تثبيت JDK (على أنظمة التشغيل الثلاثة)

Written Explanation

What the JDK is

Java is a programming language and a platform for running programs written in it. Your Java source files (.java) are not run directly. First, the compiler (javac) translates them into .class files, which contain bytecode: instructions for the Java Virtual Machine (JVM). Then the java command starts a JVM, which runs the bytecode.

The pieces fit in a simple picture:

Hello.java  --javac-->  Hello.class  --java-->  program runs on the JVM
 (you write)            (compiled)              (output appears)

The JDK (Java Development Kit) is the full toolbox for developers. It contains the compiler javac, the java launcher that runs programs, the standard class libraries, and other command-line tools you will meet over time (for packaging files into a .jar, for example).

The JRE (Java Runtime Environment) was the smaller package that could run Java programs but not compile them. So the old advice was: end users install the JRE, developers install the JDK. Since JDK 11, Oracle no longer provides a separate JRE download, and the JDK is the normal thing to install. Other vendors may still offer runtime-only packages, and some Linux package managers offer a "runtime" package separately from a "development" package. That is why the rule for this course is simple:

To write and compile Java you need a JDK. A runtime-only package is not enough.

If you install a runtime-only package by mistake, java -version works but javac -version says the command is not found. That asymmetry is the sign.

Which version, and from whom

Java has a new feature release every six months, and some releases are marked LTS, for long-term support. Vendors keep updating an LTS release with fixes for years, so most companies standardize on an LTS version. This course uses one baseline JDK, an LTS release, so that everyone sees the same behaviour. Use the version named in the course materials. If you are unsure which one is current, check the vendor's download page, because I will not guess a version number for you that may be out of date by the time you read this. As examples, the Oracle installation guide I read uses JDK 21 in its sample paths, and a current JetBrains tutorial asks for Java 25 or higher; both are LTS-style choices that you may see in practice.

The JDK is built from the open-source OpenJDK project. Several organizations publish their own builds of it. Microsoft's VS Code documentation names Amazon Corretto, Azul Zulu, Eclipse Temurin, and the Microsoft Build of OpenJDK among recommended sources, and Oracle publishes its own Oracle JDK. For learning, they are interchangeable: the same language, the same java and javac commands. Pick one, install it from its official site, and do not download a JDK from a random mirror or an unofficial "download manager".

Installing on Windows

1. Open your chosen vendor's download page and choose the Windows, 64-bit installer for the baseline version. Installers usually come as .msi or .exe files. Most home computers use a 64-bit processor. If you are unsure, Windows Settings (System, About) shows the system type.
2. Run the installer and follow the steps. Note the folder it installs to. Oracle's documentation gives C:\Program Files\Java\jdk-<version> as its default location. Other vendors use different folders, such as one under C:\Program Files\Eclipse Adoptium or C:\Program Files\Microsoft. You will need this folder in lesson 3.
3. Look for installer options such as "Add to PATH" and "Set JAVA_HOME variable". Where the installer offers them, enabling them is the easy path. Some installers set PATH by default and some do not, so you must check by running the commands.
4. Open a new terminal (Command Prompt or PowerShell) and run the two version commands.

Installing on macOS

1. Download the installer package for your Mac's processor type. Oracle's documentation says macOS is supported for both Intel (x64) and Apple Silicon (AArch64) Macs. If you do not know which you have, the Apple menu, then "About This Mac", shows it.
2. Run the package and follow the steps. Installers on macOS normally place the JDK in a system folder for Java virtual machines, and the java command is found automatically on most setups. Some distributions also install through a package manager, such as Homebrew. If you use one, follow that vendor's instructions, because a package manager may install the JDK in a folder it manages and may need extra steps to make it the default.
3. Open a new Terminal and run the two version commands.

Installing on Linux

On Linux, the usual way is your distribution's package manager, which downloads and installs the JDK from the distribution's own repositories. The command and package name depend on the distribution and version. For example, Debian and Ubuntu systems typically use apt with a package named like openjdk-<version>-jdk, and Fedora-family systems use dnf with java-<version>-openjdk-devel. Check your distribution's documentation for the exact name, and make sure the package ends in jdk or devel, not only jre or runtime.

A second way is to download an archive (a .tar.gz file) from a vendor, unpack it to a folder you choose, and add that folder's bin directory to your PATH, which you will do in lesson 3.

Installing system packages usually needs administrator rights, so the command often starts with sudo. Type it only if you understand what the command installs.

Checking the installation

Open a new terminal window. This matters, and lesson 3 explains why. Then run:

java -version
javac -version

You should see a version line for each. The java output typically looks like this (the numbers and the vendor name will differ on your machine):

openjdk version "<version>" <date>
OpenJDK Runtime Environment (<vendor build>)
OpenJDK 64-Bit Server VM (<vendor build>, mixed mode)

and the javac output is a single line:

javac <version>

Three small facts help you read the result:

• javac -version prints just the compiler's version, according to Oracle's documentation. The same page says javac --version works too.
• Oracle's documentation for java says that -version prints the version to the error stream, while --version prints to the standard output stream. For you, both appear in the terminal. The difference matters later when a script or tool captures the output.
• The two version numbers should match. If java reports one version and javac another, two JDKs are installed, and your system is mixing them. Lesson 3 shows how to find out which one is first on PATH.

The first real test: compile and run

Version numbers prove that the tools are present. To prove that they work, compile and run the program from lesson 1. Open a terminal in the greeter folder, which contains Greeter.java, and run:

javac Greeter.java
java Greeter

The first command writes Greeter.class into the folder and prints nothing if all is well. The second command starts the JVM, runs the class, and prints Workshop ready.. Notice that you type java Greeter with the class name and no .class or .java. The documented form is java [options] mainclass, so the launcher expects the class's name.