5java Io

1 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
core
programming
Java Input/Output

Input/Output2 www.corewebprogramming.com
Agenda
• Handling files and directories through the
File class
• Understanding which streams to use for
character-based or byte-based streams
• Character File input and output
• Formatting output
• Reading data from the console
• Binary File input and output

File Class
• A File object can refer to either a file or a
directory
File file1 = new File("data.txt");
File file1 = new File("C:java");
– To obtain the path to the current working directory use
System.getProperty("user.dir");
– To obtain the file or path separator use
System.getProperty("file.separator");
System.getProperty("path.separator");
or
File.separator()
File.pathSeparator()

Useful File Methods
• isFile/isDirectory
• canRead/canWrite
• length
– Length of the file in bytes (long) or 0 if nonexistant
• list
– If the File object is a directory, returns a String array of all the
files and directories contained in the directory; otherwise, null
• mkdir
– Creates a new subdirectory
• delete
– Deletes the directory and returns true if successful
• toURL
– Converts the file path to a URL object

Directory Listing, Example
import java.io.*;
public class DirListing {
public static void main(String[] args) {
File dir = new File(System.getProperty("user.dir"));
if(dir.isDirectory()){
System.out.println("Directory of " + dir);
String[] listing = dir.list();
for(int i=0; i<listing.length; i++) {
System.out.println("t" + listing[i]);
}
}
}
}

DirectoryListing, Result
> java DirListing
Directory of C:java
DirListing.class
DirListing.java
test
TryCatchExample.class
TryCatchExample.java
XslTransformer.class
XslTransformer.java

Input/Output
• The java.io package provides over 60
input/output classes (streams)
• Streams are combined (piped together) to
create a desired data source or sink
• Streams are either byte-oriented or
character-oriented
– Use DataStreams for byte-oriented I/O
– Use Readers and Writers for character-based I/O
• Character I/O uses an encoding scheme
• Note: An IOException may occur during
any I/O operation

Character File Output
Desired … Methods Construction
Character File Ouput FileWriter File file = new File("filename");
write(int char) FileWriter fout = new FileWriter(file);
write(byte[] buffer) or
write(String str) FileWriter fout = new FileWriter("filename");
Buffered Character BufferedWriter File file = new File("filename");
File Output write(int char) FileWriter fout = new FileWriter(file);
write(char[] buffer) BufferedWriter bout = new BufferedWriter(fout);
write(String str) or
newLine() BufferedWriter bout = new BufferedWriter(
new FileWriter(
new File("filename")));

Character File Output, cont.
Character Output PrintWriter FileWriter fout = new FileWriter("filename");
write(int char) PrintWriter pout = new PrintWriter(fout);
write(char[] buffer) or
writer(String str) PrintWriter pout = new PrintWriter(
print( … ) new FileWriter("filename"));
println( … ) or
PrintWriter pout = new PrintWriter(
new BufferedWriter(
new FileWriter("filename")));

FileWriter
• Constructors
– FileWriter(String filename)/FileWriter(File file)
• Creates a output stream using the default encoding
– FileWriter(String filename, boolean append)
• Creates a new output stream or appends to the existing output
stream (append = true)
• Useful Methods
– write(String str)/write(char[] buffer)
• Writes string or array of chars to the file
– write(int char)
• Writes a character (int) to the file
– flush
• Writes any buffered characters to the file
– close
• Closes the file stream after performing a flush
– getEncoding
• Returns the character encoding used by the file stream

CharacterFileOutput, Example
import java.io.*;
public class CharacterFileOutput {
FileWriter out = null;
try {
out = new FileWriter("book.txt");
System.out.println("Encoding: " + out.getEncoding());
out.write("Core Web Programming");
out.close();
out = null;
} catch(IOException ioe) {
System.out.println("IO problem: " + ioe);
ioe.printStackTrace();
try {
if (out != null) {
out.close();
}
} catch(IOException ioe2) { }
}
}
}

CharacterFileOutput, Result
> java CharacterFileOutput
Encoding: Cp1252
> type book.txt
Core Web Programming
• Note: Cp1252 is Windows Western Europe / Latin-1
– To change the system default encoding use
System.setProperty("file.encoding", "encoding");
– To specify the encoding when creating the output steam, use an
OutputStreamWriter
OutputStreamWriter out =
new OutputStreamWriter(
new FileOutputStream("book.txt", "8859_1"));

Formatting Output
• Use DecimalFormat to control spacing
and formatting
– Java has no printf method
• Approach
1. Create a DecimalFormat object describing the
formatting
DecimalFormat formatter =
new DecimalFormat("#,###.##");
2. Then use the format method to convert values into
formatted strings
formatter.format(24.99);

Formatting Characters
Symbol Meaning
0 Placeholder for a digit.
# Placeholder for a digit.
If the digit is leading or trailing zero, then don't display.
. Location of decimal point.
, Display comma at this location.
- Minus sign.
E Scientific notation.
Indicates the location to separate the mattissa from the exponent.
% Multipy the value by 100 and display as a percent.

NumFormat, Example
import java.text.*;
public class NumFormat {
public static void main (String[] args) {
DecimalFormat science = new DecimalFormat("0.000E0");
DecimalFormat plain = new DecimalFormat("0.0000");
for(double d=100.0; d<140.0; d*=1.10) {
System.out.println("Scientific: " + science.format(d) +
" and Plain: " + plain.format(d));
}
}
}

NumFormat, Result
> java NumFormat
Scientific: 1.000E2 and Plain: 100.0000

Character File Input
Character File Input FileReader File file = new File("filename");
read() FileReader fin = new FileReader(file);
read(char[] buffer) or
FileReader fin = new FileReader("filename");
Buffered Character BufferedReader File file = new File("filename");
File Input read() FileReader fin = new FileReader(file);
read(char[] buffer) BufferedReader bin = new BufferedReader(fin);
readLine() or
BufferedReader bin = new BufferedReader(
new FileReader(
new File("filename")));

FileReader
• Constructors
– FileReader(String filename)/FileReader(File file)
• Creates a input stream using the default encoding
• Useful Methods
– read/read(char[] buffer)
• Reads a single character or array of characters
• Returns –1 if the end of the steam is reached
– reset
• Moves to beginning of stream (file)
– skip
• Advances the number of characters
• Note: Wrap a BufferedReader around the FileReader to
read full lines of text using readLine

CharacterFileInput, Example
import java.io.*;
public class CharacterFileInput {
File file = new File("book.txt");
FileReader in = null;
if(file.exists()) {
try {
in = new FileReader(file);
System.out.println("Encoding: " + in.getEncoding());
char[] buffer = new char[(int)file.length()];
in.read(buffer);
System.out.println(buffer);
in.close();
...
}
}
}
}

CharacterFileInput, Result
> java CharacterFileInput
Encoding: Cp1252
Core Web Programming
• Alternatively, could read file one line at a time:
BufferedReader in =
new BufferedReader(new FileReader(file));
String lineIn;
while ((lineIn = in.readLine()) != null) {
System.out.println(lineIn);
}

Console Input
• To read input from the console, a stream
must be associated with the standard input,
System.in
import java.io.*;
public class IOInput{
BufferedReader keyboard;
String line;
try {
System.out.print("Enter value: ");
System.out.flush();
keyboard = new BufferedReader(
new InputStreamReader(System.in));
line = keyboard.readLine();
} catch(IOException e) {
System.out.println("Error reading input!"); }
}
}
}

Binary File Input and Output
• Handle byte-based I/O using a
DataInputStream or DataOutputStream
– The readFully method blocks until all bytes are read or an EOF occurs
– Values are written in big-endian fashion regardless of computer platform
DataType DataInputStream DataOutputStream
byte readByte writeByte
short readShort writeShort
int readInt writeInt
long readLong writeLong
float readFloat writeFloat
double readDouble writeDouble
boolean readBoolean writeBoolean
char readChar writeChar
String readUTF writeUTF
byte[] readFully

UCS Transformation Format –
UTF-8
• UTF encoding represents a 2-byte Unicode
character in 1-3 bytes
– Benefit of backward compatibility with existing ASCII
data (one-byte over two-byte Unicode)
– Disadvantage of different byte sizes for character
representation
UTF Encoding
Bit Pattern Representation
0xxxxxxx ASCII (0x0000 - 0x007F)
10xxxxxx Second or third byte
110xxxxx First byte in a 2-byte sequence (0x0080 - 0x07FF)
1110xxxx First byte in a 3-byte sequence (0x0800 - 0xFFFF)

Binary File Output
Binary File Output FileOutputStream File file = new File("filename");
bytes write(byte) FileOutputStream fout = new FileOutputStream(file);
write(byte[] buffer) or
FileOutputStream fout = new FileOutputStream("filename");
Binary File Output DataOutputStream File file = new File("filename");
byte writeByte(byte) FileOutputStream fout = new FileOutputStream(file);
short writeShort(short) DataOutputStream dout = new DataOutputStream(fout);
int writeInt(int)
long writeLong(long) or
float writeFloat(float)
double writeDouble(double) DataOutputStream dout = new DataOutputStream(
char writechar(char) new FileOutputStream(
boolean writeBoolean(boolean) new File("filename")));
writeUTF(string)
writeBytes(string)
writeChars(string)

Binary File Output, cont.
Buffered Binary BufferedOutputStream File file = new File("filename");
File Output flush() FileOutputStream fout = new FileOutputStream(file);
write(byte) BufferedOutputStream bout = new BufferedOutputStream(fout);
write(byte[] buffer, int off, int len) DataOutputStream dout = new DataOutputStream(bout);
or
DataOutputStream dout = new DataOutputStream(
new BufferedOutputStream(
new FileOutputStream(
new File("filename"))));

BinaryFileOutput, Example
import java.io.*;
public class BinaryFileOutput {
int[] primes = { 1, 2, 3, 5, 11, 17, 19, 23 };
DataOutputStream out = null;
try {
out = new DataOutputStream(
new FileOutputStream("primes.bin"));
for(int i=0; i<primes.length; i++) {
out.writeInt(primes[i]);
}
out.close();
}
}
}

Binary File Input
Binary File Input FileInputStream File file = new File("filename");
bytes read() FileInputStream fin = new FileInputStream(file);
read(byte[] buffer) or
FileInputStream fin = new FileInputStream("filename");
Binary File Input DataInputStream File file = new File("filename");
byte readByte() FileInputStream fin = new FileInputStream(file);
short readShort() DataInputStream din = new DataInputStream(fin);
int readInt()
long readLong() or
float readFloat()
double readDouble() DataInputStream din = new DataInputStream(
char readchar() new FileInputStream(
boolean readBoolean() new File("filename")));
readUTF()
readFully(byte[] buffer)

Binary File Input, cont.
Bufferred Binary BufferedInputStream File file = new File("filename");
File Input read() FileInputStream fin = new FileInputStream(file);
read(byte[] buffer, int off, int len) BufferedInputStream bin = new BufferedInputStream(fin);
skip(long) DataInputStream din = new DataInputStream(bin);
or
DataInputStream din = new DataInputStream(
new BufferedInputStream(
new FileInputStream(
new File("filename"))));

BinaryFileInput, Example
import java.io.*;
public class BinaryFileInput {
DataInputStream in = null;
File file = new File("primes.bin");
try {
in = new DataInputStream(
new FileInputStream(file));
int prime;
long size = file.length()/4; // 4 bytes per int
for(long i=0; i<size; i++) {
prime = in.readInt();
System.out.println(prime);
}
in.close();
}
}
}

Summary
• A File can refer to either a file or a
directory
• Use Readers and Writers for character-
based I/O
– A BufferedReader is required for readLine
– Java provides no printf; use DecimalFormat for formatted
output
• Use DataStreams for byte-based I/O
– Chain a FileOutputStream to a DataOutputStream for
binary file output
– Chain a FileInputStream to a DataInputStream for binary
file input

5java Io

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