عرض عملي: ترجمة Hello World وتشغيله وتعطيله وإصلاحه في الطرفية

Written Explanation

Do each step in a fresh folder. The exact message text and layout can differ between JDK versions and operating systems, so the messages below are typical, not guaranteed. What matters is the shape and when it appears.

Setup

mkdir demo-007
cd demo-007

Create Hello.java:

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

Step 1: write, compile, run

javac Hello.java
java Hello

You expect no output from javac, then Hello, backend! from java. List the folder: you should see Hello.java and Hello.class.

If instead you get "command not found" or similar, return to BEG-005 and check your PATH and your JDK install, using javac -version as the test.

Step 2: look inside the .class file

Open Hello.class in a plain text editor. The way to open it differs by operating system and editor, so use "Open with" from your file manager or open it from inside a text editor, and do not save any changes. You will see mostly unreadable symbols, with a few fragments of text, such as the name of the class or your message, possibly visible between them.

This is expected. A .class file is binary bytecode, a structured format for the JVM, as you saw in L02. A text editor tries to show bytes as characters and fails for most of them. It is not damaged. Close the file without saving.

Optional, if you want to see the raw bytes: a hex viewer tool shows them as numbers. The tool name differs by system (for example xxd or hexdump on many Linux and macOS setups, Format-Hex in PowerShell). The first four bytes of a valid class file are CA FE BA BE, the magic number described in the JVM Specification. Treat this as optional curiosity.

Classify: nothing went wrong in steps 1 and 2. This is the normal pipeline.

Step 3: a compile-time error (remove a semicolon)

In Hello.java, delete the semicolon after the println(...) call and save. Then compile:

javac Hello.java

A typical result:

Hello.java:3: error: ';' expected
        System.out.println("Hello, backend!")
                                             ^
1 error

Record three things, exactly as the lab asks:

1. The message: ';' expected.
2. The location: file Hello.java, line 3.
3. The stage: compile-time error. It appeared while running javac, before any program ran.

What about the old Hello.class from step 1? Failed compilation does not produce a new class file for this source, so the old one is still sitting in the folder. If you now ran java Hello, you would see the old output. This is a trap: a failed compile leaves the last successful .class behind. Read the compile output first.

Fix: put the semicolon back, compile again, and confirm the silent success.

Step 4: a runtime error (divide by zero)

Replace the body of main so that the file reads:

public class Hello {
    public static void main(String[] args) {
        System.out.println("Starting...");
        int zero = 0;
        int x = 5 / zero;
        System.out.println("Result: " + x);
    }
}

Compile and run:

javac Hello.java
java Hello

javac stays silent. The program has valid syntax, so the compiler accepts it. Then java Hello prints something like:

Starting...
Exception in thread "main" java.lang.ArithmeticException: / by zero
        at Hello.main(Hello.java:5)

Record:

1. The message: ArithmeticException: / by zero.
2. The location: line 5 of Hello.java, inside main. Notice that this line reference appears under at, which is the JVM telling you where the program was when it stopped.
3. The stage: runtime error. Compilation had succeeded, and the output Starting... shows the program ran up to the failing line. The line Result: ... never printed.

The language specification states that integer division by zero throws an ArithmeticException. It is not an error you can see by reading the grammar: the compiler does not know what zero will contain when the program runs (here you can see it, but in a real program the value could come from a user or a database).

Why did we use a variable and not write 5 / 0? Writing the constant directly is also valid code, and some compilers print a warning for it. Using a variable models the real situation, where the divisor is only known at run time.

Fix: decide what the program should do when the divisor is zero. A simple fix for the demo is to check before dividing:

public class Hello {
    public static void main(String[] args) {
        System.out.println("Starting...");
        int zero = 0;
        if (zero == 0) {
            System.out.println("Cannot divide by zero.");
        } else {
            System.out.println("Result: " + (5 / zero));
        }
    }
}

Compile and run again. The program now handles the case and finishes normally. Notice that the fix is about the logic, not about syntax.

Step 5: the planned mistake (file name does not match)

Rename the file so that it does not match the class, for example from Hello.java to hello.java. Use the terminal rename command from BEG-006 (mv in Bash, Rename-Item or mv in PowerShell). Then compile:

javac hello.java

A typical message, which can vary by version:

hello.java:1: error: class Hello is public, should be declared in a file named Hello.java
public class Hello {
       ^
1 error

Record: the message says exactly what is wrong, and it is a compile-time error, because it came from javac.

Fix: rename the file back to Hello.java, compile, and run. (On systems whose file names ignore capitalisation the rename may need two steps, so check the folder listing afterwards.)

The mental checklist

When you see a message, ask in this order:

1. Which command printed it: javac or java?
2. If javac: it is a compile-time error. Read the file, the line, the reason.
3. If java: did the JVM fail to find the class (a launch problem), or did the program start and then fail (a runtime error)? An Exception in thread "main" line followed by at ... lines means the program was running.

Then run it in IntelliJ

For the lab you also run the same program in IntelliJ IDEA (installed in BEG-005). Menus and button names differ between versions, so look for the equivalent in yours: create a project, add a class named Hello, paste the program, and use the green run button or the run option in the class's context menu. The IDE compiles and runs behind the scenes. Typos and errors appear in the editor as you type, and the output appears in a panel at the bottom. The same two error kinds exist there. They are just displayed differently.