1. Software Development Handing Errors and Creating Documentation
Computer Science 209
Software Development
Handing Errors and Creating Documentation

2. Error Handling: Who Is Responsible?
The client (ultimately, the human user)
The server (ultimately, the programmer of the basic software components)

3. Example Errors
Invalid data used to create a Student object (negative numbers, etc.)
The position is out of range during the retrieval or replacement of a student’s test score
A stack is empty before a pop

4. Handling Errors, v1
A method returns true to indicate success or false to indicate failure
Works well when there is only one kind of error and no other value must be returned
The client checks the return value and takes action if necessary

5. Handling Errors, v2
A method returns null to indicate success or a string to indicate failure
The string is also a message indicating the type of error (good for 2 or more types of errors)
The client checks the return value and takes action if necessary

6. Handling Errors, v3
When Java detects an error at run time, an exception is thrown
For example, when a program attempts to divide by zero, Java throws an ArithmeticException
This method guarantees that errors are detected

7. Example: Divide by 0
int dividend = 15;
int divisor = 0;
System.out.println("The quotient is " + dividend / divisor);
Java throws an exception and the JVM halts with an error message:
A new instance of ArithmeticException is created.
This object contains the error message that is displayed in the terminal window.

8. Example: Divide by 0 int dividend = 15; int divisor = 0;
System.out.println("The quotient is " + dividend / divisor);

9. Example: Index Out of Bounds
public int getScore(int i){
return scores[i – 1]; }
An ArrayIndexOutOfBoundsException is thrown if i < 1 or i > scores.length

10. Throwing Your Own Exception
public int getScore(int i){
if (i < 1 || i > scores.length)
throw new IllegalArgumentException("i must be between " +
"1 and " +
scores.length);
return scores[i – 1]; }
Throw an exception of the appropriate type
Java’s hierarchy of exception classes is listed in Oracle’s documentation, under Exception Summary in java.lang

11. Error Conditions Requiring Exceptions
A class’s interface gives the client enough information to use a class
To use a class properly, the client must be aware of possible error conditions
These conditions are specified by placing preconditions and postconditions in the interface

12. The Interface Is a Contract Between Client and Server
Preconditions - state what must be true before a method is run to produce the expected results
Postconditions - state what the expected results are when the preconditions are satisfied

13. Guidelines for Error Handling
Servers: Methods should enforce preconditions by throwing exceptions
Clients: There is no need to worry about potential exceptions if you obey the preconditions first
Some methods require exceptions to be caught (more on this when we examine I/O)

14. When You Don’t Know the Type
public int getScore(int i){
if (i < 1 || i >= scores.length)
throw new RuntimeException("i must be between " +
"1 and " +
scores.length);
return scores[i – 1]; }

15. javadoc
Java includes a tool, javadoc, and a special notation for program comments that allow you to generate Web-based documentation
These comments highlight parameters, returned values, and possible exceptions for methods, as well as prefatory info on the class

16. Syntax of Javadoc Comments
/**
* text
* goes
* here */
Use /** to begin the comment and * at the beginning of each line.

17. Documenting the Class
/**
* A <code>Student</code> object represents a name and a set of
* test scores for a student..
Ken Lambert
Martin Osborne
1.0 */
class Student extends Object implements Comparable<Student>{
// Blah blah blah }
symbol is a prefix for specialized comment tags

18. Documenting a Method /**
* Returns the score at the specified position.
i - the position of the score
the score
IllegalArgumentException if i less than 1 or i greater than
* number of scores */
public int getScore(int i)

19. Running javadoc
> javadoc *.java
This will create Web pages for all of the classes in the current working directory
There are many options as well

20. Packages Easy GUIs with BreezySwing
For Wednesday
Packages
Easy GUIs with BreezySwing