Chapter 12
Handling Errors

Overview

Chapter 12 discusses defensive programming, exception throwing and handling, error reporting and simple file processing.

12.4 Exception-throwing principles

exception - an object representing details of a program failure. An exception is thrown to indicate a failure has occured.

Java uses exceptions to signal failure, such as division by 0, as in the example below.

 

We have used special return values to indicate failure. In the Zuul game, false is returned when no door from the Room exists in the direction.

There are several problems with this approach:

1. false can be ignored as a failure in the Game.
2. No magic value is used by everyone to indicate failure, making it harder to recognize failure indicators in a program.

Game	Player
private void goRoom(Command command) {     String direction = command.getSecondWord();     if (!player.go(direction))    // No door       System.out.println("There is no door!");     else                                     // Door       System.out.println(player.roomDescription()); }	public boolean go(String direction) {    Room nextRoom = currentRoom.getExit(direction);    if (nextRoom == null)    // No door       return false;    else {                           // Door       currentRoom = nextRoom;       return true;    } }

A better approach is to use exceptions, a formal means of indicating failure with the advantages:

1. Impossible to ignore,
2. Part of the language, purpose recognized by everyone.

Game	Player
private void goRoom(Command command) {    String direction = command.getSecondWord();    try {       player.go(direction);       System.out.println(player.roomDescription());    }    catch(Exception ex) {       System.out.println("Exception " + ex);    } }	public void go(String direction) throws Exception {    Room nextRoom = currentRoom.getExit(direction);    if (nextRoom == null)       throw new Exception("No door");    else       currentRoom = nextRoom; }

12.4.1-12.5.2 Exception details

Five keys points of exceptions are illustrated below:

1. Line 12 - The go() Player method is defined to throw Exception, means that it contains a throw and any caller of go() must be prepared to handle the exception thrown.
2. Line 16 - Where the exception is thrown, no other statements are executed in go().
3. Line 4 - The try is required because the go() method throws Exception. If an exception is not thrown, statements following are executed (Line 6). If an exception is thrown, the catch is executed.
4. Line 8 - The catch is executed when an execution is thrown, when trying the go() method at Line 5.
5. Line 9 - The exception parameter ex has the value defined by the Exception("No door") construction.

Game	Player
private void goRoom(Command command) {    String direction = command.getSecondWord();      try {       player.go(direction);       System.out.println(player.roomDescription());    }    catch(Exception ex) {       System.out.println("Exception: " + ex);    } }	public void go(String direction) throws Exception {      Room nextRoom = currentRoom.getExit(direction);      if (nextRoom == null)       throw new Exception("No door");    else       currentRoom = nextRoom; }

Exercise 1 - Exceptions in Zuul Assume you're in a room with north and south exits only, the "library" to the north. Trace the lines executed starting at Line 1 given the command go north. What is the output? Trace the lines executed starting at Line 1 given the command go east. What is the output?

12.7 Using assertions

assertion - a statement of a fact that should be true for normal program execution.

Assert statements serve as checks that the program is behaving as expected.

When the assertion is true, the assert does nothing; when false, the assert causes an error, stopping program execution.

The assert statement is turned on during testing, and off when the program is released for production to users.

 

The result at right occurs when y is 0.

 

12.9 Case study: text input/output

We will examine reading and writing text to and from a file as a case study of exception handling, such as when a file to be read does not exist.

Reading a file

• BufferedReader input - Declares input as BufferedReader, which can open and read a file one character, one line, etc. at a time.
• input = new BufferedReader(new FileReader("J:/C201/Projects/chapter07/zuul-better/Game.java")); - Attempts to open a file and assign the object to input. If the file is not found, an exception is thrown, caught and "File not found" printed.
• input.ready() - True when there is more to read from the file.
• input.readLine() - Reads on line from the file.

import java.io.*; public class IO {    BufferedReader inputFile;    public IO() {       try {          input = new BufferedReader(new FileReader("J:/C201/Projects/chapter07/zuul-better/Game.java"));       }       catch(Exception ex) { System.out.println("File not found"); }           }    public void readFile() {       int line = 0;       try {          while( input.ready() ) {             line++;             System.out.println( line + " " + input.readLine() );          }       }       catch(Exception ex) { System.out.println("Exception " + ex); }    } }

Exercise 2 - File IO Create a new project named Ch12. Create a new class named, IO. Copy the above, paste, compile, and create an IO object. Should "File not found" print in the Terminal window: What happened and why? Check that J:/C201/Projects/chapter07/zuul-better/Game.java actually exists. If not, replace with another file path. Call the readFile method on the IO object.  What does the method do? Change the path to a file that does not exist, compile and attempt to create a new IO object. What happens, where and why? Call the readFile method on the IO object.  What happens, where and why?

Writing a file

The above program will be modified to read one file and write to another, a file copy operation.

import java.io.*; public class IOcopy {    BufferedReader input;    PrintWriter output;    public IOcopy() {       try {          input = new BufferedReader(new FileReader("C:/d/C201/BlueJ/Projects/ch12/IO.java"));          output = new PrintWriter(new FileWriter("C:/d/C201/BlueJ/Projects/ch12/IOCopy.java"));       }       catch(Exception ex) { System.out.println("File not found"); }    }    public void copyFile() {       String line;       try {          while( input.ready() ) {             line = input.readLine();             output.println( line );             System.out.println(line);          }          output.close();       }       catch(Exception ex) { System.out.println("Exception " + ex); }    } }

Exercise 3 - File IOcopy Create a new class named, IOcopy. Copy the above, paste, compile, and create an IOcopy object. Should "File not found" print in the Terminal window: What happened and why? Check that J:/C201/Projects/chapter07/zuul-better/Game.java actually exists. If not, replace with another file path. Call the copyFile method on the IOcopy object.  What does the method do?

12.10 Summary