Types of Errors

Things To Do First

• Download the notes for this session.
• Re-read chapter 1.10 and 2.19 in your textbook, do all of the exercises, Self-Check questions and Checkpoint questions.

Things To Do Later

• Review your notes from today's class:
  • What are the three different kinds of errors?
  • Give two examples each of the three different kinds of errors.

Types of Errors

At this point, errors are nothing new to you as a programmer. This is not necessarily a bad thing, since you learn a lot by debugging a program and fixing errors. In order to understand what Exceptions are, you need to understand the different kinds of errors that can occur in a program and how we go about solving each one.

There are three categories of errors in programming (these are covered in Chapter 1.10 of your textbook):

Compile-time Errors

(or just "Compile/Compilation Errors")

• Errors that are caught by the compiler when you build your source code.
• Also often referred to as syntax errors because they usually involve incorrect syntax.
• You should remember that when you get compile errors after compiling a program, your .class file is not created/updated.
• To fix these kinds of errors, you need to read the error messages produced by the compiler and fix the appropriate statements.

Examples of Compilation Errors

• Missing semi-colon at the end of a statement.
• Incorrectly typing a variable name or method name, such that the compiler can't find the item you've referred to ("Cannot find symbol!")
• Missing import statement, so the compiler can't find a class (such as JOptionPane or Scanner).
• Forgetting to include all required arguments in a method or giving a method the wrong number of arguments.
• Using incorrect case on a reserved word (e.g. While instead of while or Else instead of else).

Run-time Errors

(or "Execution Errors")

• Errors that occur while a program is running.
• Usually they cause the program to "crash".
• These kinds of errors don't get noticed at compile time, but will show up with error messages while the program runs.
• The program might run fine for a while before it crashes.

Examples of Run-Time Errors:

• Input errors, e.g. the user types a String when your input variable is an int or double.
• Using an incorrect format specifier or using the wrong kind of format specifier in a printf() or format() statement.

Logic Errors

• Errors in the logic of the program.
• These kinds of errors don't yield error messages.
• These are usually the fault of the programmer :P
• Usually they cause things like endless loops, by-passed code, incorrect output, etc.
• Solving these kinds of errors is the most difficult, because sometimes you only know that the program doesn't function, but don't know exactly why or where the problem is.
• More sophisticated editors like NetBeans and Eclipse have useful debugging tools that help you to solve these kinds of errors.

Examples of Logic Errors:

• A loop that never ends because you forgot the counter.
• An if-statement condition was written incorrectly, causing the body of the if to never execute.
• Forgetting to cast a value when dividing two integers.
• Writing a calculation incorrectly.
• Adding a semi-colon to the end of an if-statement header, such as if (x > 3);
• Columns of values in your program output are not aligned or formatted correctly.

Exercise

For each of the following programs below:
a. Identify any errors in the program.
b. Indicate the category of each error (Compilation Error, Run-Time error, or Logic Error)

1. Example A:

   public class SomeErrors {
   
       public void main(String[] args) {
   
           System.out.print("Hi");
           System.out.printlm("Hello");
           System.out.print("Bye" \n);
       }
   }

2. Example B:

   public class SomeErrors {
   
       public static void main(String[] args) {
   
           int number;
           int value = 1;
           number++;
           double result = number / value;
           System.out.println("Result: " result);
       }
   }

3. Example C:

   public class SomeErrors {
   
       public static void main(String[] args) {
   
           Scanner in = new Scanner();
           System.out.println("Enter number of cats.");
           number = in.nextInt();
           System.out.printf("%4.2f is too many cats.", number);
       }
   }

[solutions]