Java for Enterprise Applications: Key Interview Questions

Introduction

• Java is a popular choice for enterprise-level applications, making it crucial for job seekers in big MNCs.
• Success stories from students can be motivating; like, subscribe, and share your experiences.
• Here are 30 foundational Java interview questions to prepare for.

Chapter 1: Java Basics

Key Concepts Covered:

• JDK, JRE, JVM
• Main Method, Byte Code, Data Types

What is Java?

• Java: High-level, object-oriented programming language
• High-level because it's compiled into byte code (intermediate) and then into machine/native code (low-level).
• Object-oriented because it uses objects and classes to structure data and functions.
• OOP Concepts: Encapsulation, Abstraction, Polymorphism, Inheritance

JDK, JRE, and JVM

• JDK: Java Development Kit containing tools like the Java compiler.
  • Compiles Java code into byte code and checks for compile-time errors.
• JRE: Java Runtime Environment includes class libraries and JVM.
• JVM: Java Virtual Machine executes byte code, converts it to native code using JIT compiler.

Main Method

• Entry point for Java programs.
• Keywords: public, static, void
  • public: Accessible from outside the class.
  • static: Can be called without creating an object.
  • void: Doesn’t return any value.

Java Byte Code

• Intermediate code generated by the Java compiler.
• Java is high-level, byte code is intermediate, machine code is low-level.

Variables and Data Types

• Purpose: To manage and manipulate data.
• Declaring and Initializing variable: int x = 10;
• Types of Data Types:
  • Primitive: byte, int, float, etc.
  • Reference: classes, arrays
• Memory Storage:
  • Primitive stored directly in stack memory.
  • Reference stored as address in stack and value in heap.

Primitive vs Reference Data Types

• Primitive: Stores actual values, single values, fixed storage, in stack memory.
• Reference: Stores references, can hold multiple values, variable storage sizes, stack & heap memory.

Chapter 2: Control Statements, Strings & Arrays

Control Statements

• Conditional: if-else, switch, ternary operator
• Looping: for, while, do-while, for-each
• Branching: break, continue, return

Conditional Statements

• Use if-else for complex conditions and multi-line execution.
• Use ternary operator for single condition and single value results.
• Use switch for consistent comparisons and readability.

String vs String Builder

• String: Immutable (creates a new object for each modification).
• StringBuilder: Mutable (modifies the same object).

Arrays

• Purpose: To organize related data in a structured way.
• Example: String[] fruits = {"apple", "banana", "orange"};
• Arrays index data starting from 0.

Chapter 3: Object-Oriented Programming (OOP) Concepts

Key Concepts Covered

• Classes, Objects
• Access Specifiers, this keyword
• Getter and Setter Methods

What is OOP?

• OOP: Create software around objects.
• Provides a clear structure for software and applications.
• Main Concepts: Object, Class, Inheritance, Polymorphism, Abstraction, Encapsulation

Access Specifiers

• Types: public, private, protected, default
• Public: Accessible inside and outside the class.
• Private: Accessible only within the class.

this Keyword

• Refers to the current instance of the class.
• Differentiates between class field variables and parameters.

Getter and Setter Methods

• Getter: Retrieves the value of a private field.
• Setter: Sets or modifies the value of a private field.
• Usage ensures encapsulation and security.

Chapter 4: Inheritance and Polymorphism

Inheritance

• Purpose: For code reusability, creating a parent-child class relationship.
• Types:
  • Single Inheritance
  • Multi-Level Inheritance
  • Hierarchical Inheritance
  • Hybrid Inheritance
  • Multiple Inheritance (Not supported in Java)

Multiple Inheritance

• Unsupported: Child class cannot extend more than one parent class.
• Alternative: Use multiple inheritance of interfaces (implements).

Polymorphism

• Types:
  • Compile-Time (Method Overloading)
  • Runtime (Method Overriding)
• Method Overloading: Same method name, different parameters.
• Method Overriding: Child class redefines a method in the parent class.

Differences: Overloading vs Overriding

• Overloading: Same class, different signatures.
• Overriding: Different classes (parent and child), same signature.
• Overloading: Compile-time polymorphism.
• Overriding: Runtime polymorphism.

Chapter 5: Encapsulation and Abstraction

Encapsulation

• Bundling data and methods that operate on the data into a single unit.
• Implemented Using: Private fields, getter and setter methods.

Abstraction

• Showing only necessary details to the user; hiding the implementation.
• Implemented Using: Abstract classes, interfaces.

Differences: Encapsulation vs Abstraction

• Encapsulation: Bundles data and methods.
• Abstraction: Hides complexity.
• Encapsulation: Achieved using private fields and getters/setters.
• Abstraction: Achieved using abstract classes and interfaces.

Differences: Abstract Class vs Interface

• Abstract Class: Can have concrete methods and state.
• Interface: Only abstract methods and static final variables (except default methods).
• Abstract Class: Used when behavior is common to multiple classes.
• Interface: Used for defining a contract that multiple classes can implement.

Final Advice

• Start applying and giving interviews.
• Remember, rejection is part of the process. Never give up!