Chapter 9 1

Streams and File I/O

Chapter 9

Chapter 9 2

Reminders

• Project 6 released: due Nov 03 @ 10:30 pm

• Project 4 regrades due by midnight tonight

• Discussion groups now twice a week (7-9 pm: M in CS G066, W in Phys 11)

Chapter 9 3

Exam 2

• Tuesday, October 25

• 7:00 – 8:00 PM

• Physics 112

• Covers chapters 5-9 (and material from 1-4 as well)

• Similar format as Exam 1– 20 MC– 4 programming

Chapter 9 4

Why Use Files for I/O?

• Keyboard input and screen output deal only with temporary data, which is lost when the program ends.

• Files permit data to be stored permanently (or at least until a program changes the file).

• Input files can be used over and over by different programs.

• Files also provide convenient storage and retrieval of large quantities of data.

Chapter 9 5

Text Files and Binary Files

• All data in a file is stored as binary digits.– Files with contents that must be treated as

sequences of binary digits are called binary files; binary files can be read only by machines.

Chapter 9 6

Text Files and Binary Files,cont.

• Sometimes, it is more convenient to think of a file’s contents as a sequence of characters.– Files with streams and methods to make

them look like sequences of characters are called text files; text files can be read by people.

Chapter 9 7

Text Files and Binary Files, cont.

• However, all files are binary in the computer– Just a sequence of 0s and 1s: it’s how we

(or programs) interpret those 0s and 1s

Chapter 9 8

Text File Output Example

Chapter 9 9

Use toString for Output

Chapter 9 10

Text File Input Example

Chapter 9 11

The StringTokenizer Class, cont.

• Tokens are nonwhitespace characters.• example

StringTokenizer tokenizer = new

StringTokenizer(“Read my lips!”)

while (tokenizer.hasMoreTokens())

{

System.out.println

(tokenizer.nextToken());

}

Chapter 9 12

The StringTokenizer Class, cont.

• This will produceRead

my

lips!

Chapter 9 13

The StringTokenizer Class, cont.

• Separators are whitespace characters unless otherwise specified.

• To specify a set of separators, a string consisting of all the separator characters is given as a second argument to the constructor.

• example… new StringTokenizer(“Read my lips!”, “\n.,!”);

Chapter 9 14

Some Methods in Class StringTokenizer

• constructorspublic StringTokenizer(String

theString)

public StringTokenizer(String

theString, String delimiters)

• more tokens?public boolean hasMoreTokens()

• next tokenpublic String nextToken()

• remaining tokenspublic int countTokens()

Chapter 9 15

Example: StringTokenizer

• Display the words separated by any of the following characters: space, new line (\n), period (.) or comma (,).

String inputLine = keyboard.nextLine();StringTokenizer wordFinder = new StringTokenizer(inputLine, " \n.,");

while(wordFinder.hasMoreTokens()){ System.out.println(wordFinder.nextToken());}

Chapter 9 16

Example: StringTokenizer

String inputLine = keyboard.nextLine();StringTokenizer wordFinder = new StringTokenizer(inputLine, " \n.,");

while(wordFinder.hasMoreTokens()){ System.out.println(wordFinder.nextToken());}

Question2bor!tooBee

Entering "Question,2b.or !tooBee." gives this output:

Chapter 9 17

Java Tip: Testing for the End of a Text File

• When method readLine in class BufferedReader attempts to read beyond the end of a file, the method returns the value null.

• When method read attempts to read beyond the end of a file, the method returns the value -1.

Chapter 9 18

Using the File Class, cont.

Chapter 9 19

Binary Files

• Binary files store data in the same format used for main memory.

• Bytes in main memory and bytes in binary files are read similarly, which leads to efficiency.

• Binary files created by a Java program on one computer can be read by a Java program on a different computer.

Chapter 9 20

Binary Files, cont.

• Class ObjectInputStream and class ObjectOutputStream are used to process binary files.– Data is read or written, one byte at a time.– Numbers and characters are converted

automatically to bytes for storage in a binary file.

– Data in files can be treated as Java primitive data types, as strings, or as other objects.

Chapter 9 21

Opening a Binary File

• syntaxObjectOutputStream Output_Stream_Name =

new ObjectOutputStream

(new FileOutputStream(File_Name));

• exampleObjectOutputStream myOutputStream =

new ObjectOutputStream

(new FileOutputStream

(“myfile.dat”));

Chapter 9 22

ObjectOutputStream

Chapter 9 23

Output to Binary Files Using ObjectOutputStream,

cont.• The numbers are not in human-readable form

because there are no lines or other separators.

Chapter 9 24

Some Methods in Class ObjectOutputStream

• to createpublic ObjectOutputStream(OutputStream

streamObject)

• to create a streamnew ObjectOutputStream

(new FileOutputStream

(File_Name_or_File_Object))

• to write a primitive typepublic void writeInt(int n) throws

IOException

Chapter 9 25

Some Methods in Class ObjectOutputStream,

cont.• to write a primitive type, cont.

public void writeLong(long n) throws

IOException

public void writeDouble(double x)

throws IOException

public void writeFloat(float x)

throws IOException

Chapter 9 26

Some Methods in Class ObjectOutputStream,

cont.public void writeChar(int n)

throws IOException

public void writeBoolean(boolean b)

throws IOException

• to write a Stringpublic void writeUTF(String aString)

throws IOException

Chapter 9 27

Some Methods in Class ObjectOutputStream,

cont.• There is no method writeString.• Instead, use method writeUTF.• UTF stands for Unicode Text Format.• UTF provides short, efficient codes for ASCII

characters.

Chapter 9 28

Some Methods in Class ObjectOutputStream,

cont.• To write an object

public void writeObject(Object

anObject)

throws IOException,

NotSerializableException,

InvalidClassException

• to closepublic void close() throws IOException

Chapter 9 29

Different Types in the Same File

• Different types can be written to the same file.• However, the different types must be read

from the file just as they were written to the file.

Chapter 9 30

Reading Input from a Binary File Using

ObjectInputStream• A file written using ObjectOutputStream can be

read using ObjectInputStream.

• The methods in class ObjectInputStream correspond to the methods in class ObjectOutputStream.

Chapter 9 31

Some Methods in Class ObjectInputStream

• to createObjectInputStream

(InputStream streamObject)

• to create a streamnew ObjectInputStream (new

FileInputStream

(File_Name_or_File_Object)

• to read a primitive typepublic int readInt() throws IOException

Chapter 9 32

Some Methods in Class ObjectInputStream, cont.• to read a primitive type, cont.

public long readLong()

throws IOException

public double readDouble()

throws IOException

public float readFloat()

throws IOException

public char readChar()

throws IOException

public boolean ReadBoolean()

throws IOException

Chapter 9 33

Some Methods in Class ObjectInputStream, cont.• to read a String

public String readUTF()

throws IOException

• to read an objectpublic Object readObject()

throws ClassNotFoundException,

InvalidClassException,

OptionalDataException, IOException

• to closepublic void close() throws IOException

Chapter 9 34

Opening an Input File

• syntaxObjectInputStream Input_Stream_Name =

new ObjectInputStream(new

FileInputStream(File_Name));

• exampleObjectInputStream myInputStream = new

ObjectInputStream(new

FileInputStream(“myfile.dat”));

Chapter 9 35

The EOFException Class

• ObjectInputStream methods that read from a binary file throw an EOFException when they try to read beyond the end of the file.

• When using class ObjectInputStream, the class EOFException can test for the end of a file.

Chapter 9 36

The EOFException Class

Chapter 9 37

Checking for the End of File

• Different classes with file reading methods check for the end of a file in different ways.– Binary files throw an exception in the class EOFException.

– A text file returns a special value, such as null.

• Be sure to test for the end of the file in the correct way.

Chapter 9 38

The Classes FileInputStream and FileOutputStream

• We used stream class FileInputStream when we created a stream of class ObjectInputStream.

• We used stream class FileOutputStream when we created a stream of class ObjectOutputStream.

Chapter 9 39

Binary I/O of Class Objects

• Using method writeObject of class ObjectOutputStream you can output class objects to a binary file, and then read objects from the file using method readObject of class ObjectInputStream.

• However, the class being written and read must be serializable.

Chapter 9 40

Binary I/O of Class Objects, cont.

• To make a class serializable, add implements Serializable to the class heading.

• examplepublic class SomeClass implements

Serializable

• The Serializable interface is available after importing java.io.*

Chapter 9 41

Binary I/O of Class Objects,

Chapter 9 42

Chapter 9 43

Files and toString

• Method toString provides convenient output to the screen and to a text file.

• However, method toString is not needed for object I/O to a binary file.

Chapter 9 44

The Serializable Interface

• A class which is serializable affects how Java performs file I/O with objects of the class.– Java assigns a serial number to each

object of the class that it writes to a stream of type ObjectOutputStream.

– If the object is written more than once, Java writes only the serial number for the object.

Chapter 9 45

Graphics Supplement

Chapter 9 46

Graphics Supplement, cont.

Chapter 9 47

Graphics Supplement, cont.

Chapter 9 48

Graphics Supplement, cont.

Chapter 9 49

Summary

• You have become familiar with the concept of an I/O stream.

• You now understand the difference between binary files and text files.

• You have learned how to save data in a file.• You have learned how to read data from a

file.

Chapter 9 50

Summary, cont.

• You have learned how use the classes ObjectOutputStream and ObjectInputStream to read and write class objects with binary files.