Core Java Interview Questions

Ultimate Guide to Java Interview Questions

If you're preparing for a Java interview, mastering these questions will give you a strong foundation. This post covers all critical Java topics, from Core Java concepts to Advanced topics, including coding challenges. Let's dive in!

Here’s a detailed explanation of the requested topics:

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Core Java Basics

Key Features of Java:

1. Platform Independent: Java code runs on any platform using the JVM.
2. Object-Oriented: Encourages encapsulation, inheritance, and polymorphism.
3. Simple and Easy to Learn: Syntax similar to C++ but without complexity like pointers.
4. Secure: Provides runtime checking, bytecode verification, and a security manager.
5. Robust: Exception handling and garbage collection help avoid errors.
6. Multithreaded: Supports concurrent programming with built-in thread management.
7. High Performance: Uses Just-In-Time (JIT) compiler.
8. Distributed and Networked: Simplifies distributed system development using APIs.
9. Dynamic and Extensible: Adapts to evolving requirements through dynamic linking.

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Difference Between JDK, JRE, and JVM:

1. JDK (Java Development Kit): Tools for developing and debugging Java programs, including JRE and compilers.
2. JRE (Java Runtime Environment): Executes Java applications; includes JVM and libraries.
3. JVM (Java Virtual Machine): Runs Java bytecode; ensures platform independence.

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Difference Between == and .equals():

• ==: Compares references (memory locations) for objects.
• .equals(): Compares content or state of objects. Can be overridden.

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Class and Object in Java:

• Class: Blueprint defining the properties (fields) and behaviors (methods).
• Object: Instance of a class, representing a real-world entity.

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public static void main(String[] args) Explained:

• public: Accessible from anywhere.
• static: No need to instantiate the class.
• void: No return value.
• main: Entry point of the program.
• String[] args: Command-line arguments.

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Difference Between String, StringBuilder, and StringBuffer:

1. String: Immutable; thread-safe.
2. StringBuilder: Mutable; not thread-safe but faster.
3. StringBuffer: Mutable; thread-safe with synchronized methods.

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Access Modifiers:

1. Private: Visible within the class.
2. Default: Visible within the package.
3. Protected: Visible within the package and subclasses.
4. Public: Visible everywhere.

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Difference Between final, finally, and finalize:

• final: Used to declare constants, prevent inheritance, or override.
• finally: Block to ensure cleanup after a try block.
• finalize(): Method invoked by the garbage collector before destroying an object.

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Object-Oriented Programming (OOP)

Four Pillars of OOP:

1. Encapsulation: Restricts access to internal details. Example: Private fields with public getters/setters.
2. Abstraction: Hides complexity using interfaces or abstract classes. Example: List interface.
3. Inheritance: Reuse code by extending parent classes. Example: class Dog extends Animal.
4. Polymorphism: Behavior changes based on the object. Example: Method overriding.

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Difference Between Abstraction and Encapsulation:

• Abstraction: Focuses on "what" an object does.
• Encapsulation: Focuses on "how" the data is protected.

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Method Overloading vs. Overriding:

• Overloading: Same method name, different parameters.
• Overriding: Subclass modifies a parent class method.

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Interface vs. Abstract Class:

• Interface: 100% abstraction; methods are implicitly abstract.
• Abstract Class: Can have abstract and concrete methods.

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Multiple Interfaces: A class can implement multiple interfaces using the implements keyword.

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Constructor Overloading Example:

class Example {
    Example() { }
    Example(int a) { }
    Example(String s) { }
}

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super and this:

• this: Refers to the current object.
• super: Refers to the parent class object.

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Collections and Data Structures

ArrayList vs. LinkedList:

• ArrayList: Faster for random access.
• LinkedList: Faster for insertions/deletions.

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HashMap, LinkedHashMap, TreeMap:

• HashMap: Unordered, fastest.
• LinkedHashMap: Maintains insertion order.
• TreeMap: Sorted order.

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Significance of hashCode() and equals() in HashMap:

• Ensures proper key-value mapping by preventing duplicates.

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Iterator vs. ListIterator:

• Iterator: Traverses in one direction.
• ListIterator: Traverses in both directions.

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Fail-Fast vs. Fail-Safe:

• Fail-Fast: Concurrent modification triggers ConcurrentModificationException.
• Fail-Safe: Works on a copy, no exception.

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Internal Working of HashMap: Uses buckets for storage, hashing for key-value mapping, and linked lists to resolve collisions.

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Multithreading and Concurrency

Creating Threads in Java:

• Extending Thread:

  class MyThread extends Thread { public void run() { } }

• Implementing Runnable:

  class MyRunnable implements Runnable { public void run() { } }

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synchronized Method vs. Block:

• Method: Locks entire method.
• Block: Locks a specific section.

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wait(), notify(), notifyAll():

• Used for inter-thread communication.

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Volatile Keyword: Ensures visibility of changes across threads.

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Exception Handling

Checked vs. Unchecked Exceptions:

• Checked: Caught during compile-time (IOException).
• Unchecked: Occurs during runtime (ArithmeticException).

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Custom Exception Example:

class MyException extends Exception { MyException(String msg) { super(msg); } }

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Java 8+ Features

Lambda Expression:

(numbers) -> numbers.stream().mapToInt(Integer::intValue).sum();

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Need a deeper dive into specific areas? Let me know!
Common Coding Questions

• Write a program to check if a string is a palindrome.
• Write a program to find the second-largest element in an array.
• Write a program to reverse a linked list.
• Write a program to implement a thread-safe Singleton class.
• Write a program to find the frequency of characters in a string.
• Write a program to demonstrate deadlock and resolve it.
• Write a program to check if two strings are anagrams.