MIT AITI 2003Lecture 15Streams

Input and Output data from/to other sources

Goals

1. Read bytes/characters from a file into a program

2. Create a new file

3. Write bytes/characters from a program to a file

Traditional I/O

The traditional approach uses different schemes depending on the type of the source or destination, e.g.,– keyboard input– screen output– files– interprocess pipes– network sockets

Java I/O

Java’s preferred approach is to handle I/O using streams (pioneered in C++)

Java provides– set of abstract stream classes that define the

stream interfaces– hierarchy of stream implementations

Input vs Output Streams

We use streams to save data or retrieve dataA stream is an object that takes information from

one source and sends it to another Input streams - read data from a sourceOutput streams – write data to a source If you want to read and write the same

destination, you use 2 streams

Byte Streams vs. Character Streams

Byte Streams are used to read and write data in byte format (such as executable programs, word-processing documents, MP3 music files, etc)

Character Streams are used to read and write data in the form of characters – individual letters, numbers, punctuation, and the like (such as text files, word processing documents, web pages and the like).

Using streams to input/output data

1. Create a stream object associated with the data

2. Call methods of the stream to either put information in the stream or take information out of it

3. Close the stream by calling the object’s close() method

Streams and I/O Channels

Usually the other end of a stream leads to or arises from a platform-specific media service, for instance, a file system

FileSystem

Output Stream

Program

Input Stream

FileSystem

What Streams Share

Java Streams are FIFO queues– Streams deliver information in the order it was

inserted into the underlying channel

Standard Java streams only provide sequential access without rewind, backup, or random access

Coupling Streams

Java streams may be combined by using one stream as a constructor argument to another

This is usually done to add functionality and/or convert the data

Stream pipelines are constructed– from the data source to the program or– from the data destination back to the program

Stream Pipeline – Reading I

FileInputStream

Input Stream Reader

Buffered Reader

Stream Tokenizer

Stream Pipeline – Reading II

A FileInputStream reads bytes from a file

An InputStreamReader converts a byte stream to characters

A BufferedReader buffers a character stream for efficiency

A StreamTokenizer parses a character stream into tokens

Stream Pipeline - Reading III

FileInputStream f =

new FileInputStream( path );

InputStreamReader i =

new InputStreamReader( f );

BufferedReader b =

new BufferedReader( i );

StreamTokenizer t =

new StreamTokenizer( b );

StreamTokenizer (1)

Java supplies a special class called StreamTokenizer that accepts a Reader as a constructor argument.

It breaks the input character stream into tokens, sequences of 1 or more contiguous characters that "belong" together.

The user accesses these tokens by calling the nextToken() method, which always returns an int representing the type of the next token:– word,– number,– end of file (EOF),– end of line (EOL, optional), and– otherCharacter , returned as int value of the 16 bit character

code

StreamTokenizer (2)

When a StreamTokenizer recognizes a word, the public member sval contains the String representing the word.

When a number token is recognized, public member nval contains its value.

StreamTokenizer also ignores whitespace (blanks and tabs) and C, C++, and Java style comments by default.

StreamTokenizer instance methods can change the definition of “word” or “number” and can turn on or off features like ignoring comments.

StringTokenizer

Similarly, a StringTokenizer breaks a string into tokens. The tokenization method is much simpler than the one used

by the StreamTokenizer class. The following is one example of the use of the tokenizer. The

code: StringTokenizer st = new StringTokenizer("this is a test"); while (st.hasMoreTokens()) {

println(st.nextToken()); } prints the following output:

this is a test

Reading primitive Java data types

FileInputStream

DataInput

Stream

A FileInputStream reads bytes from a file

An DataInputStream reads primitive Java data types from a byte stream in a machine-independent way.

Reading character files

Buffered Reader

Stream Tokenizer

A FileReader reads a character stream from a file (equivalent to a InputStreamReader on a FileInputStream, assuming the default character encoding and the default byte-buffer size are appropriate)

A BufferedReader buffers a character stream for efficiency

A StreamTokenizer parses a character stream into tokens

FileReader

Stream Pipeline – Writing I

Buffered Writer

Ouput StreamWriter

FileOutputStream

Stream Pipeline – Writing II

A BufferedWriter buffers a character stream for efficiency (buffering characters so as to provide for the efficient writing of single characters, arrays, and strings)

A OuputStreamWriter converts characters to a byte stream according to a specified character encoding. (The encoding that it uses may be specified by name, or the platform's default encoding may be accepted. )

A FileOutputStream writes bytes to a file

Write to a byte file

A DataOutputStream writes primitive Java data types to a byte stream.

A FileOutputStream writes bytes to a file.

DataOutputStream

File OutputStream

Write to a character file

A PrintWriter print formatted representations of objects to a text-output stream

A FileWriter writes to character files.

PrintWriter

File Writer

A slightly complicated example

First File: Employee_May.dat

Name, SSN, hourly rate, salary to datePaul Njoroge, 555-12-3456, 65, 20000

Evelyn Eastmond, 555-22-2222, 70, 30000

Peilei Fan, 555-33-4444, 60, 15000

Ethan Howe, 555-44-5555, 80, 40000

Naveen Goela, 555-66-8888, 75, 20000

Second file: Hours.dat, contains 5 integers, which are the number of hours each employee has worked for that month. The integers have the same sequence as that of the employee records. Content: 50 60 40 50 70

What we need to do:

Our program 1. reads the number of hours worked from

Hours.dat, 2. calculates the monthly salary for that

employee, 3. updates her salary to date, 4. and print the new data to a file called

Employee_June.dat

Five steps for the program

1. Import the Headers

2. Read the Data File (Hours.dat)

3. Read the Text File (Employee_May.dat)

4. Update the Data

5. Output the File

1. Import the Headers

import java.io.*;import java.util.*;

2. Read the Data File (Hours.dat)

1. Create a File object representing Hours.dat 2. Connect the file object to an input stream (FileInputStream) 3. Attach a filter stream (DataInputStream) to the input stream 4. Read 5 integers from the data input stream (call readInt() method of

DataInputStream) 5. Close the input stream

File f = new File("Hours.dat"); FileInputStream fin= new FileInputStream(f); DataInputStream din = new DataInputStream(fin); int[] hours = new int[10]; for (int i=0; i<5; i++) hours[i] = din.readInt(); din.close();

3. Read the Text File (Employee_May.dat)

6. Connect Employee_May.dat to a FileReader7. Attach a BufferedReader to the file reader8. Read 5 strings from the buffered reader9. Close the buffered reader

FileReader fr = new FileReader("Employee_May.dat"); BufferedReader in = new BufferedReader(fr); String[] records = new String[5]; for (int j=0; j<5; j++) records[j] = in.readLine(); in.close();

4. Update the Data (steps)

The readLine() function return a string that contains all 4 fields of an employee record. We need to find the hourly rate and salary to date.

10. Assign the string to a StringTokenizer11. Find the 3rd and 4th tokens in the string12. Calculate the salary for this month and add it

to the salary to date In this process, we used a utility class called StringTokenizer which

breaks the string into individual pieces (tokens) based on the delimiter. In our example,the delimiter is ", "

4. Update the Data (code)

StringTokenizer st; String name, ssn; double hourlyRate, salary; for (int k=0; k<5; k++) {

st = new StringTokenizer(records[k], ", "); name = st.nextToken(); ssn = st.nextToken(); hourlyRate = Double.parseDouble(st.nextToken()); salary = Double.parseDouble(st.nextToken()); salary += hourlyRate * hours[k]; records[k] = name + ", " + ssn + ", " + hourlyRate + ", " + salary }

5. Output the Data

13. Create a new FileWriter with the file name Employee_June.dat

14. Attach a PrintWriter to the file writer15. Write the string array to the PrintWriter16. Close the PrintWriter FileWriter fw = new FileWriter("Employee_June.dat"); PrintWriter out = new PrintWriter(fw); for (int i=0; i<5; i++) out.println(records[i]); out.close();

Pop Quiz

What package do you need to import in order to write/read a file?

What package do you need to import in order to use StringTokenizer?

To read a byte file, what pipes do you need to use, for instance, read a byte file “MyInt.dat” which contains integer data types?

To read a text file (“MyText.txt”), what pipes do you need to use?

Pop Quiz (2)

1. What do you need to use to write a text file?2. What do you need to use to write a byte file?3. Can you use a text editor such as word pad, note

pad, or microsoft word to see the content of a byte file? What about a text file?

4. Do you know the name of the person who sits next to you?

5. What kind of Exception do you have to deal with when you read/write a file? How are you going to deal with it?