Execution errors

 

Exceptional conditions can stop a program abruptly before it completes its intended work. 

           

Examples of exceptions include:

 

·          Issue a command to read a non-existent file.

·          User enters invalid data

·          Divide by zero

·          Subscript that is too large for the array

·          Calculate a value that is too large for the variable’s type.

 

 

Programs “throw” errors

 

When a Java program meets a condition it is not prepared to solve, it “throws” an “error” or an “exception.” 

 

 

“Errors” are normally generated by the System:

 

·          bugs in the Java VM, or hardware limitations (out of memory or other resources).

·          these errors are not handled within a program.

 

 

“Exceptions” are generally Programmer errors:

 

·          Bugs within the program or unexpected data supplied to the program

·          can be caught and handled within a program

 

                        

“Exception” is also a Java class

 

that creates an object describing the “exceptional” situation

 

 

Some approaches for handling error conditions

 

·          Ignore potential errors. 

 

Most of the time, let Java terminate the program when an error occurs.  While the program is “down,” the programmer will have to analyze the cause, fix the condition, test program changes, then rerun the program.

 

Advantage:  The program code is minimal and clearly focused toward what should be accomplished.

 

Disadvantage:  If the program is crucial, workflow of users may be impaired, schedules may not be met, employees may be idle, or laid-off.

 

·          Test for all possible errors before processing the data.

 

Advantage:  Potential errors will be recognized and avoided.  The program will be “down” a minimum amount of time.

 

Disadvantage:  Program code becomes larger and the processing time of the program is increased as it checks all data and conditions for possible error.   If the error checking tasks is intermixed with the regular processing tasks,  the program code becomes harder to understand.

 

·          Try to execute code.  If it doesn’t work, catch the errors and process them.

 

Advantage:  A block of code will be clearly focused toward achieving a given task.  Exceptional events are grouped and not in the way of the primary task.  Errors will only be processed if they occur.  The program will be “down” a minimum amount of time.

 

Disadvantage:  Program code becomes larger. 

 

 

Java Exception Handling:  Try block, one or more catch blocks and an optional finally block

 

 

 

try {

// Statements that may cause Exception1 or Exception2

// If no statement causes an exception, then the

// catch blocks will not be executed.  If an exception

// occurs, statement processing finds the first

// appropriate catch block.

}

catch (Exception1 e1) { // catch can handle one type of exception

// handle Exception1 – what to do about it

System.out.println(“Error is “ + e1.getMessage());

}

catch (Exception2 e2) {

// handle Exception2

System.out.println(“Error is “ + e2.getMessage());

}

catch (Exception error) {

     // a generic “catch-all” block to handle any Exception

// other than Exception1 or Exception2

System.out.println(“Error is “ + error.getMessage());

// put catch-all blocks after the more specific error blocks

}

finally {
     // optional code always executed after try block completes
     // whether an exception is thrown or not

// typically used to do program clean up operations like

// closing files which were opened for processing

}

 

                There must be at least one catch block following a try block.

 

 

Program Example:  ProductCodes.java

 

 

import java.io.*;

public class ProductCodes {

   public static void main  (String[] args) throws IOException {

      String code = " ";

      char zone = '\t';

      int district = 0, entered = 0, banned = 0;

      BufferedReader br =

         new BufferedReader

            (new InputStreamReader (System.in));

      System.out.print ("Enter product code (XXX to quit): ");

      code = br.readLine();

      while (!code.equals ("XXX")) {

        try {

          zone = code.charAt(4);

          district = Integer.parseInt(code.substring(0, 4));

          entered++;

          if (zone == 'R' && district > 2000)

            banned++;

        } // end of try

        catch (StringIndexOutOfBoundsException exception) {

          System.out.println ("\t" + exception.getMessage()

          + " Product Code should be at least 5 characters:");

        } // end of catch

        catch (NumberFormatException exception) {

          System.out.println ("\tCharacters 1 though 4 should be numeric: "

           + " " + exception.getMessage());

        } // end of catch

        System.out.print ("Enter product code (XXX to quit): ");

        code = br.readLine();

      } // end of while

      System.out.println ("# of non-exception codes entered: " + entered);

      System.out.println ("# of banned codes entered: " + banned);

   }

}

 

 

Class Structure to Define Your Own Exception

 

public class MyException extends Exception {
  public MyException(String msg) {
     super(msg);
  }
}

 

 

Program Example:  OutOfRangeException

 

public class OutOfRangeException extends Exception {

   //  Sets up the exception object with a particular message.

   OutOfRangeException (String message) {

      super (message);

   }

}

 

 

Throw Statement and Throws Clause

 

 

 

 

The programmer can generate an exception by executing a “throw” statement.

 

throw anException

 

If your program might throw a customized exception, use a “throws” clause.   

 

public class MyClass {
  public void aMethod(int i, int j) throws MyException {
    // ...
    throw new MyException(reason);
    // ...
  }
}

 

Program Example:  StoppedByException.java

 

import java.io.*;

public class StoppedByException {

 

   public static void main (String[] args)

     throws IOException, OutOfRangeException {

       BufferedReader br =

           new BufferedReader

(new InputStreamReader (System.in));

       final int MIN = 25, MAX = 40;

       OutOfRangeException problem = new OutOfRangeException

("Input value is out of range.");

       System.out.print ("Enter an integer value between "

+ MIN + " and " + MAX + ", inclusive: ");

       int value = Integer.parseInt(br.readLine());

       if (value < MIN || value > MAX)

         throw problem;

       System.out.println ("End of main method."); 

// may never reach the above println statement

   }

}

 

Program Example:  CatchException.java

 

import java.io.*;

public class CatchException {

 

   public static void main (String[] args)

     throws IOException {

       BufferedReader br =

           new BufferedReader

(new InputStreamReader (System.in));

       final int MIN = 25, MAX = 40;

       OutOfRangeException problem =

           new OutOfRangeException

("Input value is out of range.");

       System.out.print ("Enter an integer value between "

+ MIN + " and " + MAX + ", inclusive: ");

       try {

         int value = Integer.parseInt(br.readLine());

         if (value < MIN || value > MAX)

           throw problem;

       }

       catch (OutOfRangeException exception) {

         System.out.println(exception.getMessage());

       }

       System.out.println ("End of main method."); 

// should reach println statement above

   }

}

 

 

I/O Streams

 

·          A stream is a sequence of bytes.

 

·          In a program, we read information from an input stream and write information to an output stream

 

·          An I/O stream is either a

o         character stream, which deals with text data

o         byte stream, which deal with binary data

 

 

 

Two Types of I/O

 

Stream-based I/O supports reading or writing data sequentially.

 

A stream may be opened for either reading or writing, but not both.

 

 

Random access I/O supports reading and writing data at any position of a file.

           

A random access file may be opened for both reading and writing.

 

 

 

 

Some classes used for I/O

 

 

FileInputStream

Reads bytes from a file

Reader

Abstract class for reading character streams

InputStreamReader

Reads bytes & translates into characters

BufferedReader

Read character-input and builds lines or arrays

StreamTokenizer

Parse strings into tokens

OutputStream

Abstract class to outputting bytes

FileOutputStream

Write data to a file

Writer

Write character streams

 

 

If an I/O class does not do everything needed to process input from a file,

“pipe” the output of that class to another class that further refines the data.

Continue building a pipe until the data is in the form needed.

 

 

            Example of connecting classes via a “piped” process:

           

text input file

ŕ

FileInputStream

ŕ

InputStreamReader

ŕ

 

 

extracts bytes from file

and outputs bytes

 

gets bytes from Stream

and outputs characters

 

 

 

                                              

     

Common Class Input Combinations

   

Read character at a time from a text file

 

 

FileInputStream(fileName)

InputStreamReader

 

 

Read one line at a time from a text file

 

FileInputStream (fileName)

InputStreamReader

BufferedReader

 

 

Read one word at a time from a text file

 

 

FileInputStream (fileName)

InputStreamReader

BufferedReader

StreamTokenizer

 

 

Read Keyboard input

 

 

InputStreamReader(System.in)

BufferedReader

 

 

Read int, double, etc. from a non-text file

 

 

FileInputStream(fileName)

DataInputStream

 

 

   

 

 

I/O Program Examples

 

 

Inventory.dat

 

Widget 14 3.35

Spoke 132 0.32

Wrap 58 1.92

Thing 28 4.17

Brace 25 1.75

Clip 409 0.12

Cog 142 2.08

 

 

ReadCharacters.java  (read the file one character at a time)

 

 

import java.io.*;

public class ReadCharacters{

  public static void main (String[] args) {

    String file="C:/bluej/read/inventory.dat";

    int asciiValue;

    char ch;

    try {

        FileInputStream stream = new FileInputStream(file);

        InputStreamReader reader = new InputStreamReader (stream);

        asciiValue = reader.read();

        while (asciiValue != -1) {

            ch  =  (char) asciiValue;

            System.out.print(String.valueOf(ch));

            asciiValue = reader.read();

        }   

        stream.close();

    } catch (FileNotFoundException exception) {

        System.out.println("The file " + file + " was not found.");

    } catch (IOException exception) {

        System.out.println(exception);

    }

  }

}

 

ReadLines.java

 

import java.io.*;

public class ReadLines{

  public static void main (String[] args) {

    String line, file="C:/bluej/read/inventory.dat";

    try {

        FileInputStream stream = new FileInputStream(file);

        InputStreamReader reader = new InputStreamReader (stream);

        BufferedReader buffer = new BufferedReader (reader);

        line = buffer.readLine();

        while (line != null){

            System.out.println(line);

            line = buffer.readLine();

        }   

        stream.close();

    } catch (FileNotFoundException exception) {

        System.out.println("The file " + file + " was not found.");

    } catch (IOException exception) {

        System.out.println(exception);

    }

  }

}

 

 

 

ReadWords.java

 

import java.io.*;

import java.util.StringTokenizer;

public class ReadWords{

  public static void main (String[] args) {

    String word, line, file="C:/bluej/read/inventory.dat";

    String name;

    float price;

    int units;

    try {

      FileInputStream stream = new FileInputStream(file);

      InputStreamReader reader = new InputStreamReader (stream);

      BufferedReader buffer = new BufferedReader (reader);

      line = buffer.readLine();

      while (line != null){

        StringTokenizer tokenizer = new StringTokenizer (line);

        name = tokenizer.nextToken();

        try

          {

           units = Integer.parseInt(tokenizer.nextToken());

           price = Float.parseFloat(tokenizer.nextToken());

           System.out.println(name + " " + units + " " + price );

        } catch (NumberFormatException exception) {

           System.out.println("Error in input. Line ignored:");

           System.out.println(line);

          }

        line = buffer.readLine();

       }   

        stream.close();

    } catch (FileNotFoundException exception) {

        System.out.println("The file " + file + " was not found.");

    } catch (IOException exception) {

        System.out.println(exception);

    }

  }

}

 

 

 

 

InventoryItem.java                                       

 

import java.text.DecimalFormat;

public class InventoryItem {

  private String name;

  private int units;   

  private float price; 

  private DecimalFormat fmt;

  public InventoryItem(String itemName, int numUnits,

                       float cost) {

    name = itemName;

    units = numUnits;

    price = cost;

    fmt = new DecimalFormat ("0.##");

  }

  public String toString() {

    return name + ":\t" + units + " at " + price +

      " = " + fmt.format ((units * price));

   }

}

 

 

 

Inventory.java

 

 

import java.util.StringTokenizer;

import java.io.*;

public class Inventory {

  public static void main (String[] args) {

    final int MAX = 100;

    InventoryItem[] items = new InventoryItem[MAX];

    StringTokenizer tokenizer;

    String line, name, file="C:/bluej/read/inventory.dat";

    int units, count = 0;

    float price;

    try {

      FileReader fr = new FileReader (file);

      BufferedReader inFile = new BufferedReader(fr);

      line = inFile.readLine();

      while (line != null) {

        tokenizer = new StringTokenizer (line);

        name = tokenizer.nextToken();

        try {

          units = Integer.parseInt(tokenizer.nextToken());

          price = Float.parseFloat(tokenizer.nextToken());

          items[count++] =

          new InventoryItem(name, units, price);

        }

        catch (NumberFormatException exception) {

          System.out.println("Error in input. Line ignored:");

          System.out.println(line);

          }

      line = inFile.readLine();

      }

      inFile.close();

      for (int scan = 0; scan < count; scan++)

        System.out.println(items[scan]);

        }

      catch (FileNotFoundException exception) {

        System.out.println("The file " + file + " was not found.");

      }

      catch (IOException exception) {

        System.out.println(exception);

      }

  }

}

 

ReadLocalFileAsURLStream.java

 

 

import java.io.*;

import java.net.*;

public class ReadLocalFileAsURLStream{

  public static void main (String[] args) {

    String line, file="C:/bluej/read/inventory.dat";

    try {

        URL URLfile = new URL ("file:///C:/bluej/read/inventory.dat");

        InputStream input = URLfile.openStream();

        BufferedReader buffer = new BufferedReader(

                                new InputStreamReader(input));

        line = buffer.readLine();

        while (line != null)

        {

            System.out.println(line);

            line = buffer.readLine();

        }   

    }  

    catch (FileNotFoundException exception) {

        System.out.println("The file " + file + " was not found.");

    }

    catch (IOException exception) {

        System.out.println(exception);

    }

  }

}

 

 

 

ReadRemoteURLFile.java

 

 

import java.io.*;

import java.net.*;

public class ReadRemoteURLFile{

  public static void main (String[] args) {

    String line, file="http://condor.depaul.edu/~jpetlick/extra/java/inventory.dat";

    try {

        URL URLfile = new URL (file);

        InputStream input = URLfile.openStream();

        BufferedReader buffer = new BufferedReader(

                                new InputStreamReader(input));

        line = buffer.readLine();

        while (line != null) {

            System.out.println(line);

            line = buffer.readLine();

        }   

    }  

    catch (FileNotFoundException exception) {

        System.out.println("The file " + file + " was not found.");

    }

    catch (IOException exception) {

        System.out.println(exception);

    }

  }

}

 

 

Output to a file

 

 

//********************************************************************

//  TestData.java       Author: Lewis and Loftus

//

//  Demonstrates the use of a character file output stream.

//********************************************************************

 

import java.io.*;

public class TestData {

   //-----------------------------------------------------------------

   //  Creates a file of test data that consists of ten lines each

   //  containing ten integer values in the range 0 to 99.

   //-----------------------------------------------------------------

   public static void main (String[] args) throws IOException {

      final int MAX = 10;

 

      int value;

      String file = "test.dat";

 

      FileWriter fw = new FileWriter (file);

      BufferedWriter bw = new BufferedWriter (fw);

      PrintWriter outFile = new PrintWriter (bw);

 

      for (int line=1; line <= MAX; line++) {

         for (int num=1; num <= MAX; num++) {

            value = (int) (Math.random() * 100);

            outFile.print (value + "   ");

         }

         outFile.println ();

      }

 

      outFile.close();

      System.out.println ("Output file has been created: " + file);

   }

}