Java

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important links

Version 21 API Specification

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Java is a high-level, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It is a general-purpose programming language intended to let programmers write once, run anywhere (compiled Java code can run on all platforms that support Java without the need to recompile).
Java applications are typically compiled to bytecode that can run on any Java virtual machine (JVM) regardless of the underlying computer architecture.
The syntax of Java is similar to C and C++, but has fewer low-level facilities than either of them. The Java runtime provides dynamic capabilities (such as reflection and runtime code modification) that are typically not available in traditional compiled languages.

OpenJDK

note

OpenJDK (Open Java Development Kit) is an open-source implementation of the Java Platform, Standard Edition (Java SE).
It is the "official reference implementation" for Java SE, meaning it's the version of Java that most Java-based software projects rely on.

OpenJDK includes the Java Development Kit (JDK), which consists of:

• Java Runtime Environment (JRE): Contains the "core libraries" and the "Java Virtual Machine (JVM)", which is responsible for running Java applications.
• Java Compiler (javac): Translates "Java source code" into "bytecode", which can be executed by the "JVM".
• Tools and Libraries: Includes tools like "debugging", "profiling", and "performance tuning", as well as "core Java libraries" (like collections, streams, etc.).

Key Features of OpenJDK:

• Open Source: It is licensed under the "GNU General Public License (GPL)", allowing developers to view, modify, and contribute to its code.
• Reference Implementation: OpenJDK serves as the "primary version of Java SE", maintained by contributors including Oracle, Red Hat, IBM, and others.
• Cross-Platform: It works on various operating systems (Windows, macOS, Linux, etc.).
• Frequent Updates: Regular updates are provided for security and bug fixes.

Relationship to Oracle JDK:

"Oracle JDK", which was traditionally the "official JDK", is based on OpenJDK but has "proprietary components" such as "commercial tools and features" that Oracle adds for its customers.
Since "Java 11", "Oracle JDK" and OpenJDK are functionally almost identical, with "Oracle JDK" mainly adding commercial support and licensing differences.
OpenJDK is widely used in enterprise, open-source, and personal Java development because of its open nature and active community.

Compiling

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Is the process of translating human-readable code (source code) into machine-readable code, which computers can then understand and execute. In Java it is translated into "byte code" which requires the "JVM" for execution.

The "Java SE platform" is essentially just a specification and not an implementation.
In order for programs to be compiled and executed, we need a:

• specific compiler,
• a runtime environment and
• an implementation of the libraries.

There are different implementations from different manufacturers. If the implementations pass a series of tests called the Technology Compatibility Kit (TCK), they can call themselves "Java SE compatible".

Compiling on the CLI with Javac

• Create a file with the "example code" below, any text editor which does not add formatting can be used. Note that the "file name" has to match the "class name" (HelloWorld).

HelloWorld.java

public class HelloWorld {
	public static void main(String[] args) {
		System.out.println("Hello World!");
	}
}

• Compile the "source code" into "byte code" with the javac command. HelloWorlds.java --> HelloWold.class.
  (all .java files following the command are compiled, use *)
  javac HelloWorld.java

• Execute the program with the java command (the file/class containing the main method), without the file extension .class.
  java HelloWorld

Hello World!

java is a command line tool that is part of a "java runtime environment" (JRE) that knows how to start a "java virtual machine", load and execute your class file.