1. 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

2. I/O Streams
A stream is a data structure facilitating handling of an information source for sequential access
Source
reads
Program
Stream
Program
writes
Destination
Programs read from and write to streams, and not
the actual source or destination

3. 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

4. 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.

5. 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

6. 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

7. 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

8. 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.

9. 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

10. 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

11. 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

12. 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); }

13. 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

14. Example Output Output is in red: 82% javac Echo.java 83% java Echo
now is the time[Return]
now is the time
for all good men[Return]
for all good men
to come to the aid[Return]
to come to the aid
[ctl-D]
84%

15. Unix Redirection 84% java Echo > myfile now is the time
for all good men
to come to the aid
[ctl-D]
85% cat myfile
86% java Echo < myfile > newfile
87% cat newfile
forall good men
88%

16. 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

17. Readers Think of the Reader class like InputStream that
has been specialized to deal with characters.
Reader is a direct, abstract subclass of Object.
Note similarity to the InputStream hierarchy.

18. Writers Think of the Writer class like OutputStream that
has been specialized to deal with characters.
Writer is a direct, abstract subclass of Object.
Note similarity to the OutputStream hierarchy.

19. 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(); }

20. 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

21. New Echo Example Output
Output is in red:
93% java Echo
now is the time[Return]
for all good men[Return]
to come to the aid[Return]
[ctl-D]
now is the time
for all good men
to come to the aid
94%

22. 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(); }

23. 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

24. FileCopy Example Output
105% ls
FileCopy.class FileCopy.java infile
106% cat infile
Here is a test file
to show that the FileCopy
class really works.
107% java FileCopy
108% ls
outfile
109% cat outfile
110%

25. Exception Example Note: The FileNotFoundException class is a
111% rm outfile
112% rm infile
113% ls
FileCopy.class FileCopy.java
114% java FileCopy
Exception 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.

26. 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(); }

27. Example Output 121% ls FileCopy.class FileCopy.java 122% java FileCopy
Source file name: FileCopy.java
Destination file name: X.java
123% ls
FileCopy.class FileCopy.java X.java
124% cat X.java
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();
... }

28. 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

29. 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;
s = br.readLine(); }
System.out.println("The sum is " + sum);

30. 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

31. Example Output 136% java ConvertTest Enter an integer: 3
Enter an integer: [ctl-D]
The sum is 1683
137% java ConvertTest
Enter an integer: trash
Exception 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%

32. 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++];
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(sum + " total");

33. Output 144% java ColumnTest Enter an integer: 3 Enter an integer: 417
Enter an integer: [ctl-D]
Numbers 3
417 29
1234
1683 total
145%

34. 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 `` '' indicates a string with three leading blanks then an integer in 4-digit field

35. Decimal Formatting Example
import 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++];
s = br.readLine(); }
DecimalFormat f = new DecimalFormat(" ");
System.out.println("Numbers");
System.out.println(" ");
for (int j = 0; j < i; j++)
System.out.println(f.format(nums[j]));
System.out.println(f.format(sum) + " total");

36. Decimal Formatting Example Output
144% java ColumnTest
Enter an integer: 3
Enter an integer: 417
Enter an integer: 29
Enter an integer: 1234
Enter an integer: [ctl-D]
Numbers
0003
0417
0029
1234
1683 total
145%
If you can figure out how to get the leading zeros to
print as blanks, please let me know.