Java I/O

● Input: information brought to program from an external source

● Output: information sent from program to an external destination

● External source or destination can be:– Disk file or memory– Another program (piping)– Network location (URL)

● Information type can be:– Primitive types or objects– Images or sounds

I/O Streams

● A stream is a data structure facilitating handling of an information source for sequential access

Source Programreads

DestinationProgramwrites

Stream

Programs read from and write to streams, and notthe actual source or destination

Standard Input and Output Streams

● Java provides references to the standard input stream (keyboard) and standard output stream (display)

● System.in: an instance of the standard Java Platform InputStream class

● System.out: an instance of the PrintStream class

● InputStream and PrintStream are in the java.io package

InputStream Class

An input stream provides 8-bit bytes.

The InputStream class is a direct descendant of the Object class.

The shaded subclasses implement data sink streams;unshaded subclasses implement processing streams.

Input Streams Used for Processing

● InputStream: abstract superclass of all input stream classes

● All subclasses of InputStream must implement read() method that returns one byte as an int:– FilterInputStream: designed to be extended to

provide extra functionality involving the transformation (filtering) of data

– SequenceInputStream: concatenates several streams into one logical stream

– ObjectInputStream: reads data and objects that have been serialized to an output destination

Subclasses of FilterInputStream● DataInputStream: reads bytes that are parts

of primitive types (int, double, etc.) and returns a specific value.

● BufferedInputStream: buffers input bytes for efficiency

● PushbackInputStream: adds the ability to ``push back'' or ``unread'' one byte

● LineNumberInputStream: deprecated. Use LineNumberReader class instead

Input Streams Used on Data Sinks (all subclasses of InputStream)

● FileInputStream: obtains input bytes from a file in a file system (intended for raw data, as for image files, only)

● PipedInputStream: gets bytes from a piped output stream, usually from another program thread

● ByteArrayInputStream: gets bytes from an internal buffer which is a byte array

● StringBufferInputStream: deprecated. Use StringReader class instead

OutputStream Class

An output stream outputs 8-bit bytes.

The OutputStream class is a direct descendant of the Object class.

The shaded subclasses implement data sink streams;unshaded subclasses implement processing streams.

Output Streams Used for Processing

● OutputStream: abstract superclass of all output stream classes

● All subclasses of OutputStream must implement write() method that writes one byte stored as an int:– FilterOutputStream: designed to be extended to

provide extra functionality involving the transformation (filtering) of data

– ObjectOutputStream: writes data and objects that have been serialized to an output destination

Subclasses of FilterOutputStream

● DataOutputStream: writes primitive data types (int, double, etc.)

● BufferedOutputStream: buffers output bytes for efficiency

● PrintStream: converts primitive data types to string representation for printing– can be created to perform automatic flushing

Output Streams Used on Data Sinks (all subclasses of OutputStream)

● FileOutputStream: sends output bytes to a file in a file system (intended for raw data, as for image files, only)

● PipedOutputStream: sends bytes to a piped input stream, usually from another program thread

● ByteArrayOutputStream: sends bytes to an internal buffer which is a byte array

Example: Echoing Bytes from Standard Input to Standard Output

import java.io.*;

public class Echo {

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

int c;

while ((c = System.in.read()) != -1) System.out.write(c);

}}

Notes on the Example

● Must import the I/O package, of which all the input and output stream classes are part

● Since the read() method may encounter an I/O error, the main() method must either:– catch and deal with an IOException, or– throw the IOException to the calling procedure

● Even though the streams read and write bytes, the internal storage is an int

● The read() method returns -1 if EOF is encountered

Example Output

82% javac Echo.java83% java Echonow is the time[Return]now is the timefor all good men[Return]for all good mento come to the aid[Return]to come to the aid[ctl-D]84%

Output is in red:

Unix Redirection

84% java Echo > myfilenow is the timefor all good mento come to the aid[ctl-D]85% cat myfilenow is the timefor all good mento come to the aid86% java Echo < myfile > newfile87% cat newfilenow is the timeforall good mento come to the aid88%

Streams vs. Readers/Writers

● The InputStream and OutputStream methods work on bytes

● The Echo example notwithstanding, the stream classes are used primarily for binary data as in sound and images

● In Java, characters are represented using 16-bit Unicode

● To insure internationalization, programs dealing with character I/O should use descendants of the Reader and Writer classes

Readers

Think of the Reader class like InputStream thathas been specialized to deal with characters.

Reader is a direct, abstract subclass of Object.

Note similarity to the InputStream hierarchy.

Writers

Think of the Writer class like OutputStream thathas been specialized to deal with characters.

Writer is a direct, abstract subclass of Object.

Note similarity to the OutputStream hierarchy.

New Echo Class

import java.io.*;

public class Echo {

public static void main(String[] args) throws IOException { InputStreamReader r = new InputStreamReader(System.in); OutputStreamWriter w = new OutputStreamWriter(System.out);

int c;

while ((c = r.read()) != -1) w.write(c); w.flush();

}}

Notes on the New Echo Class

● InputStreamReader and OutputStreamWriter are built on top of InputStream and OutputStream– so their constructors require streams as arguments

● Internal character storage is an int ● Processing loop is exactly the same as for streams● The writer requires flushing

New Echo Example Output

93% java Echonow is the time[Return]for all good men[Return]to come to the aid[Return][ctl-D]now is the timefor all good mento come to the aid94%

Output is in red:

File Handling Example

import java.io.*;

public class FileCopy {

public static void main(String[] args) throws IOException { FileReader r = new FileReader("infile"); FileWriter w = new FileWriter("outfile");

int c;

while ((c = r.read()) != -1) w.write(c); w.flush();

}}

Notes on FileCopy

● FileReader and FileWriter constructors can take file names as strings for arguments

● Processing loop and flush call the same as before● This class is of limited use since the file names

are hard coded

FileCopy Example Output

105% lsFileCopy.class FileCopy.java infile106% cat infileHere is a test fileto show that the FileCopyclass really works.107% java FileCopy108% lsFileCopy.class FileCopy.java infileoutfile109% cat outfileHere is a test fileto show that the FileCopyclass really works.110%

Exception Example

111% rm outfile112% rm infile113% lsFileCopy.classFileCopy.java114% java FileCopyException in thread "main" java.io.FileNotFoundException: infile (No such file or directory)

at java.io.FileInputStream.open(Native Method)at

java.io.FileInputStream.<init>(FileInputStream.java:103)at

java.io.FileInputStream.<init>(FileInputStream.java:66)at java.io.FileReader.<init>(FileReader.java:39)at FileCopy.main(FileCopy.java:7)

115% Note: The FileNotFoundException class is a subclass of IOException.

The exception generates a stack trace with line numbers of offending code.

Reading Strings● The BufferedReader class has a readLine() method for reading strings

import java.io.*;

public class FileCopy {

public static void main(String[] args) throws IOException { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); System.out.print("Source file name: "); String inFileName = br.readLine(); System.out.print("Destination file name: "); String outFileName = br.readLine();

FileReader r = new FileReader(inFileName); FileWriter w = new FileWriter(outFileName); int c; while ((c = r.read()) != -1) w.write(c); w.flush(); }}

Example Output121% lsFileCopy.class FileCopy.java

122% java FileCopySource file name: FileCopy.javaDestination file name: X.java

123% lsFileCopy.class FileCopy.java X.java

124% cat X.javaimport java.io.*;

public class FileCopy {

public static void main(String[] args) throws IOException { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); System.out.print("Source file name: "); String inFileName = br.readLine(); System.out.print("Destination file name: "); String outFileName = br.readLine(); ... }}

Reading Strings and Converting to Numbers

● The java.lang package has classes Integer, Double, etc., whose instances have int, double, etc. as data members

● These classes have static methods for converting strings to numbers

● For example, the Integer class has a parseInt(<string>) method that returns an int given a String

● Similarly, the Double class has a parseDouble(<string>) method that returns a double

Example

import java.io.*;

public class ConvertTest {

public static void main(String[] args) throws IOException { int sum = 0; BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); System.out.print("Enter an integer: "); String s = br.readLine(); while ( s != null ) { int num = Integer.parseInt(s); sum += num; System.out.print("Enter an integer: "); s = br.readLine(); } System.out.println("The sum is " + sum); }}

Notes on the Example

● The readLine() method returns a null string if end of stream is reached

● The parseInt() method must be fully qualified with its class name since the method is static

● The parseInt() method might throw a NumberFormatException if its argument is not syntactically correct

● Unlike an IOException, a NumberFormatException is not checked by the compiler

Example Output136% java ConvertTest Enter an integer: 3Enter an integer: 417Enter an integer: 29Enter an integer: 1234Enter an integer: [ctl-D]The sum is 1683

137% java ConvertTest

Enter an integer: 3Enter an integer: 417Enter an integer: trashException in thread "main" java.lang.NumberFormatException: trash

at java.lang.Integer.parseInt(Integer.java:426)at java.lang.Integer.parseInt(Integer.java:476)at ConvertTest.main(ConvertTest.java:12)

138%

Lining Output Up in Columns

import java.io.*;

public class ColumnTest {

public static void main(String[] args) throws IOException { int sum = 0; int[] nums = new int[10]; int i = 0; BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); System.out.print("Enter an integer: "); String s = br.readLine(); while ( s != null ) { nums[i] = Integer.parseInt(s); sum += nums[i++]; System.out.print("Enter an integer: "); s = br.readLine(); } System.out.println("Numbers"); System.out.println("-------"); for (int j = 0; j < i; j++) System.out.println(nums[j]); System.out.println("-------"); System.out.println(sum + " total"); }}

Output

144% java ColumnTestEnter an integer: 3Enter an integer: 417Enter an integer: 29Enter an integer: 1234Enter an integer: [ctl-D]Numbers-------3417291234-------1683 total145%

Decimal Formatting● The java.text package has a DecimalFormat

class for formatting decimals as strings● The DecimalFormat class has a format() method

that takes an integer and formats it according to a pattern string given to the DecimalFormat object's constructor

● The pattern string can have regular characters as well as special formatting characters

● The special character ``0'' indicates a decimal digit● E.g. the pattern string `` 0000'' indicates a string

with three leading blanks then an integer in 4-digit field

Decimal Formatting Exampleimport java.io.*;import java.text.*;

public class ColumnTest { public static void main(String[] args) throws IOException { int sum = 0; int[] nums = new int[10]; int i = 0; BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); System.out.print("Enter an integer: "); String s = br.readLine(); while ( s != null ) { nums[i] = Integer.parseInt(s); sum += nums[i++]; System.out.print("Enter an integer: "); s = br.readLine(); } DecimalFormat f = new DecimalFormat(" 0000"); System.out.println("Numbers"); System.out.println("-------"); for (int j = 0; j < i; j++) System.out.println(f.format(nums[j])); System.out.println("-------"); System.out.println(f.format(sum) + " total"); }}

Decimal Formatting Example Output

144% java ColumnTestEnter an integer: 3Enter an integer: 417Enter an integer: 29Enter an integer: 1234Enter an integer: [ctl-D]Numbers------- 0003 0417 0029 1234------- 1683 total145%

If you can figure out how to get the leading zeros toprint as blanks, please let me know.