عرض عملي: إصلاح خمسة برامج معطّلة من رسائل المترجم

Written Explanation

Setup

Make a new folder for this demo and open a terminal in it. Create the five files below. For each one, the file name is the class name plus .java. The messages shown are what JDK 21 printed for these exact files. Yours may differ slightly in wording, spacing, or line positions.

You will work in this order:

1. Compile (javac P1.java).
2. Read the first message.
3. Fix one thing.
4. Compile again.
5. When javac is silent, run (java P1) and check the output.

Keep a small log: file, message, line, fix.

Program 1: the missing semicolon

public class P1 {
    public static void main(String[] args) {
        System.out.println("Ready");
        System.out.println("Cairo")
    }
}

Compile with javac P1.java. You should see:

P1.java:4: error: ';' expected
        System.out.println("Cairo")
                                   ^
1 error

Read it: file P1.java, line 4, the compiler expected a semicolon, and the caret sits right after the closing parenthesis. The fix is to add ; at the end of line 4. Compile again: silent. Run java P1:

Ready
Cairo

Program 2: the misspelled name

public class P2 {
    public static void main(String[] args) {
        system.out.println("Ready");
    }
}

javac P2.java says:

P2.java:3: error: package system does not exist
        system.out.println("Ready");
              ^
1 error

Read it: Java met the name system followed by a dot, took it for a package, and found none. The message names system, so check its spelling and capital letters against the working line from Lesson 3: it is System. Change the first letter to a capital S, compile (silent), and run: Ready.

If your JDK says cannot find symbol here instead, the fix is the same. Both messages mean "I do not know this name".

Program 3: the missing closing brace

public class P3 {
    public static void main(String[] args) {
        System.out.println("Ready");
    }

P3.java:4: error: reached end of file while parsing
    }
     ^
1 error

Read it: the compiler hit the end of the file while a block was still open. The caret is at the last line, but the cause is above. Use the brace count from Lesson 2: the file has two { (class and main) and only one }. The closing brace of the class is missing. Add } on a new line at the end. Compile, run: Ready.

Program 4: the wrong main line

public class P4 {
    public static void main(String args) {
        System.out.println("Ready");
    }
}

javac P4.java prints nothing. The program is valid Java, so it compiles. Does that mean it works? Run it:

Error: Main method not found in class P4, please define the main method as:
   public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application

Read it: the message came from java, not from javac, so it is a launch error. The launcher wants exactly the line it shows, and ours has String args without the square brackets. Change it to String[] args, compile, run: Ready.

Program 5: the unclosed string

public class P5 {
    public static void main(String[] args) {
        System.out.println("Ready);
    }
}

P5.java:3: error: unclosed string literal
        System.out.println("Ready);
                           ^
1 error

Read it: the caret is under the quote that opened the text. A string must close on the same line, and this one did not. Add the closing " before the ): System.out.println("Ready");. Compile, run: Ready.

Program 6: the planned mistake

Now the trap. Create P6.java:

public class P6 {
    public static void main(String[] args) {
        System.out.println("Ready")
        System.out.printn("Go");
    }
}

Compile it, read the message, and fix only what it names:

P6.java:3: error: ';' expected
        System.out.println("Ready")
                                   ^
1 error

Add the semicolon after println("Ready") and stop there, thinking you are finished. A beginner would now run java P6. But recompile first, as the loop says, and you see:

P6.java:4: error: cannot find symbol
        System.out.printn("Go");
                  ^
  symbol:   method printn(String)
  location: variable out of type PrintStream
1 error

The second mistake was hidden behind the first. The message says Java found no method named printn in the type of System.out, so it is a typo. Change printn to println, compile (silent), and run:

Ready
Go

The message at the first compile said "1 error", yet the file had two mistakes. The error count tells you what the compiler has found so far, not what is wrong with the file. That is why the habit is: fix, recompile, and read again, until javac is silent and the output is what you intended.

Your log

Here is what a good log looks like after the demo:

Using the IDE

In IntelliJ IDEA the same faults are marked in the editor as you type, usually with a red underline, and the message appears when you hover over it. The build output panel shows the compiler messages when you run the program. The idea is the same: read the first message, change one thing, and run again. The exact colours, menu names, and wording of IDE messages differ from javac's and vary by version, so confirm each fix with the real compiler output when you are unsure.