10 Must-Know Java Coding Interview Questions

Easy Questions

1. What are the 5 pillars of OOP?

• Encapsulation
• Abstraction
• Inheritance
• Polymorphism
• Reusability

2. What is the difference between a constructor and a method?

• A constructor is a special method that is used to initialize an object.
• A method is a function that is defined inside a class.

3. What is the difference between a static and a non-static method?

• A static method is a method that is defined within a class, but does not require an instance of the class to be called.
• A non-static method is a method that is defined within a class and requires an instance of the class to be called.

Medium Questions

4. What is the difference between an interface and an abstract class?

• An interface is a contract that defines a set of methods that a class must implement.
• An abstract class is a class that defines a set of abstract methods that must be implemented by its subclasses.

5. What is the difference between a final class and a non-final class?

• A final class cannot be extended by other classes.
• A non-final class can be extended by other classes.

6. What is the difference between a private, protected, and public member?

• A private member can only be accessed by the class in which it is defined.
• A protected member can be accessed by the class in which it is defined and its subclasses.
• A public member can be accessed by any class.

Hard Questions

7. What is the difference between a stack and a queue?

• A stack is a data structure that follows the Last In First Out (LIFO) principle, meaning the last item added is the first item removed.
• A queue is a data structure that follows the First In First Out (FIFO) principle, meaning the first item added is the first item removed.

8. What is a binary search tree?

• A binary search tree is a data structure that stores data in a way that allows for efficient searching and retrieval.
• It is implemented using a binary tree, which is a tree in which each node has a maximum of two children.

9. What is a hashtable?

• A hashtable is a data structure that maps keys to values.
• It is implemented using an array of buckets, each of which contains a list of key-value pairs.

10. What is a linked list?

• A linked list is a data structure that stores data in a linear fashion.
• It is implemented using a series of nodes, each of which contains a value and a pointer to the next node.