3. Debugging¶

3.1. Objectives¶

Recognize and correct syntax errors in a Java program.

3.2. Time Goal¶

25 minutes on this section

3.3. Key Terms¶

syntax error: A mistake in the source code that violates the rules of the language
compile time error: An error that is found during the compilation. These are also called syntax errors.
logic error: mistake in the program that causes incorrect behavior (although it violates no language rules)
bug: An error in a program.
debugging: The process of finding and removing errors.

3.4. Exercises¶

Note

This part of the lab will give you some practice in reading and interpreting Java. As you work through each question, pay particular attention to the message produced, and in some cases, a single error will cause several other errors.

Note

Remember that the Java source file has to be translated into a class file before it can be run. The compiler tries to make sense of your code, but if your code is incorrect, which means it has syntax errors, you will see error messages displayed below the code. A syntax error is an error in the specification of the program. An example of a syntax error is if the code has a open curly brace {, but no close curly brace }.

The error messages will tell the line number that the compiler found the error and the type of error. The error messages are not always easy to understand and sometimes the actual error is before the line that the complier says is the problem.

Note

Click on the Run button below to try and run the following code. Look for an error message after the code. This is called a compile time error because it is an error detected by the compiler.

What is wrong? Can you fix it? The error message will tell you the line number that it thinks is causing the error (SecondClass.java:5: error: unclosed string literal). Check line 5 to make sure that everything looks correct. One good thing to check is that all { have a matching } and all ( have a matching ) and all starting " have a ending " as well.

Note

Notice that the compiler claims that there are 3 errors, but all the errors are caused by the same problem (the missing end "). Fix the code and run it again.

Note

Click on the Run button below to try and run the following code. Look for an error message after the code. What is wrong this time? Can you fix it? One good thing to check is that all { have a matching } and all ( have a matching ) and all starting " have a ending " as well.

Note

Use the code editor below to run your java program in the web browser windows. Or you can click on the editor link here to open the code editor in a new window. You can have your partner work along in a separate instance of the code editor if that helps. Note that you can use the pencil icon to edit a file again if you need to go back and make changes after you’ve used the triangular “run” button.

Note

What is the name of the source file in this program?

Q-66: Class name different from file name.

Delete the beginning ‘H’ from the name of the class (so the first non-comment line is public class ello) and save the program.

Now compile and run your program. By clicking the triangle icon. What error message do you get during the compile?

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

Correct!

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

It looks like you changed the name of the file instead of the class inside the file. Please try again.

Hello.java:3: error: cannot find symbol
    System.out.printl("Hello World!");
              ^
  symbol:   method printl(String)
  location: variable out of type PrintStream
1 error

If you're getting this message, please ask your TA for help to get you back on track.

Error: Main method is not static in class Hello, please define the main method as:
   public static void main(String[] args)

If you're getting this message, please ask your TA for help to get you back on track.

All compiler messages will begin with the name of the source file (Hello.java) and the line number in that file that contains the error.

What line number was your error on?

Q-67: Misspelling inside a string literal.

Correct the mistake above, save, and compile. Next, delete one letter ‘l’ from the word Hello in the message to be printed (inside the quotation marks). Save the program and recompile it.

Why is there no error message?

The compiler was run in silent error mode.
Incorrect. Try again.
The code is logically correct.
Try again
The code is syntactically correct.
Correct
The class file was not executed.
Incorrect--the class file was executed. Try again.
The class was marked as public.
While this is true, it isn't relevant. Try again.

Q-68: No ending quote mark for a string literal.

Correct the spelling in the string, then delete the ending quotation mark enclosing the “Hello World!” Save the program and recompile it.

What error message(s) do you get?

Hello.java:3: error: unclosed string literal
    System.out.println("Hello World!);
                       ^

Correct!

Hello.java:3: error: ';' expected
    System.out.println("Hello World!);

Incorrect--this is a bogus error message. Try again.

Hello.java:5: error: reached end of file while parsing
}
 ^

Incorrect--you may need to fix a missing close brace before you try again.

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

Incorrect--you may need to fix a missing semicolon and try again.

Q-69: No beginning quote mark for a string literal.

Put the ending quotation mark back, then take out the beginning one. Save and recompile.

Which of the following are correct?

The errors messages were identical
The error messages were different
The error occured on line 3
The error occured on line 5
This is a logic error

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