Core Java Complete Roadmap

What You Will Learn

Core Java is the foundation for backend development, Spring Boot, Android, data engineering, and enterprise systems. This guide gives you the order in which to learn Java so you do not jump randomly between topics.

Prerequisites

• Basic programming understanding
• JDK installed locally
• A code editor such as IntelliJ IDEA or VS Code
• Comfort using terminal commands

Concept Overview

Java is a statically typed, object-oriented language that runs on the JVM. You write .java files, compile them into bytecode, and the JVM executes that bytecode across operating systems.

The most important Core Java areas are:

• Syntax and data types
• Classes and objects
• Inheritance, abstraction, encapsulation, and polymorphism
• Exceptions
• Collections
• Generics
• Streams and lambda expressions
• Multithreading
• JVM memory basics

Step-by-Step Learning Path

1. Start with variables, operators, conditions, loops, arrays, and methods.
2. Learn classes, constructors, access modifiers, static members, and packages.
3. Understand the four OOP pillars with small examples.
4. Practice exceptions using checked and unchecked exception cases.
5. Learn collections: List, Set, Map, Queue, and when to use each one.
6. Add generics so your collections and utility methods are type safe.
7. Learn lambda expressions and streams for data transformation.
8. Study threads, executors, synchronization, and common concurrency mistakes.
9. Understand stack, heap, garbage collection, and class loading at a high level.

Code Example

import java.util.List;

record Course(String name, int lessons) {}

public class RoadmapExample {
  public static void main(String[] args) {
    List<Course> courses = List.of(
        new Course("OOP", 8),
        new Course("Collections", 10),
        new Course("Streams", 6)
    );

    courses.stream()
        .filter(course -> course.lessons() >= 8)
        .map(Course::name)
        .forEach(System.out::println);
  }
}

Real-World Use Cases

• Building Spring Boot APIs
• Writing backend services
• Creating automation tools
• Solving coding interview problems
• Understanding enterprise Java codebases

Best Practices

• Prefer clear class names and small methods.
• Use interfaces for contracts and classes for implementation details.
• Handle exceptions where you can add context or recover.
• Choose collections based on access pattern, ordering, uniqueness, and lookup needs.
• Avoid shared mutable state in concurrent code.

Common Mistakes

• Memorizing syntax without writing programs
• Using inheritance where composition is simpler
• Catching Exception everywhere without meaningful handling
• Using ArrayList or HashMap for every problem without thinking
• Ignoring JVM and memory basics

Interview Questions

• What is the difference between JDK, JRE, and JVM?
• How does HashMap work internally?
• What is the difference between checked and unchecked exceptions?
• Why are strings immutable in Java?
• What is the difference between synchronized and Lock?

Summary

Core Java is the base layer. Once you are comfortable with OOP, collections, exceptions, streams, and threading, Spring Boot and enterprise Java become much easier to understand.