Chapter 10 Exceptions and File I/O. © 2004 Pearson Addison-Wesley. All rights reserved10-2...

Chapter 10

Exceptions and File I/O

© 2004 Pearson Addison-Wesley. All rights reserved 10-2

Exceptions

• Exception handling is an important aspect of object-oriented design

• Chapter 10 focuses on:

the purpose of exceptions exception messages the try-catch statement propagating exceptions the exception class hierarchy GUI mnemonics and tool tips more GUI components and containers


Outline

Exception Handling

The try-catch Statement

Exception Propagation

Exception Classes

I/O Exceptions

Tool Tips and Mnemonics

Combo Boxes

Scroll Panes and Split Panes


File I/O Reading in from a file

• Java has several classes that handle input from a file.

• One of these is FileReader. Declaring an object of type FileReader will allow a program to read in one character at a time with the read method

• But generally, one wants to read in more than one character at a time, say a line at a time. For this we use the BufferedReader class together with the FileReader class. It has the method readLine(), which will read in a line of a file.


File I/O -Reading in from a file

• Declaring an object of type BufferedReader: BufferedReader filein=new BufferedReader(new

FileReader( “data.dat”)); You can now use the filein object to readin a line of a file

with a statement String input = filein.readLine();

• Read in an entire file as follows: BufferedReader filein = new BufferedReader(new

FileReader(“data.dat”)); //declaring object to read in while((str =filein.readLine()) != null) //reading in a line and

//checking whether at the end of file System.out.println(str); //printing out the line

• Remember the method with these statements will need a throws IOException in the header or you will have to have a try…catch block around them


File I/O Writing out to a file

• Java has a number of classes that can be used to write data out to a file.

• We will use two of them together, PrintWriter and FileWriter

• FileWriter allows writing to a file one character at a time and PrintWriter allows writing a line at a time, if used with FileWriter.

• The methods that PrintWriter has to write with are print(), and println()

• Because print() and println(), as we have seen already are overloaded, you can write out ints, doubles, Strings or concatenated combinations.


File I/O Writing out to a file

• To declare an object of PrintWriter Class :• PrintWriter fileout = new PrintWriter(new

FileWriter(“dataout.dat”));• To to write a line to your file with this object:

fileout.println(“My name is “+name);

• To read write out an array of ints, one number per line, after declaring fileout as above: int arr [] = { 1,2,3,4,5,6,7,8,9,10,11,13,15,17,20,30,23}; for(int index=0;index<arr.length;index++)

fileout.println( “arr[index] + “ is on the “ +index + “ line”);

• Remember, again you would need a throws IOException in the header of the method with these statements or have to surround the statements with a try ..catch block.


File I\O -Reading in and Writing out

• To read in from a file data.dat and write out what you have read in to a file dataout.dat: BufferedReader input = new BufferedReader(new

FileReader(“data.dat”)); PrintWriter output = new PrintWriter(new

FileWriter(“dataout.dat”)); String str; try { while((str = input.readLine()) != null)

• output.println(str); }catch(FileNotFoundException fnfe) {

• System.out.println(fnfe); }catch(IOException oe) {

• System.out.println(oe); }


File I/O - parsing the line of input

• Once you have read in a line from a data file, say with the statement BufferedReader input = new BufferedReader(new

FileReader(“data.dat”)); String str = input.readLine();

You have a String variable, str, with a line from data.dat.

If the line just contains a statement that you want to print out you can just say System.out.println(str); and be finished

But suppose instead the line contains a set of numbers

That you want to add up and take an average of. In this case you need some way of separating the numbers.

In Java, there is a class that allows you to do that, the String Tokenizer class.


Separating the data in a String

• The String Tokenizer class allows you to parse a String (line) of data and extract each data item in the String.

• To declare a String Tokenizer object: StringTokenizer tokenizer = new StringTokenizer(str); //where str is a line of data read in. So you declare and

instantiate a new tokenizer object for each line (String) of data read in from a file.

• Then you use a loop to search the String str to extract each data item. while(tokenizer.hasMoreTokens()) { //checking if at end

• String word = tokenizer.nextToken(); //getting next word• numwords++; //counting the words}


Separating the data in a String

• When you declare a String Tokenizer object like this: StringTokenizer tokenizer = new StringTokenizer(str);

• The separator for the data will automatically be a blank, so words or numbers separated by blanks can be extracted

• You can specify what the separator will be: StringTokenizer=new StringTokenizer(str, “,”); Now the

separator will be a comma.


Reading in an array of data in a file with 10 numbers per line

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

BufferedReader input = new BufferedReader(new FileReader(“data.dat”)); //declaring input objectint arr [] = new int[10][30]; //declaring array to hold inputint I = 0; String str;while((str =input.readLine()) != null) //reading until eof{

StringTokenizer tok=new StringTokenizer(str); //data //separated by blanks

for(int j = 0; j<arr[I]length;j++)while(tok.hasMoreTokens()) //at end of line

arr[ I][j]=Integer.parseInt(tok.nextToken());

I++; }

}


File I/O- Reading in text file and counting the words• Suppose we have a file with the following words in

it:

“Out, out, brief candle, life’s but a walking shadow, a poor player who struts and frets his hour upon the stage and then is heard no more; it is a tale told by an idiot, full of sound and fury signifying nothing”


File I/O -reading in text file and counting words cont. • Then to read in that file and count the number of words in it:

BufferedReader input=new BufferedReader(new FileReader(“data.dat”)); //get an object to use for readingString str; //String variable to hold inputint numwords=0; //int to hold word countwhile((str=input.readLine()) != null) //loop reading in lines{

StringTokenizer tok = new StringTokenizer(str); //Tokenizer for //each line

while(tok.hasMoreTokens()) //loop for getting each word on //line

{tok.nextToken(); //getting next wordnumwords++; //adding another word to count

}

}


Summary

• Chapter 10 has focused on:

• the purpose of exceptions

• exception messages

• the try-catch statement

• propagating exceptions

• the exception class hierarchy

• GUI mnemonics and tool tips

• more GUI components and containers

Chapter 10 Exceptions and File I/O. © 2004 Pearson Addison-Wesley. All rights reserved10-2...

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