1. Workshop for Programming And Systems Management Teachers
Chapter 11
Input / Output and Exceptions
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Learning Goals
Understand at a conceptual and practical level
Standard input and output
Streams
How to handle exceptions
How to read from a character file
How to write to a character file
How to read and write objects (serialization)
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3. Standard Input and Output
We have been using System.out.println to print output to a PrintStream (standard output).
System.out.println(“First Name: “ + firstName);
There is also a System.err PrintStream that can be used to write to the standard error output.
System.err.println(“Error: no file name given”);
You can use System.in to read a byte or bytes from an InputStream (standard input).
int numGrades = System.in.read();
PrintStreams and InputStreams are in the java.io package.
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4. Input and Output Streams - java.io
Java handles input and output through sequential streams of bits
Programs can read from a stream or write to a stream
Source or
Destination
Program
100110
Byte Data
Character Data
File
The source or destination of the stream may be a file, a network socket, a string, an array of bytes, etc.
The bit stream can be made up of objects, binary data, character data, images, sounds, etc.
There is a tutorial on input and output streams at
String
Array
Pipe
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5. Character versus Byte Streams
Java input and output classes are categorized by the type of data they handle
Character
use Reader/Writer classes
Byte
use Stream classes
Java input and output classes can also be categorized by
data sink classes (read and write data from different sources like files, pipes, memory)
processing classes (process the data, filter it, buffer it, print it)
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Data Sink Classes
You can read from files, memory in the form of character arrays, strings, or byte arrays, and from pipes.
Pipes are ways that two threads can communicate.
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Processing Classes
The buffering process makes input and output more efficient since memory access is much faster than secondary storage (disk) access.
You can extend the Filter classes to do your own special filtering of the data.
Object serialization is a way of “freeze-drying” an object to pass it in a stream and reconstitute it on the other side back into an object.
DataInputStream and DataOutputStream allow you to read and write java types in a machine independent way.
The LineNumber classes keep track of the number of lines read from a file.
The Pushback classes let you peek ahead at the next character or byte and yet still read it normally.
The Print classes make it easy to print data.
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8. Chaining Input and Output Classes
Often input or output classes are chained
Passing one type of input/output class to the constructor for another
One common thing is to chain a processing class with a data sink class
Like a BufferedReader or BufferedWriter and a FileReader or FileWriter
new BufferedReader(new FileReader(fileName));
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Exceptions
Exceptions are disruptions in the normal flow of a program. Exception is short for exceptional event.
The programmer is required to handle checked exceptions in Java
like trying to read from a file that doesn’t exist
Run-time exceptions do not have to be handled by the programmer
like trying to invoke a method on a object reference that is null
Children of RuntimeException
Exceptions are unusual conditions that a program might want to handle.
The compiler will give you an error message if you fail to catch a checked exception.
Run-time exceptions are problems that are detected at run-time like divide by zero or null pointer exception. Runtime exceptions can occur anywhere in a program and in a typical program can be very numerous. Java does not require the programmer to handle them since to do so would require too much additional effort.
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Exception Exercise
Find the documentation for the Exception class
What package is it in?
What is the parent class?
Find the documentation for the RuntimeException class
What subclasses does it have?
Find the documentation for the FileNotFoundException
Does this exception need to be caught?
Find the documentation for NullPointerException
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11. Causing Exceptions Exercise
Try the following in DrJava interaction’s pane
> String temp = null;
> temp.toLowerCase()
> int[] numberArray = {9, 8, 3};
> numberArray[5]
> FileReader reader = new FileReader("test.txt");
> FileWriter writer = new FileWriter("test.txt");
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Try and Catch
Use a try & catch clause to catch an exception
try {
code that can cause exceptions
} catch (ExceptionClassName varName) {
code to handle the exception }
You can catch several exceptions
Make the most general one last
All exceptions are children of the class Exception
There can be multiple catch clauses that each catch a different exception. If you do this be sure to catch the most general class last.
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Try and Catch Example
What if you want to know if a file isn’t found
That you are trying to read from
If this occurs you might want to use a JFileChooser to let the user pick the file
You also want to handle any other error
try {
code that can cause the exception
} catch (FileNotFoundException ex) {
code to handle when the file isn’t found
} catch (Exception ex) {
code to handle the exception }
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Catching Exceptions
A catch clause will catch the given Exception class and any subclasses of it.
So to catch all exceptions use:
try {
code that can throw the exception
} catch (Exception e) {
System.err.println(“Exception: “ + e.getMessage());
System.err.println(“Stack Trace is:”);
e.printStackTrace(); }
You can create your own exceptions by subclassing Exception or a child of Exception.
You shouldn’t subclass RuntimeException.
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15. The optional finally clause
A try and catch statement can have a finally clause
Which will always be executed
Will happened if no exceptions
Will happened even if exceptions occur
Used for releasing resources
try {
code that can cause the exception
} catch (FileNotFoundException ex) {
code to handle when the file isn’t found
} finally {
code to always be executed }
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Throwing an Exception
Use a throw clause to throw an exception
/** Method that throws an exception */
public retValue methodName() throws ExceptionClassName {
...
if (errorCondition) throw new ExceptionClassName(); }
The method must report all exceptions that it throws
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17. Reading Lines of Character Data
Enclose the code in a try and catch clause
Catch FileNotFoundException if the file doesn’t exist
And you want to give the user a chance to specify a new file using a JFileChooser
Catch Exception to handle all other errors
Create a buffered reader from a file reader for more efficient reading
File names are relative to the current directory
Loop reading lines from the buffered reader until the line is null
Do something with the data
Close the buffered reader
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18. Reading from File Example
BufferedReader reader = null;
String line = null;
// try to read the file
try {
// create the buffered reader
reader = new BufferedReader(new FileReader(fileName));
// loop reading lines till the line is null (end of file)
while ((line = reader.readLine()) != null) {
// do something with the line }
// close the buffered reader
reader.close();
} catch (Exception ex) {
// handle exception
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19. Read From File Exercise
Modify FortuneTellerPanel.java to add a constructor that takes a file name and reads the items for the array of fortunes from that file
The first item in the file is the number of fortunes
To use to create the array
If there is any error tell the user and use the original array of fortunes
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Writing to a File
Use a try-catch clause to catch exceptions
Create a buffered writer from a file writer
writer = new BufferedWriter(new FileWriter(fileName));
Write the data
writer.write(data);
Close the buffered writer
writer.close();
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21. Writing to a File Example
BufferedWriter writer = null;
PictureInfo info = null;
String endOfLine = System.getProperty("line.separator");
// Try to write the picture information
try {
// create a writer
writer = new BufferedWriter(new FileWriter(directoryName + infoFileName));
// write out the number of picture information objects
writer.write(pictureInfoArray.length + endOfLine);
// loop writing the picture info to the file
for (int i = 0; i < pictureInfoArray.length; i++) {
info = pictureInfoArray[i];
writer.write(info.getFileName() + "|" + info.getCaption() + "|" + endOfLine); }
// close the writer
writer.close();
} catch (Exception ex) {
System.out.println("Trouble writing the picture information to " + infoFileName);
See the code in PhotoAlbum.java.
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22. Saving and Restoring Objects
Applications often need to save the current objects to a file.
So they aren’t lost when the application is stopped
Applications often read objects from files.
In the main method you call a method to read in the objects from a file
Java provides classes to read objects and write objects
ObjectInputStream, ObjectOutputStream
The input and output classes are in the package java.io.
To use the input and output classes import the java.io package
import java.io.*;
File input and output streams are specialized classes that handle reading and writing files.
Piped input and output streams are specialized classes that handle reading and writing pipes. Pipes are used in network connections.
Object input and output streams are specialized classes that handle reading and writing objects.
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Object Output
Import the package java.io
import java.io.*;
Make the object serializable
public class Name implements Serializable
Open the file for output using a FileOutputStream
FileOutputStream fOut = new FileOutputStream(“fileName”);
Create an ObjectOutputStream using the FileOutputStream as a parameter to the constructor.
ObjectOutputStream oOut = new ObjectOutputStream(fOut);
Output the object
oOut.writeObject(theObject);
Flush to make sure all objects are written
oOut.flush();
The Serializable interface allows us to “freeze-dry” objects in a form that they can be reconstituted from later.
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24. Write Objects Exercise
Modify WriteObjects/Person.java
Create a class method that takes an array of Person objects and writes them to a file with the passed name
Modify the main method to create an array of Person objects and call the method to write them to a file
Make objects for yourself and your family
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Object Input
To input an object we need to
Import the package java.io
import java.io.*;
Make the object serializable
public class Name implements Serializable
Open the file for input using a FileInputStream
FileInputStream fIn = new FileInputStream(“fileName”);
Create an ObjectInputStream using the FileInputStream as a parameter to the constructor.
ObjectInputStream oIn = new ObjectInputStream(fIn);
Input the object - must cast to class
objectName = (ClassName) oIn.readObject();
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Read Objects Exercise
Edit ReadObjects/Person.java
Create a class method to read Person objects from a passed file name
And return an array of Person objects
Modify the main method to call this method
And loop through the array of Person objects and use System.out.println(person) to print each
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Summary
There are many classes in java.io for handling input and output
Use a buffer to make input or output more efficient
Checked exceptions in java must be caught with a try and catch (optional finally) clause
The code will not compile if the exception isn’t caught or thrown to the caller
Runtime exceptions (children of RuntimeException) do not have to be in a try and catch clause
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