Java
Java Core
Essential Java programming concepts and fundamentals
Java Core
This section outlines the essential Java Core knowledge required to progress from a Junior to a Senior Java Developer. The skills are categorized into three levels of importance: must-know (fundamental), should-know (intermediate), and nice-to-know (advanced).
Must-Know (Fundamental)
| Skill | Description | System Design Application |
|---|---|---|
| Object-Oriented Programming | Master class design, inheritance, polymorphism, encapsulation, and abstraction. Understand when to use interfaces vs abstract classes, and how to apply SOLID principles effectively. | Forms the foundation for creating maintainable, extensible systems. Proper OOP design leads to systems that can evolve with changing requirements. |
| Collections Framework | Deep understanding of List, Set, Map implementations and their performance characteristics. Know when to use ArrayList vs LinkedList, HashMap vs TreeMap, etc. Understand the internal workings of these collections. | Critical for data structure selection in performance-sensitive applications. Proper collection choice can significantly impact memory usage and processing speed. |
| Concurrency & Multithreading | Master threads, ExecutorService, CompletableFuture, locks, atomic variables, and concurrent collections. Understand thread safety issues, deadlocks, race conditions, and how to avoid them. | Essential for high-performance systems that need to handle multiple operations simultaneously. Proper concurrency management prevents data corruption and system instability. |
| Exception Handling | Develop sophisticated exception handling strategies. Know the difference between checked and unchecked exceptions, when to use each, and how to create custom exception hierarchies. | Robust error handling is crucial for system resilience. Well-designed exception hierarchies make systems more maintainable and debuggable. |
| Streams & Functional Programming | Master Stream API operations, lambda expressions, method references, and functional interfaces. Understand the benefits of immutability and side-effect-free programming. | Enables more concise, readable code for data processing pipelines. Functional approaches simplify parallel processing and improve code maintainability. |
| Generics | Deep understanding of type parameters, wildcards (? extends T, ? super T), type erasure, and generic methods. Know how to design flexible, type-safe APIs using generics. | Creates more reusable and type-safe components. Properly designed generic interfaces reduce code duplication and improve API usability. |
| I/O & NIO | Master file operations, serialization, and NIO for non-blocking I/O. Understand performance implications of different I/O approaches. | Critical for systems that handle large volumes of data or require high throughput. NIO is essential for scalable network applications. |
| Testing with JUnit & Mockito | Advanced unit testing techniques, test-driven development, mocking, and test coverage analysis. Know how to test complex scenarios and edge cases. | Ensures system reliability and facilitates refactoring. Comprehensive test suites are a hallmark of well-designed systems. |
| Java Memory Model | Understand heap vs stack, garbage collection algorithms, memory leaks, and how to profile and optimize memory usage. | Essential for designing high-performance, resource-efficient systems. Memory optimization is critical for systems that need to handle large data volumes. |
Should-Know (Intermediate)
| Skill | Description | System Design Application |
|---|---|---|
| JVM Tuning & Optimization | Understand JVM architecture, garbage collection algorithms, and performance tuning parameters. Know how to analyze heap dumps and optimize memory usage. | Critical for high-load systems. Proper JVM tuning can significantly improve application performance and stability under load. |
| Reactive Programming | Master reactive streams (Project Reactor, RxJava), understand backpressure, and non-blocking programming models. | Enables building highly responsive, resilient systems that can handle concurrent operations efficiently. Essential for microservices architectures. |
| Java Modules (JPMS) | Understand module system introduced in Java 9, how to design modular applications, and resolve module dependencies. | Improves application security and maintainability by enforcing explicit dependencies and encapsulation at the module level. |
| Advanced Concurrency Patterns | Master fork/join framework, phaser, cyclic barrier, and other advanced concurrency utilities. Understand thread pools and their configuration. | Essential for complex parallel processing systems. These patterns enable efficient utilization of multi-core processors. |
| Reflection & Annotations | Deep understanding of runtime type information, dynamic proxy, custom annotations, and annotation processors. | Powers many frameworks like Spring. Understanding these concepts helps in creating flexible, metadata-driven architectures. |
| Java Security | Master security manager, cryptography APIs, secure coding practices, and common vulnerabilities (OWASP). | Critical for systems handling sensitive data. Security must be designed into the system from the ground up. |
| Advanced Collections | Understand concurrent collections, custom collection implementations, and collection performance optimization. | Enables building high-performance data processing systems. Custom collections can be tailored to specific access patterns. |
| Java Native Interface (JNI) | Know how to integrate Java with native code (C/C++) when necessary for performance-critical operations. | Allows leveraging native libraries for performance-critical components while maintaining the overall system in Java. |
| Internationalization & Localization | Master resource bundles, locale-specific formatting, and character encoding issues. | Essential for global applications. Properly designed i18n architecture simplifies adding support for new languages and regions. |
Nice-to-Know (Advanced)
| Skill | Description | System Design Application |
|---|---|---|
| GraalVM & Native Image | Understand ahead-of-time compilation, native image generation, and polyglot programming capabilities of GraalVM. | Enables creating applications with faster startup and lower memory footprint, critical for serverless and containerized deployments. |
| Java 21 Virtual Threads | Master Project Loom's virtual threads for high-throughput concurrent applications without the overhead of OS threads. | Revolutionary for high-concurrency systems. Virtual threads enable writing simple blocking code that scales to millions of concurrent operations. |
| Bytecode Analysis & Manipulation | Understand JVM bytecode, and libraries like ASM or ByteBuddy for runtime code generation and transformation. | Powers advanced frameworks and performance tools. Understanding bytecode enables creating sophisticated runtime optimizations. |
| Advanced Garbage Collection | Deep knowledge of Z Garbage Collector (ZGC), Shenandoah, and G1 collector tuning for large heaps with low pause times. | Critical for latency-sensitive applications. Advanced GC tuning minimizes application pauses and improves overall responsiveness. |
| Java Flight Recorder & Mission Control | Profiling and monitoring JVM applications in production with minimal overhead. | Enables performance analysis of production systems. JFR provides insights into runtime behavior without significant performance impact. |
| Java 21+ Features | Stay current with pattern matching, sealed classes, records, and other cutting-edge Java features. | Modern Java features often simplify common programming patterns, leading to more maintainable and less error-prone code. |
| CompletableFuture Advanced Patterns | Master complex asynchronous workflows, error handling, and composition of asynchronous operations. | Essential for building responsive, non-blocking systems that can handle complex processing workflows efficiently. |
| Custom ClassLoaders | Understand the classloading mechanism and how to implement custom classloaders for specialized use cases. | Enables advanced plugin systems and dynamic code loading scenarios. Custom classloaders are the foundation of many modular architectures. |
| Java Agent & Instrumentation | Know how to use Java agents for bytecode instrumentation and application monitoring. | Powers APM tools and advanced debugging solutions. Understanding instrumentation enables creating sophisticated monitoring systems. |
Learning Resources
Books
- Effective Java (3rd Edition) by Joshua Bloch - Essential patterns and best practices
- Java Concurrency in Practice by Brian Goetz - Definitive guide to Java concurrency
- Modern Java in Action by Raoul-Gabriel Urma - Covers functional programming and modern Java features
- Optimizing Java by Benjamin Evans - Deep dive into JVM performance tuning
- Java Performance: The Definitive Guide by Scott Oaks - Comprehensive guide to performance optimization
Online Courses
- Java Multithreading, Concurrency & Performance Optimization (Udemy)
- Advanced Java Programming (Pluralsight)
- Reactive Programming with Java and Spring WebFlux (Udemy)
- Java Memory Management (Udemy)
- Java Performance Tuning (Oracle University)
Free Resources
- Java Design Patterns (https://java-design-patterns.com/)
- Inside the Java Virtual Machine (Oracle Documentation)
- Java Language Updates (https://docs.oracle.com/en/java/javase/21/language/java-language-changes.html)
- Java Specialists Newsletter (https://www.javaspecialists.eu/archive/)
- Baeldung Java Tutorials (https://www.baeldung.com/)
System Design Applications
Java Core skills directly impact system design in numerous ways:
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Performance-Critical Systems: Advanced knowledge of Java concurrency, memory management, and JVM tuning enables building systems that can handle high loads with minimal resource usage.
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Scalable Architectures: Understanding CompletableFuture, virtual threads, and reactive programming allows designing systems that scale horizontally with increasing load.
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Resilient Systems: Proper exception handling, defensive programming, and thread safety knowledge leads to systems that can recover from failures gracefully.
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Maintainable Codebases: Applying SOLID principles, effective use of generics, and functional programming patterns results in systems that are easier to maintain and extend over time.
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Resource-Efficient Applications: Knowledge of memory management, garbage collection, and efficient data structures enables creating applications that make optimal use of available resources.
A Senior Java Developer must be able to connect these core language features to broader system design decisions, understanding how low-level implementation choices impact high-level architecture goals.