الترجمة والتشغيل من الطرفية

Written Explanation

Step 0: a working folder

Use the terminal skills from BEG-006. The commands below create a folder, enter it, and show the current location. The exact commands differ a little between shells, so use the version that matches yours.

mkdir hello-java
cd hello-java
pwd

These work in Bash and in PowerShell (pwd is accepted in both). In the Windows Command Prompt use cd with no argument to show the folder. Keep the terminal open in this folder: both javac and java will look here.

Step 1: write the source file

Create a file named exactly Hello.java containing this program. Use any plain-text editor (an IDE is fine, as is the editor of your choice). On Windows, make sure the editor does not secretly save it as Hello.java.txt: recall from BEG-005 that hidden file extensions can cause this.

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

Read it once so that nothing is mysterious:

• public class Hello declares a class named Hello. A class is the basic container for Java code.
• public static void main(String[] args) is the entry point: the method the java command calls first. You will study methods properly later; for now treat this line as a fixed phrase that must be typed exactly.
• System.out.println(...) prints a line of text. The text in double quotes is what appears.
• Curly braces { } mark where the class and the method start and end. Each statement ends with a semicolon.

Capitalisation matters: Hello and hello are different names.

The file-name rule

A public class named Hello must live in a file named Hello.java: the class name plus .java, capital letters included. The Java Language Specification describes this as something a system that stores code in files may enforce, and the standard javac does enforce it. If you save the program above as Greeting.java, compilation fails. The message typically says that class Hello is public and should be declared in a file named Hello.java (the exact wording can vary between versions).

Use this as a habit: one public class per file, and the file name equals the class name.

Step 2: compile with javac

Check that the file is there, then compile:

ls              (Bash or PowerShell: use dir in Command Prompt)
javac Hello.java
ls

• You give javac the file name, including .java.
• If the code is valid, javac prints nothing at all. In the terminal, silence is success.
• Listing the folder afterwards should show a new file, Hello.class. By default javac puts each class file in the same folder as the source, and names it after the class.

Now you have both files: Hello.java (source, text) and Hello.class (bytecode, binary).

Step 3: run with java

java Hello

The output should be:

Hello from the terminal!

Note what you typed: Hello, not Hello.class and not Hello.java. The java command expects the name of a class, and it finds that class's .class file for you. The documentation for the launcher describes its argument as a class name, and its examples never include an extension.

So the two commands differ in what they take:

If you type java Hello.class, the launcher looks for a class literally named Hello.class, finds nothing, and reports that it could not find or load the main class. The fix is to drop the extension.

A shortcut you will see

Recent JDKs can also run a single source file in one step: java Hello.java compiles it in memory and runs it, without leaving a .class file behind. The java documentation calls this source-file mode. It is handy for quick experiments. In this module, use the two-step version, because the point is to see the compile stage and its output file.

Reading a compile error

Suppose you delete the semicolon after the println(...) call and compile. Compilation fails, no new .class file is produced from this run, and a message appears. It typically has this shape (wording may vary by JDK version):

Hello.java:3: error: ';' expected
        System.out.println("Hello from the terminal!")
                                                      ^
1 error

Read it in parts:

• Hello.java:3 is the file and the line number where javac noticed the problem.
• error: ';' expected says what it believes is wrong.
• The caret ^ points to the spot.
• 1 error counts the problems.

Sometimes the problem is on the line before the reported one, because the compiler only noticed when it kept reading. If the reported line looks fine, check the line above.

If an old Hello.class already exists from an earlier successful compile, it stays in the folder and java Hello would still run that old version. That is a good reason to read the compile output before running.

Compiling is not the same as working

A program can compile cleanly and still fail when run. For example, this compiles without complaint:

public class Divide {
    public static void main(String[] args) {
        int people = 0;
        System.out.println(90 / people);
    }
}

At run time the JVM stops with a message that begins with Exception in thread "main" java.lang.ArithmeticException: / by zero and shows where it happened. Integer division by zero is specified by the Java language to throw an ArithmeticException. This is a runtime error, and the demo in the next lesson walks through it step by step. For now, the point is the two-stage nature of the pipeline: javac checks the rules, and the JVM meets the data.

The full cycle

Every change follows the same loop:

1. Edit and save the .java file.
2. javac Hello.java. Fix errors until it is silent.
3. java Hello. Check the output.
4. Back to 1.