parent class object.

Why used:

• Access parent fields/methods
• Call parent constructor

Use case: Extending framework classes.

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30. What is a Package in Java?

Answer: Package groups related classes.

Why used:

• Avoid naming conflicts
• Better project organization
• Access control

Use case: java.util, java.lang

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31. What is Access Modifier?

Answer: Access modifiers control visibility.

Modifier	Scope
private	Same class
default	Same package
protected	Package + subclass
public	Everywhere

Why important: Security and clean design.

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32. What is a Java Bean?

Answer: A Java Bean is a reusable class with:

• Private variables
• Public getters/setters
• No-arg constructor

Use case: Data transfer between layers (DTOs).

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33. What is Serialization?

Answer: Serialization converts object into byte stream.

Why used:

• Save object to file
• Send object over network

Use case: Session management, caching.

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34. What is transient keyword?

Answer: transient prevents a variable from being serialized.

Use case: Passwords, sensitive data.

transient String password;

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35. What is HashMap vs Hashtable?

Answer:

• HashMap → Not synchronized, faster
• Hashtable → Synchronized, slower

Use case:

• HashMap → Single-threaded apps
• Hashtable → Legacy systems

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36. What is ConcurrentHashMap?

Answer: Thread-safe map with better performance than Hashtable.

Use case: High-concurrency server applications.

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37. What is Synchronization?

Answer: Synchronization ensures one thread accesses shared resource at a time.

Why needed: Avoid data inconsistency.

Use case: Bank transactions.

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38. What is volatile keyword?

Answer: Ensures visibility of variable changes across threads.

Why important: Prevents thread caching issues.

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39. What is Deadlock?

Answer: Deadlock occurs when two threads wait for each other forever.

Why dangerous: Application freezes.

Use case: Improper lock ordering.

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40. Why Java does not support Multiple Inheritance?

Answer: To avoid diamond problem ambiguity.

Solution provided by Java: Interfaces allow multiple inheritance safely.

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