Question 1

What are the main principles of Object-Oriented Programming in Java?

Accepted Answer

OOP principles include Encapsulation, Inheritance, Polymorphism, and Abstraction. They help structure code into reusable and maintainable components.

Question 2

What is the difference between Thread and Runnable in Java?

Accepted Answer

`Thread` is a class, while `Runnable` is an interface. Implementing Runnable is preferred for sharing resources and extending another class.

Question 3

What is the difference between ArrayList and LinkedList?

Accepted Answer

ArrayList is backed by a dynamic array, offering fast random access, while LinkedList uses a doubly linked list, better for frequent insertions/deletions.

Question 4

What is the difference between checked and unchecked exceptions?

Accepted Answer

Checked exceptions must be declared or handled (e.g., IOException), while unchecked exceptions (e.g., NullPointerException) occur at runtime.

Question 5

What is the Java Memory Model and how does it relate to concurrency?

Accepted Answer

The Java Memory Model defines how threads interact with memory. It ensures visibility and atomicity, especially in multithreaded environments.

Question 6

What is polymorphism in Java?

Accepted Answer

Polymorphism allows objects to take many forms. It includes method overloading (compile-time) and overriding (runtime).

Question 7

Explain the purpose of the `final` keyword.

Accepted Answer

`final` can be applied to: variables (constant), methods (cannot be overridden), classes (cannot be extended).

Question 8

What are abstract classes and interfaces? Compare them.

Accepted Answer

Abstract classes can have concrete methods and state. Interfaces (pre-Java 8) had only abstract methods; Java 8+ allows default/static methods. A class can implement multiple interfaces but extend only one abstract class.

Question 9

What is the diamond problem in Java?

Accepted Answer

The diamond problem occurs with multiple inheritance of classes. Java avoids it by not allowing multiple class inheritance, but it can occur with interfaces (resolved by explicit overriding).

Question 10

What is the difference between `synchronized` and `Lock`?

Accepted Answer

`synchronized` is a language keyword; `Lock` is an interface (ReentrantLock) offering more flexibility like tryLock, interruptible locks, and fairness.

Java Interview Questions & Answers

What are the main principles of Object-Oriented Programming in Java?

What is the difference between Thread and Runnable in Java?

What is the difference between ArrayList and LinkedList?

What is the difference between checked and unchecked exceptions?

What is the Java Memory Model and how does it relate to concurrency?

What is polymorphism in Java?

Explain the purpose of the `final` keyword.

What are abstract classes and interfaces? Compare them.

What is the diamond problem in Java?

What is the difference between `synchronized` and `Lock`?

Explain the concept of deadlock and how to prevent it.

What is the difference between `wait()` and `sleep()`?

What is the difference between `HashMap` and `Hashtable`?

Explain how `HashMap` works internally.

What are the differences between `Set`, `List`, and `Map`?

What is the `try-with-resources` statement?

What is the difference between `throw` and `throws`?

What are generics and why are they used?

Explain the concept of reflection in Java.

What is the difference between `String`, `StringBuilder`, and `StringBuffer`?

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