Java Interview Guide - Part 1: Core Questions (1–10)

A comprehensive list of essential Java interview questions covering OOP, memory management, exceptions, collections

Java Interview Guide - Part 1: Core Questions (1–10)

☕ Java Programming Essentials

1. What is Java?

Java is a high-level, object-oriented, and platform-independent programming language developed by Sun Microsystems (now owned by Oracle) in 1995. It is widely used for building enterprise-scale applications, mobile apps, and web-based software.

🔑 Key Features of Java

  • Platform Independent:
    Java code is compiled into bytecode, which can run on any system with a Java Virtual Machine (JVM). This is known as Write Once, Run Anywhere (WORA).

  • Object-Oriented:
    Java follows the principles of Object-Oriented Programming (OOP) such as encapsulation, inheritance, and polymorphism.

  • Robust:
    Java has strong memory management, automatic garbage collection, and exception handling to create reliable applications.

  • Multithreaded:
    Java supports multithreading, allowing concurrent execution of two or more threads for maximum CPU utilization.

  • Secure:
    Java eliminates the use of explicit pointers and runs inside a secure virtual machine environment.


2. 🌐 Principles of Object-Oriented Programming (OOP)

PrincipleDescription
EncapsulationBundling data and methods that operate on the data into a single unit (class).
AbstractionHiding internal implementation and showing only essential features.
InheritanceMechanism where one class acquires properties and behaviors of another.
PolymorphismAbility to take many forms—same interface, different implementations.

3. 🧰 Difference Between JDK, JRE, and JVM

ComponentDescription
JDK (Java Development Kit)Full-featured software development kit including compiler, debugger, and tools to develop Java applications.
JRE (Java Runtime Environment)Provides libraries and JVM to run Java applications. It does not include development tools.
JVM (Java Virtual Machine)Executes Java bytecode and provides platform independence.

4. 🖥️ Platform Independence in Java

Java achieves platform independence through bytecode. The Java compiler converts source code into bytecode, which is interpreted by the JVM. Since JVMs are available for many platforms, the same bytecode can run on any OS.


5. 🏁 Significance of the main() Method

The main() method is the entry point of any standalone Java application.

public static void main(String[] args)

Breakdown:

  • public: Accessible from anywhere.
  • static: Can be called without creating an object.
  • void: Does not return any value.
  • String[] args: Accepts command-line arguments.

6. 🧠 Java Memory Management

Java handles memory through automatic garbage collection. Key points:

  • Objects are created in heap memory.
  • The Garbage Collector (GC) automatically deallocates memory for objects no longer in use.
  • Developers don’t need to manually free memory, reducing memory leaks and errors.

7. 🏗️ Constructors in Java

Constructors are special methods used to initialize objects.

Characteristics:

  • Same name as the class.
  • No return type (not even void).
  • Called automatically when an object is created.

Difference from Methods:

ConstructorsMethods
Initialize objectsPerform actions
No return typeMay return a value
Called once during object creationCan be called multiple times

8. 🔁 Method Overloading vs Overriding

✅ Method Overloading (Compile-time Polymorphism)

Same method name with different parameter lists.

class Example {
    void display(int a) { }
    void display(String b) { }
}

🔄 Method Overriding (Runtime Polymorphism)

Subclass provides a specific implementation of a method already defined in the superclass.

class Parent {
    void display() { }
}

class Child extends Parent {
    @Override
    void display() { }
}

9. 🧬 Inheritance in Java

Inheritance allows a class to reuse the fields and methods of another class using the extends keyword.

Types of Inheritance:

TypeDescription
SingleOne class inherits from another.
MultilevelA class inherits from a class which itself inherits from another.
HierarchicalMultiple classes inherit from a single superclass.
Multiple (via Interfaces)A class implements multiple interfaces (Java does not support multiple class inheritance).

10. 🔣 Polymorphism in Java

Polymorphism allows objects to be treated as instances of their parent class, enabling flexibility and reusability.

Types:

TypeDescriptionExample
Compile-timeMethod Overloadingdisplay(int) vs display(String)
RuntimeMethod OverridingSubclass overrides superclass method

> 💡 Java's design philosophy emphasizes **portability**, **security**, and **simplicity**, making it one of the most enduring and widely-used programming languages in the world.

📌 What’s Next?

👉 Explore detailed answers, code samples, use cases, and illustrations in upcoming parts of this series:

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