Transcript Exceptions and IO

Exceptions and IO
Dr. Andrew Wallace PhD BEng(hons) EurIng
[email protected]
Overview
• Problems
• Handling problems
• Input / output
• Streams
Problems
• What can go wrong?
• Boundaries
• Problems
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User inputs
Memory limits
Resource limits
Impossible maths
IO errors
Problems
• User inputs
• public class ParseException extends Exception
• public class DataFormatException extends Exception
Problems
• Memory limits
• public class ArrayIndexOutOfBoundsException
• public class NullPointerException
• public class OutOfMemoryError
Problems
• Resource limits
• public class FileSystemNotFoundException
• Public class RunTimeErrorException
Problems
• Impossible maths
• public class ArithmeticException
Problems
• IO errors
• public class FileNotFoundException
• public UnknownHostException
Quiz
• Name five problems that can occur in any java program
• How does Java handle the problems?
Handling problems
• Two types of problem handling classes
• Checked
• Anticipate and recover
• Try / catch block
• Method throws
• Unchecked
• No special handling
• Errors in the machine or run time errors
Object
Handling problems
Throwable
unchecked
Error
Exception
checked
unchecked
RuntimeException
Handling problems
• Create your own exceptions
• Create a subclass of one of the Java defined classes
public class EndOfTheWorldException extends Exception
• Add information on the error
• Parameter to the constructor
• Use the “throw” key word to throw your exception
if(bError)
{
throw new EndOfTheWorldException();
}
public void func() throws Exception
Handling problems
• Example
try
{
…
}
catch(IOException e)
{
…
}
catch(Exception e)
{
…
}
finally
{
…
}
Handling problems
• Try with resources
BufferedReader br = new BufferedReader(new FileReader(path));
try
{
try (BufferedReader br =
new BufferedReader(new FileReader(path)))
{
Throwable.getSuppressed
Quiz
• What does checked and uncheck mean in regards to java
exceptions?
• How do you create you own exceptions?
Input / Output
• Communicate with the outside world
• Java allows many ways to get data in a send data out
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GUI (graphic inputs / outputs)
Keyboard / screen
Database input / output
File input / output
Input / Output
Input / Output
• File IO is an abstraction
• “File” is the data
• Place to read and write to is a “stream”
Input / Output
• Java IO package
• Standard streams
• System.in
• System.out
• System.err
- keyboard
- console
- console
• Redirect
• System.setIn (Out) (Err)
- InputStream
- PrintStream
- PrintStream
Input / Output
• Java InputStream is abstract
• System.in is a specific implementation
• Read strings
• Read numbers
• Reads in a whole line
• Use Scanner to break up the string
Input / Output
• Scanner class
• Breaks up a sting into “tokens”
• Uses a delimiter (like space, for example)
• Use next functions to get data out
• Use has functions to check for more tokens
Input / Output
Scanner
sc = new Scanner(System.in);
String
strMsg;
while(sc.hasNext())
{
strMsg = sc.next();
…
}
Input / Output
• File
• Save data
• Exists between program runs
• Text file
• Simple and readable with other programs
Input / Output
• The File class
• Represents information on the file
• Path
• Director
• File attributes (hidden, is it readable? Does it exists?)
• System independent
Quiz
• What is a stream in java IO?
• What is a file in java IO?
Input / Output
• Class Reader and Class Writer
• Java super classes
• Read and write text (char)
• Open a file
• Read or write to a file
• Close a file
Input / Output
try
{
FileReader fInput = new FileReader(“inputfile.txt”);
FileWriter fOutput = new FileWriter(“outputfile.txt);
nChar = fInput.read();
fOutput.write(nChar);
}
catch( …)
{
…
}
finally
{
if(fInput != null)
fInput.close();
if(fOutput != null)
fOutput .close();
}
Input / Output
• Open a file
• File object
• FileDescriptor
• String (file name)
• FileWriter allow you to append text as well
• Close
Input / Output
• Read char as int
• Read an array of char
• Write a char as int
• Write an array of char
• Write a string
• flush
Input / Output
• More advanced IO
• Connect writer to another class
• PrintWriter(new FileWriter(“file.txt”));
• println methods for all basic java objects
• No exceptions - checkError()
Input / Output
• BufferedReader
• new BufferedREader(new FileReader(“file.txt”))
• String readLine();
• Scanner
• Scanner
• Scanner
fIn = new Scanner(new File(“file.txt”));
fIn = new Scanner(new FileReader(“file.txt”));
Streams
Data streams
Character
Input
Character
Output
Byte Output
Byte Input
Processing streams
Character
Input
Character
Output
Byte Input
Byte Output
Streams
• Data Streams
• character
• Unicode
• Reader
• FileReader
• int read methods
• Char arrays
Streams
• Data Streams
• Byte
• 8 bit data
• InputStream
• FileInputStream
• int read()
• Byte arrays
Streams
• Data Streams
• Character
• Writer
• FileWriter
• Byte
• OutputStream
• FileOutputStream
• int write
• void write
Streams
• Processing Streams
• Character
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BufferedReader
BufferedWriter
InputStreamReader
LineNumberReader
• Byte
• ObjectInputStream (output)
• ZipInputStream (output)
Quiz
• What are the two types of steams in java?
• What are the basic data types used?
Questions?