streams and file i/o - purdue universitycs180/spring2007web/lecture... · 2005. 10. 20. · chapter...

Chapter 9 1

Streams and File I/O

Chapter 9

Chapter 9 6

The Concept of a Stream

• Files can be used to store– Java classes– Java programs– output from a program– input for a program.

• File I/O as well as keyboard and screen I/O are handled by streams.

Chapter 9 7

The Concept of a Stream, cont.

• A stream is a flow of data (characters, numbers, etc.).

• Data flowing into a program is called an input stream.

• Data flowing out of a program is called an output stream.

Chapter 9 8


• A stream is implemented as an object.– It delivers data to a destination such as a

file or a stream or– it takes data from a source such as a file or

the keyboard, and delivers it to a program.• System.out is the only output stream we have

used so far.• Objects of class Scanner, used for keyboard

input, are streams, too.

Chapter 9 9


• This chapter discusses streams that connect programs to files.

Chapter 9 10

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 11

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 12

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 14

Text Files and Binary Files, cont.

• However, binary files are more efficient to process than text files.

• In Java, binary files are platform-independent.– Binary files can be created by one

computer and read by another, combining portability and efficiency.

Chapter 9 15

Text Files and Binary Files, cont.

• Though text files can be read and written using an editor, binary files must be read and written by a program.

Chapter 9 17

Text-File Output with PrintWriter

• Class PrintWriter has a method println that behaves like System.out.println

• The java.io package contains the class PrintWriter and the other file I/O classes discussed in this chapter.

Chapter 9 18

Text-File Output with PrintWriter, cont.

• class TextFileOutputDemo

Chapter 9 21


• A file is opened using something similar tooutputStream = new PrintWriter(

new FileOutputStream(“out.txt”));

– An empty file is connected to a stream.– If the named file (out.txt, for example)

exists already, its old contents are lost.– If the named file does not exist, a new

empty file is created (and named out.txt, for example).

Chapter 9 22


• Class Printwriter has no constructor that takes a file name as an argument.

• So, we use class FileOutputStream to create a stream and can be used as an argument to a PrintWriter constructor.

• SyntaxPrintWriter Output_Stream_Name = new

PrintWriter (new FileOutputStream(File_Name));

Chapter 9 23


• The FileOutputStream constructor, and thus the PrintWriter constructor invocation can throw a FileNotFoundException, which means that the file could not be created.

• The PrintWriter object is declared outside the try block.– If it were declared inside the try block, it

would be local to the try block.

Chapter 9 24

Some Methods in Class PrintWriter

• constructorPrintWriter(OutputStream streamObject)

• to create a new filenew PrintWriter(new

FileOutputStream(File_Name))

• to append new text to an old filenew PrintWriter(new

FileOutputStream(File_Name, true))

Chapter 9 25

Some Methods in Class PrintWriter, cont.

• to output to the file connected to the streampublic final void

println(Almost_Anything)public final void

print(Almost_Anything)

• To close a stream’s connection to a filepublic void close()

• To flush the output streampublic void flush()

Chapter 9 26

Closing Text Files

• When a program finishes writing to or reading from a file, it should close the file.– examplesoutputStream.close();inputStream.close();

• If a program does not close a file before the program ends, Java will will close it when the program ends, provided the program ends normally.

Chapter 9 27

Closing Text Files, cont.

• The sooner a file is closed, the less likely it is to be damaged by being left open when a program ends abnormally.

• If a program writes a file, it must close the file before it attempts to read from it.

Chapter 9 29

Use toString for Text-File Output

• Classes typically include a method toString.• The methods println and print in class

PrintWriter behave like System.out.println and System.out.print, respectively.

Chapter 9 30

Use toString for Text-File Output, cont.

• class Species

Chapter 9 31

Use toString for Text-File Output, cont.

• class TextFileObjectOutputDemo

Chapter 9 33

Text-file Input with BufferedReader

• Class BufferedReader is the preferred stream class for reading from a text file.

• Class BufferedReader has no constructor that takes a filename as its argument.– Class FileReader accepts a file name as a

constructor argument and produces a stream that is a Reader object.

– The constructor for class BufferedReaderaccepts a Reader object as an argument.

Chapter 9 34

Text-file Input with BufferedReader, cont.

• syntaxBufferedReader Stream_Name = new

BufferedReader(newFileReader(File_Name));

• Methods readln and read are used to read from the file.

• The FileReader constructor, and thus theBufferedReader constructor invocation can throw a FileNotFoundException.

Chapter 9 35

Text-file Input with BufferedReader, cont.

• class TextFileInputDemo

Chapter 9 38

Some Methods in Class BufferedReader

• constructorBufferedReader(Reader, readerObject)

• to create a streamnew BufferedReader(new

FileReader(File_Name))

• to read a line of input from the filepublic String readLine() throws IOException

– If the read operation goes beyond the end of the file, null is returned.

Chapter 9 39

Some Methods in Class BufferedReader, cont.

• to read a single character from the file and return it as an int valuepublic int read() throws IOException

– If the read operation goes beyond the end of the file, -1 is returned.

• to read a single character from the file and to treat it as a characterchar next = (char)(inputStream.read());

Chapter 9 40

Some Methods in Class BufferedReader, cont.

• To read a number from a text file, the number must be read in as a string and the string must be converted to a number.

• to close a stream’s connection to a filepublic void close()

Chapter 9 45

Java Tip: Using Path Names

• When providing a file name as an argument for opening a file, a simple file name may be used if the file is in the same directory as the program being run.

• A full or relative path name also can be used.• A full path name is the complete path name,

starting from the root directory.

Chapter 9 46

Java Tip: Using Path Names, cont.

• A relative path name is the path name starting from the directory containing the program.

• The way to specify path names depends upon the operating system.

Chapter 9 49

The StringTokenizerClass

• Class BufferedReader can read entire lines or single characters, but not single words.

• Class StringTokenizer can take an entire line of text and break it into individual words.

• The class StringTokenizer is in the java.utilpackage.

• Individual words are called tokens.

Chapter 9 50

The StringTokenizerClass, cont.

• Tokens are nonwhitespace characters.• example

StringTokenizer tokenizer = new StringTokenizer(“Read my lips!”)

while (tokenizer.hasMoreTokens()){

System.out.println(tokenizer.nextToken());

}

Chapter 9 51


• This will produceReadmylips!

Chapter 9 52


• 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 53

Some Methods in Class StringTokenizer

• constructorspublic StringTokenizer(String

theString)public StringTokenizer(String

theString, String delimiters)

• more tokens?public boolean hasMoreTokens()

• next tokenpublic String nextToken()

Chapter 9 54

Some Methods in Class StringTokenizer, cont.

• remaining tokenspublic int countTokens()

Chapter 9 55

Java Tip: Testing for the End of a Text File

• When method readLine in class BufferedReaderattempts 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 56

Java Tip: Testing for the End of a Text File

• class EOFDemo

Chapter 9 57

The Classes FileReaderand FileOutputStream

• Class FileReader is used with class BufferedReader; class FileOutputStream is used with class Printwriter.

• Class FileReader and class FileOutputStreamaccept a file name as a constructor argument.

Chapter 9 58

The Classes FileReaderand FileOutputStream,

cont.

• Connecting a BufferedReader object to a file using a string name requires two steps.– First, create an object of the class FileReader.

– Then use this object to create an object of class BufferedReader.

Chapter 9 59


cont.

• exampleBufferedReader inputStream =

new BufferedReader(new FileReader(“story.txt”);

Chapter 9 60


cont.

• Producing a PrintWriter output stream from a file using FileOutputStream requires two steps.– First, create an object of the class FileOutputStream.

– Then use this object to create an object of class PrintWriter.

Chapter 9 61


cont.

• examplePrintWriter OutputStream =

new PrintWriter(new FileOutputStream

(“stuff.txt”);

Chapter 9 62

Using the File Class

• The methods of the class File can check the properties of files.– Does the named file exist?– Is the file readable?

• Typically, the operating systems lets you designate files as not readable or as readable only by certain users.

Chapter 9 63

Using the File Class, cont.

• The File class is like a wrapper class for strings which are file names.– examplenew File(“treasure.txt”)

Chapter 9 64

class FileClassDemo


Chapter 9 65


• Method canWrite determines if the operating system will let you write to the file.

• Typically, the operating systems lets you designate files as not writeable or as writeable only by certain users.

Chapter 9 66

Some Methods in the Class File

public boolean exists()public boolean canRead()public boolean canWrite()public boolean delete()public boolean length()public String getName()public String getPath()

Chapter 9 68

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 read by a Java program on a different computer.

Chapter 9 69

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 70

Opening a Binary File

• syntaxObjectOutputStream Output_Stream_Name =

new ObjectOutputStream(new FileOutputStream(File_Name));

• exampleObjectOutputStream myOutputStream =

new ObjectOutputStream(new FileOutputStream

(“myfile.dat”));

Chapter 9 71

Output to Binary Files Using ObjectOutputStream

• class BinaryOutputDemo

Chapter 9 72

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 73

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 74

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 75


cont.public void writeChar(int n)

throws IOExceptionpublic void writeBoolean(boolean b)

throws IOException

• to write a Stringpublic void writeUTF(String aString)

throws IOException

Chapter 9 76


cont.• To write an object

public void writeObject(ObjectanObject) throws IOException, NotSerializableException, InvalidClassException

• to closepublic void close() throws IOException

Chapter 9 77


cont.• to flush

public void flush() throws IOException

Chapter 9 78


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 79

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 81

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 classObjectOutputStream.

Chapter 9 83

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 84

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 IOExceptionpublic char readChar()

throws IOExceptionpublic boolean ReadBoolean()

throws IOException

Chapter 9 85

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 86

Opening an Input File

• syntaxObjectInputStream Input_Stream_Name =

new ObjectInputStream(new

FileInputStream(File_Name));

• exampleObjectInputStream myInputStream = new

ObjectInputStream(new

FileInputStream(“myfile.dat”));

Chapter 9 89

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 90

• class EOFExceptionDemo

The EOFException Class

Chapter 9 92

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 93

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 94

The Classes FileInputStream and

FileOutputStream, cont.• FileOutputStream and FileInputStream accept a

file name as a constructor argument.• Neither ObjectInputStream nor

ObjectOutputStream accepts a file name as an argument.

Chapter 9 101

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 classObjectInputStream.

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

Chapter 9 102

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 9 103


• class Species

Chapter 9 104


• class IODemo

Chapter 9 106

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 9 107

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 9 114

Graphics Supplement• class FileOrganizer

Chapter 9 115

Graphics Supplement, cont.• class FileOrganizer, cont.

Chapter 9 116

Graphics Supplement, cont.• class FileOrganizer, cont.

Chapter 9 117

Graphics Supplement, cont.

streams and file i/o - purdue universitycs180/spring2007web/lecture... · 2005. 10. 20. · chapter...

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