Learn Java Exception Handling, Streams, and Text Files

1. Chapter 8 • Overview • Learn to use try catch blocks • Learn to use streams • Learn to use text files

2. Exceptions • We have used if statements to Prevent errors • if (count != 0) { average = sum / count; } else { average = 0; System.out.println(“Error – count is zero”); • Protects against runtime error

3. Exceptions classes • There are a series of classes used to trap exceptions. • See table top of page 517 for some typical run time exceptions

4. Try-Catch statement • Some errors can not be protected against using if statements • Try-catch lets the error occur • if occurs it catches and handles the error • readInt uses this • used to catch bad user input • See example bottom of page 517

5. New Statement!!! Way Cool  • Syntax try { //statements that may throw exception} catch(exceptionType parameterName) { // statements for this exception type } ……………. finally { //statements to execute after try/catch }

6. Multiple catch blocks • In the previous code you can have multiple catch blocks • allows you to catch multiple exception types • finally block is optional executes after try-catch blocks • see example page 520 • btw only one catch

7. Catch block order • This is important if having multiple catch blocks • Need to keep in mind class hierarchy • Only first catch block that matches exception is executed. • Class Exception is the superclass of all exceptions classes • if it is first it will always be executed since all exceptions are of this type • It should be last to catch anything that may have been missed • See example page 522

8. Other helpful methods • ex.getMessage() • retrieves message indicating exception type • ex.printStackTrace() • shows which step throw error • See output bottom of page 523

9. Throwing exceptions • We can throw exceptions out of a method, back to the calling method. • Use throw clause in header of method • Tells compiler that this method can throw and exception • The calling program must then catch the exception • See example on page 525

10. Checked exceptions • Special category of exceptions called checked exceptions • You must either • catch these exceptions • or handle by throwing to calling method • If you do not you will get syntax error. • Subclasses of IOException are checked • Arithmetic are not

11. Throwing your own • You can also throw exceptions using the throw statement • Would allow you to customize the error message thrown • See bottom page 526 example

12. 8.2 Streams and Text Files • We have been using a stream off and on all semester • System.out is a stream to the system console • print • println • Streams are just a continuous “stream” of characters, both printable and non-printable.

13. Writing to Output Text File • Create Stream object and associate it with disk file • Give Stream object desired functionality • Read information from an input file or write information to an output file • Close the file

14. Write to file • See example page 531 • FileWriter outStream …….. • creates stream object • PrintWriter outs = new……. • Wraps stream object in new object • Gives desired functionality • Program then writes the data to text file

15. Input/Output Exceptions • IOException is the most general class for I/O • Not only can you output to system console • You can also read from the system console • System.in • readLine • Most useful when running from command prompt.

16. 8.3 Using Text Files • This is what we need to learn to do to complete Major assignment number 4. • FileReader inStream = new FileReader(“myData.txt”); • BufferedReader ins = new BufferedReader(inStream); • Methods we then use are: • ins.readLine(); • ins.ready();

17. How to read text file • DVD GUI • If we look on page 544 the DVD example • The method readDVDs in the middle of the page: • basically does what we need to complete the project • title going to null signifies the end of the file • can also use ready() to find out when file ends • We need to read our input file for Major 4, although we do not need to store the records in an array

18. Chapter 8 in closing • This is what we need to know out of Chapter 8 • Section 8.5 Binary files • These files are just files that are storing binary data, not text data. • Binary data is stored in native format. • Our Major 4 uses a text file that can actually be opened in notePad