Core Java Concepts
Here's a concise overview of the most important Core Java concepts every Java developer should know, especially if you're preparing for interviews or building Java applications.
☕ Core Java Concepts
1. OOP (Object-Oriented Programming) Principles
Java is an object-oriented language. It supports:
Encapsulation – Binding data and methods (e.g., using private fields + getters/setters)
Inheritance – Reusing code via parent-child relationships (extends)
Polymorphism – One method behaves differently depending on the object (method overriding)
Abstraction – Hiding implementation using abstract classes or interfaces
2. Classes and Objects
A class is a blueprint (e.g., Car)
An object is an instance of a class (Car myCar = new Car();)
Constructors initialize objects
this keyword refers to the current object
3. Data Types and Variables
Primitive types: int, char, double, boolean, etc.
Non-primitive types: String, arrays, objects
Variable types: local, instance, static
4. Control Flow Statements
Conditional: if, else, switch
Loops: for, while, do-while, for-each
Branching: break, continue, return
5. Arrays and Collections
Arrays: Fixed-size, basic structure (int[] nums)
Collections (from java.util):
List (ArrayList, LinkedList)
Set (HashSet, TreeSet)
Map (HashMap, TreeMap)
Queue, Deque
6. Exception Handling
Checked Exceptions: Must be handled (IOException)
Unchecked Exceptions: Runtime (NullPointerException)
Use: try, catch, finally, throw, throws
7. Multithreading & Concurrency
Java provides the Thread class and Runnable interface.
Use synchronized to manage shared resources.
Advanced: ExecutorService, Callable, Future
8. Interfaces and Abstract Classes
Interfaces define method signatures (interface Drawable)
Abstract classes can contain both abstract and concrete methods
Java 8+ interfaces can have default and static methods
9. Java Memory Management
Automatic garbage collection (GC)
Heap vs Stack memory
finalize() method (deprecated, not recommended)
10. Java Keywords
Important ones:
final, static, this, super, extends, implements, new, synchronized, volatile, instanceof
11. File I/O (Input/Output)
File, FileReader, BufferedReader, Scanner
FileWriter, BufferedWriter
Use try-with-resources for safe handling
12. Java 8+ Features
Lambdas: (a, b) -> a + b
Streams API: .filter(), .map(), .collect()
Functional Interfaces: Predicate, Function, Consumer
Optional class to avoid null
13. Packages and Access Modifiers
Packages help organize code
Access modifiers:
public – accessible everywhere
private – only within the class
protected – within package and subclasses
(default/no modifier) – package-private
14. Annotations
Used for metadata, e.g., @Override, @Deprecated, @FunctionalInterface
Custom annotations can be created
15. JVM, JRE, JDK
JVM (Java Virtual Machine): Executes Java bytecode
JRE (Java Runtime Environment): JVM + libraries
JDK (Java Development Kit): JRE + compiler + tools
🚀 Bonus Concepts (For Advanced Use)
Concept Description
Generics Type-safe collections (List<String>)
Reflection Inspect and manipulate classes at runtime
Enums Fixed set of constants (enum Status)
Serialization Convert object to byte stream
Java Modules Introduced in Java 9, support modular applications
Design Patterns Singleton, Factory, Observer, etc.
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