Core Java refers to the essential features and functionalities of the Java programming language that form the foundation for building Java applications. It includes basic concepts, libraries, and APIs (Application Programming Interfaces) that developers use to create Java applications. Here’s an overview of the key topics that constitute Core Java:

1. Basic Syntax

• Keywords: Java reserved words (e.g., class, public, static, void, etc.)
• Data Types: Primitive data types like int, float, char, boolean, and reference types (objects).
• Variables and Constants: Declaring and using variables, constants with final.
• Operators: Arithmetic, relational, logical, bitwise, and other operators.

2. Object-Oriented Programming (OOP) Concepts

• Classes and Objects: Definition of classes, creating objects, and using them.
• Inheritance: The concept of one class inheriting the properties and behaviors of another class.
• Polymorphism: The ability for different classes to define methods with the same name but with different implementations.
• Encapsulation: Wrapping data (variables) and methods into a single unit or class.
• Abstraction: Hiding complex implementation details and providing simple interfaces.

3. Control Flow

• Conditional Statements: if, else, switch.
• Loops: for, while, do-while.
• Exception Handling: Using try, catch, throw, throws, and finally to handle errors.

4. Classes and Objects

• Constructors: Special methods for initializing objects.
• Instance and Static Members: Differentiating between instance variables/methods and static ones.
• Access Modifiers: public, private, protected, default (package-private).

5. Collections Framework

• List, Set, Map, Queue: Interfaces and their implementations like ArrayList, HashSet, HashMap, LinkedList, etc.
• Generics: Type-safe collections, allowing the storage of objects of a specific type.
• Iterator: Interface for traversing collections.

6. Threads and Concurrency

• Thread Creation: Implementing Runnable or extending Thread.
• Thread Synchronization: Using synchronized blocks to avoid conflicts between threads.
• Executor Service: Managing thread pools.
• Concurrency Utilities: CountDownLatch, Semaphore, CyclicBarrier, and other classes in java.util.concurrent.

7. I/O (Input/Output)

• Streams: Reading and writing data using InputStream, OutputStream, Reader, and Writer.
• File Handling: Using classes like File, FileReader, FileWriter, and BufferedReader for reading and writing files.
• Serialization: Converting objects into a byte stream for storage or transmission.

8. Java API

• String Handling: String, StringBuilder, StringBuffer for manipulating strings.
• Date and Time: Working with Date, Calendar, and the java.time package.
• Math: Performing mathematical operations with classes like Math and BigDecimal.

9. Java 8 Features

• Lambda Expressions: Anonymous functions for passing behavior as parameters.
• Streams API: Processing collections in a functional way.
• Functional Interfaces: Interfaces with a single abstract method (e.g., Runnable, Callable).
• Optional: A container object for null-safe operations.

10. JVM (Java Virtual Machine) and Memory Management

• JVM Architecture: The components of JVM and how it executes Java bytecode.
• Garbage Collection: Automatic memory management in Java to reclaim unused objects.

11. Packages and APIs

• Creating and Using Packages: Organizing classes into namespaces.
• Java Standard Library: Using built-in classes for various tasks (e.g., java.util, java.io).

Core Java is the foundational layer of Java programming. Once these basics are understood, developers can build advanced Java applications, including those using Java EE (Enterprise Edition), frameworks like Spring, and technologies like JavaFX for GUI developments.