Java io

based on OOP with Java, by David J. Barnes

Input-Output 1

The java.io Package

Text files• Reader and Writer classes

Byte stream files• InputStream, FileInputStream, OutputStream,

FileOutputStream

Primitive-data stream files• DataInputStream, DataOutputStream

Object stream files• ObjectInputStream & ObjectOutputStream


Input-Output 2

The File Class

A File object represents an abstract pathname.– Represents both files and directories (folders).– Name, parent directory, size, permissions.– Constructor takes the file’s name:

•File info = new File("Letter.txt");

– No exception thrown if the file does not exist.


Input-Output 3

Methods of the File Class

File info = new File("Letter.txt");if(info.exists()){ System.out.println("Size of "+info.getName()+ " is "+info.length()); if(info.isDirectory()){ System.out.println("The file is a directory."); } if(info.canRead()){ System.out.println("The file is readable."); } if(info.canWrite()){ System.out.println("The file is writeable."); }}


Input-Output 4

FileReader and FileWriter

The two main classes for reading and writing text files.– A file is opened when its name is passed to

their constructors.– Files should be closed (via close) when

finished with.– Their read and write methods deal with char

and char[].


Input-Output 5

Opening and Closing a Filetry{ // Try to open the file. FileReader inputFile = new FileReader(filename); // Process the file's contents. ... // Close the file now that it is finished with. inputFile.close();}catch(FileNotFoundException e){ System.out.println("Unable to open "+filename);}catch(IOException e){ // The file could not be read or closed. System.out.println("Unable to process "+filename);}


Input-Output 6

Copying a Text Filestatic void copyFile(FileReader inputFile, FileWriter outputFile) throws IOException { final int bufferSize = 1024; char[] buffer = new char[bufferSize]; // Read the first chunk of characters. int numberRead = inputFile.read(buffer); while(numberRead > 0){ // Write out what was read. outputFile.write(buffer,0,numberRead); numberRead = inputFile.read(buffer); } outputFile.flush();}


Input-Output 7

Reading Single Charactersstatic void copyFile(FileReader inputFile,

FileWriter outputFile){ try{ // Read the first character. int nextChar = inputFile.read(); // Have we reached the end of file? while(nextChar != -1){ outputFile.write(nextChar); // Read the next character. nextChar = inputFile.read(); } outputFile.flush(); } catch(IOException e){ System.out.println("Unable to copy a file."); }}


Input-Output 8

Buffered Reader and Writers

try{ FileReader in = new FileReader(infile); BufferedReader reader = new BufferedReader(in); FileWriter out = new FileWriter(outfile); BufferedWriter writer = new BufferedWriter(out); ... reader.close(); writer.close();}catch(FileNotFoundException e){ System.out.println(e.getMessage());}catch(IOException e){ System.out.println(e.getMessage());}


Input-Output 9

Line-by-Line Copying

BufferedReader reader = new BufferedReader(...);// Read the first line.String line = reader.readLine();// null returned on EOF.while(line != null){ // Write the whole line. writer.write(line); // Add the newline character. writer.newLine(); // Read the next line. line = reader.readLine();}


Input-Output 10

PrintWriter

try{ FileWriter out = new FileWriter(outfile); PrintWriter writer = new PrintWriter(out);

writer.println(…);writer.format(“%d…”,…);

writer.close();}catch(IOException e){ System.out.println(e.getMessage());}


Input-Output 11

System.in

BufferedReader reader = new BufferedReader(

new InputStreamReader(System.in));


Input-Output 12

The StringTokenizer Class

Defined in the java.util package. Splits strings into separate String tokens.

– Delimiter characters are user settable (whitespace by default).

– Will also return delimiters if required.

Simple interface– public boolean hasMoreTokens()– public String nextToken()


Input-Output 13

Finding Words

String line = in.readLine();while(line != null){ StringTokenizer tokenizer = new StringTokenizer(line,",;:.\"' \t"); while(tokenizer.hasMoreTokens()){ String word = tokenizer.nextToken(); // Print the next word. System.out.println(word); } line = in.readLine();}


Input-Output 14

The Scanner Class

Defined in the java.util package. A scanner can be created from File, InputStream, or String.

Use useDelimiter to set the delimiting pattern.

Use next to retrieve the next token and use nextInt, etc. to retrieve a token of a certain type.


Input-Output 15

Finding Words

String line = in.readLine();while(line != null){ Scanner sc = new Scanner(line); sc.useDelimiter(",|;|:|\\.|\"|'| |\t"); while(sc.hasNext()){ String word = sc.next(); // Print the next word. System.out.println(word); } line = in.readLine();}


Input-Output 16

The Stream Classes

Several classes that deliver input as a stream of raw bytes, or write raw bytes.– BufferedInputStream, FileInputStream.– BufferedOutputStream,FileOutputStream.

Their read and write methods take byte[] rather than char[] arguments.


Input-Output 17

Manipulating Primitive Data Files DataInputStream

– readBoolean(),readChar(),readInt()...

DataOutputStream– writeBoolean(),writeChar(),writeInt(),....


Input-Output 18

Reading from URLsimport java.net.*;import java.io.*;public class URLReader { public static void main(String[] args) throws Exception {

URL yahoo = new URL("http://www.yahoo.com/");BufferedReader in = new BufferedReader(

new InputStreamReader(yahoo.openStream()));

String inputLine;

while ((inputLine = in.readLine()) != null) System.out.println(inputLine);

in.close();

}}


Input-Output 19

Review

Input-Output is usually difficult to perform in a platform-independent way.

The java.io package provides the required independence.

The File class supplies details of external files.

Use Reader and Writer classes for text files.


Input-Output 20

Review (cont.)

Use Buffered classes for input-output efficiency.

Use Stream classes when access to binary data is required.

Use DataInputStream and DataOutputStream when access to primitive data is required

StringTokenizer is useful for breaking up textual input.

Documents

Java io