Polymorphism in Java

📍 Polymorphism allows a single interface to be used for different types of actions.

📍 It is of two types:

Method Overloading (Compile-time Polymorphism)

Method Overriding (Runtime Polymorphism)


*Method Overloading* (Same method name, different parameters)

class MathOperations {
int add(int a, int b) {
return a + b;
}

double add(double a, double b) {
return a + b;
}
}

*Method Overriding* (Same method in parent and child class)

class Animal {
void makeSound() {
System.out.println("Animal makes a sound");
}
}

class Dog extends Animal {
void makeSound() {
System.out.println("Dog barks");
}
}

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1. What is Java?
2. What are the features of Java?
3. What are the OOP concepts?
4. What is data encapsulation, and why is it useful?
5. What is polymorphism?
6. What are the types of polymorphism, and how do they differ?
7. What does an interface in Java refer to?
8. What is a class in Java?
9. What is a Java object?
10. What is a singleton class, and how can it be used?
11. What is a constructor in Java?
12. What does the term constructor overloading mean?
13. How are non-primitive variables used in Java?
14. In Java, what is a static variable?
15. What are Java data types, and how are they grouped?
16. How do you define primitive data types and describe each by size and description?
17. What do the terms autoboxing and unboxing mean in Java?
18. What are wrapper classes in Java?
19. What is inheritance in Java?
20. What types of inheritance does Java support?
21. What is Java exception handling?
22. What are the differences between unchecked exceptions, checked exceptions, and errors?
23. What are loops in Java?
24. What are the types of loops in Java, and how are they used?
25. What is an infinite loop?
26. What is the difference between the continue and break statement?
27. What is the entry point in Java, and how is it written?
28. In Java, what’s the purpose of static methods and static variables?
29. How do you use, call, and access a static method in Java?
30. How do you use, call, and access a non-static method in Java?

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Leetcode patterns you should definitely checkout to Learn DSA(Java) from scratch

1️⃣ Arrays: Data structures, such as arrays, store elements in contiguous memory locations. They are versatile and useful for a wide variety of purposes.
LeetCode Problems:
• Search in Rotated Sorted Array (Problem #33)
• Product of Array Except Self (Problem #238)
• Find the Missing Number (Problem #268)

2️⃣Two Pointers: In Two Pointers, two pointers are maintained in the collection and can be manipulated to solve a problem efficiently.
LeetCode problems:
• Trapping Rain Water (Problem #42)
• Longest Substring Without Repeating Characters (Problem #3)
• Squares of a Sorted Array (Problem #977)

3️⃣In-place Linked List Traversal: As an explanation, in-place traversal is a technique for modifying linked list nodes without using extra space.
LeetCode Problems:
• Remove Nth Node From End of List (Problem #19)
• Reorder List (Problem #143)

4️⃣Fast & Slow Pointers: This pattern uses two pointers to traverse a sequence at different speeds (fast and slow), often used to detect cycles or find a specific position in the sequence.
LeetCode Problems:
• Happy Number (Problem #202)
• Subarray Sum Equals K (Problem #560)
• Intersection of Two Linked Lists (Problem #160)

5️⃣Merge Intervals: This pattern involves merging overlapping intervals in a collection, often used in problems dealing with intervals or ranges.
LeetCode problems:
• Non-overlapping Intervals (Problem #435)
• Minimum Number of Arrows to Burst Balloons (Problem #452)

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Typical java interview questions sorted by experience


Junior
* Name some of the characteristics of OO programming languages
* What are the access modifiers you know? What does each one do?
* What is the difference between overriding and overloading a method in Java?
* What’s the difference between an Interface and an abstract class?
* Can an Interface extend another Interface?
* What does the static word mean in Java?
* Can a static method be overridden in Java?
* What is Polymorphism? What about Inheritance?
* Can a constructor be inherited?
* Do objects get passed by reference or value in Java? Elaborate on that.
* What’s the difference between using == and .equals on a string?
* What is the hashCode() and equals() used for?
* What does the interface Serializable do? What about Parcelable in Android?
* Why are Array and ArrayList different? When would you use each?
* What’s the difference between an Integer and int?
* What is a ThreadPool? Is it better than using several “simple” threads?
* What the difference between local, instance and class variables?

Mid
* What is reflection?
* What is dependency injection? Can you name a few libraries? (Have you used any?)
* What are strong, soft and weak references in Java?
* What does the keyword synchronized mean?
* Can you have “memory leaks” on Java?
* Do you need to set references to null on Java/Android?
* What does it means to say that a String is immutable?
* What are transient and volatile modifiers?
* What is the finalize() method?
* How does the try{} finally{} works?
* What is the difference between instantiation and initialisation of an object?
* When is a static block run?
* Why are Generics are used in Java?
* Can you mention the design patterns you know? Which of those do you normally use?
* Can you mention some types of testing you know?

Senior
* How does Integer.parseInt() works?
* Do you know what is the “double check locking” problem?
* Do you know the difference between StringBuffer and StringBuilder?
* How is a StringBuilder implemented to avoid the immutable string allocation problem?
* What does Class.forName method do?
* What is Autoboxing and Unboxing?
* What’s the difference between an Enumeration and an Iterator?
* What is the difference between fail-fast and fail safe in Java?
* What is PermGen in Java?
* What is a Java priority queue?
* *s performance influenced by using the same number in different types: Int, Double and Float?
* What is the Java Heap?
* What is daemon thread?
* Can a dead thread be restarted?

Source: medium.

🌟 Applications of Java You Should Know 🌟

Java's versatility makes it a go-to language for a variety of applications. Here are the top areas where Java shines:

1️⃣ Mobile Applications:

- Powering Android apps through frameworks like Android SDK.

2️⃣ Desktop GUI Applications:

- Used to create user-friendly graphical interfaces with tools like JavaFX and Swing.

3️⃣ Web-based Applications:

- Building robust web apps using frameworks like Spring, Hibernate, and Struts.

4️⃣ Enterprise Applications:

- Ideal for large-scale business solutions like CRM, ERP systems, using Java EE.

5️⃣ Scientific Applications:

- Java’s precision and reliability make it suitable for scientific research and simulations.

6️⃣ Gaming Applications:

- Java is used for creating cross-platform games and 2D/3D game engines.

7️⃣ Big Data Technologies:

- Supporting frameworks like Hadoop and Apache Kafka for data processing.

8️⃣ Business Applications:

- Simplifies tasks like inventory management and customer service applications.

9️⃣ Distributed Applications:

- Java helps create applications that can run on multiple servers, ensuring scalability.

🔟 Cloud-based Applications:

- Java is widely used for building SaaS, IaaS, and PaaS platforms.

💡 *Why Java?*
Its platform independence, security, and scalability make it ideal for these diverse applications. 🚀

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Java Basics every beginner should learn to build a strong foundation:

1. Hello World & Setup

Install JDK and an IDE (like IntelliJ or Eclipse)

Write your first program: public class HelloWorld

2. Data Types & Variables

Primitive types: int, double, char, boolean

Non-primitive types: String, Arrays, Objects

Type casting (implicit & explicit)

3. Operators

Arithmetic: + - * / %

Comparison: == != > < >= <=

Logical: && || !

4. Control Flow

If, else if, else

Switch-case

Loops: for, while, do-while

break and continue

5. Functions (Methods)

Syntax: public static returnType methodName(params)

Method overloading

Return types & parameter passing

6. Object-Oriented Programming (OOP)

Classes & Objects

this keyword

Constructors (default & parameterized)

7. OOP Concepts

Encapsulation (private variables + getters/setters)

Inheritance (extends keyword)

Polymorphism (method overriding)

Abstraction (abstract classes & interfaces)

8. Arrays & ArrayList

Declaring and iterating arrays

ArrayList methods: add, remove, get, size

Multidimensional arrays

9. Exception Handling

Try-catch-finally blocks

throw and throws

Custom exceptions

10. Basic Input/Output

Scanner class for user input

System.out.println() for output

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