1. Chapter 12 Exception Handling and Text IO Part 2

2. Pros & Cons of Exception Handling
Advantage: Code is easier to understand and modify Exception handling separates error-handling code from normal programming tasks, thus making programs easier to read and to modify
Drawback: Slower performance, higher resource requirement Be aware, however, that exception handling usually requires more time and resources because it requires instantiating a new exception object, rolling back the call stack, and propagating the errors to the calling methods

3. When to Use Exceptions Handling?
Use try-catch blocks to deal with unexpected error conditions
Do not use them to deal with simple, expected situations
Example: 2nd block of code is preferable over the 1st
try {
System.out.println(refVar.toString()); }
catch (NullPointerException ex) {
System.out.println("refVar is null");
if (refVar != null)
System.out.println(refVar.toString());
else
System.out.println("refVar is null");

4. When to Throw Exceptions?
When an exception occurs in a method,
if we want the exception to be processed by its caller, we should create an exception object and throw it
if we can handle the exception in the method where it occurs, there is no need to throw it

5. Exceptions are objects based on the superclass java.lang.Throwable
Exception Types
Exceptions are objects based on the superclass java.lang.Throwable

6. Custom Exception Classes
Define custom exception classes only if Java’s predefined built-in classes are not sufficient
Define custom exception classes by extending Exception or a subclass of Exception

7. Example: Custom Exception Class
In Listing 12.7, setRadius() throws an exception if the radius is negative. Suppose you wish to pass the radius to the handler, you have to create a custom exception class
InvalidRadiusException
CircleWithRadiusException
TestCircleWithRadiusException
Run

8. The File Class
The File class contains the methods for obtaining the properties of a file/directory and for renaming and deleting a file/directory
File is intended to provide an abstraction that deals with most of the machine-dependent complexities of files and path names in a machine-independent fashion
File is a wrapper class for the filename and its directory path
A File object encapsulates the properties of a file or a path, but does not contain the methods for reading/writing content from/to a file

9. The File Class

10. Example: Explore File Properties
Objective: Write a program that demonstrates how to create files in a platform-independent way and use the methods in the File class to obtain their properties. The following figures show a sample run of the program on Windows and on Unix.
TestFileClass
Run

11. Text I/O
A File object does not contain the methods for reading/ writing content from/to a file
In order to perform I/O, you need to create objects using appropriate Java I/O classes
We can read/write strings and numeric values from/to a text file using Scanner and PrintWriter class objects

12. Writing Data Using PrintWriter
WriteData
Run

13. Reading Data Using Scanner
ReadData
Run

14. Example: PrintWriter & Scanner
Objective: Write a class named ReplaceText that replaces a string in a text file with a new string. The filename and strings are passed as command-line arguments as follows:
java ReplaceText sourceFile targetFile oldString newString
For example, invoking
java ReplaceText s.txt t.txt apple orange
replaces all the occurrences of apple by orange in s.txt and saves the new file in t.txt
ReplaceText
Run

15. Reading Data from the Web
Just like we can read data from a file on your computer, we can also read data from a file on the Web

16. Reading Data from the Web
URL url = new URL("
After a URL object is created, you can use openStream() defined in the URL class to open an input stream and use this stream to create a Scanner object as follows:
Scanner input = new Scanner(url.openStream());
ReadFileFromURL
Run