Chapter 10
File I/O
Streams Streams
• A stream is an object that enables the flow of data between a program and some I/O device p g or file
– If the data flows into a program then the stream – If the data flows into a program, then the stream
is called an input stream
If th d t fl t f th th
– If the data flows out of a program, then the stream is called an output stream
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Streams Streams
I fl f h k b d f
• Input streams can flow from the keyboard or from a file
S t i i i t t th t t t th
– System.inis an input stream that connects to the keyboard
Scanner keyboard = new Scanner(System.in);y ( y );
• Output streams can flow to a screen or to a file
– System.outy is an output stream that connects to the p screen
System.out.println("Output stream");
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Text Files and Binary Files Text Files and Binary Files
• Files that are designed to be read by human beings, and that can be read or written with an editor are called text files
– Text files can also be called ASCII files because the data they contain uses an ASCII encoding scheme
– An advantage of text files is that the are usually the same
ll t th t th f
on all computers, so that they can move from one computer to another
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Text Files and Binary Files Text Files and Binary Files
Fil th t d i d t b d b d
• Files that are designed to be read by programs and that consist of a sequence of binary digits are called binary files
binary files
– Binary files are designed to be read on the same type of computer and with the same programming language as the computer that created the file
the computer that created the file
– An advantage of binary files is that they are more efficient to process than text files
– Unlike most binary files, Java binary files have the advantage of being platform independent also
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Writing to a Text File Writing to a Text File
• The class PrintWriter is a stream class that can be used to write to a text file
– An object of the class PrintWriter has the methods print and println
methods print and println
– These are similar to the System.out methods of the same names but are used for text file of the same names, but are used for text file output, not screen output
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Writing to a Text File Writing to a Text File
All h fil I/O l h f ll i h k
• All the file I/O classes that follow are in the package
java.io, so a program that uses PrintWriterwill start with a set of importp statements:
import java.io.PrintWriter;
import java.io.FileOutputStream;
i t j i Fil N tF dE ti
import java.io.FileNotFoundException;
• The class PrintWriterhas no constructor that takes a file name as its argument
name as its argument
– It uses another class, FileOutputStream, to convert a file name to an object that can be used as the argument to its (the
PrintWriter) constructor PrintWriter) constructor
Writing to a Text File Writing to a Text File
• A stream of the class PrintWriter is created and connected to a text file for writing as follows:
PrintWriter outputStreamName;
outputStreamName = new PrintWriter(new
FileOutputStream(FileName));
FileOutputStream(FileName));
– The class FileOutputStreamtakes a string representing the file name as its argument
– The class PrintWritertakes the anonymous FileOutputStreamobject as its argument
Writing to a Text File Writing to a Text File
Thi d bj t f th l P i tW it
• This produces an object of the class PrintWriter that is connected to the file FileName
– The process of connecting a stream to a file is calledThe process of connecting a stream to a file is called opening the file
– If the file already exists, then doing this causes the old contents to be lost
contents to be lost
– If the file does not exist, then a new, empty file named FileNameis created
• After doing this, the methods print and println can be used to write to the file
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Writing to a Text File Writing to a Text File
• When a text file is opened in this way, a FileNotFoundExceptioncan be thrown
– In this context it actually means that the file could not be createdy – This type of exception can also be thrown when a program
attempts to open a file for reading and there is no such file
• It is therefore necessary to enclose this code in exceptionIt is therefore necessary to enclose this code in exception handling blocks
– The file should be opened inside a tryblock
Acatchblock should catch and handle the possible exception – A catchblock should catch and handle the possible exception – The variable that refers to the PrintWriterobject should be
declared outside the block (and initialized to null) so that it is not local to the block
not local to the block
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Writing to a Text File Writing to a Text File
• When a program is finished writing to a file, it should always close the stream connected to that file
outputStreamName.close();
– This allows the system to release any resources used to connect the stream to the file
– If the program does not close the file before the program
d J ill l i i ll b i i f l
ends, Java will close it automatically, but it is safest to close it explicitly
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Writing to a Text File Writing to a Text File
O d fil ll
• Output streams connected to files are usually buffered
R th th h i ll iti t th fil
– Rather than physically writing to the file as soon as possible, the data is saved in a temporary location (buffer) – When enough data accumulates or when the methodWhen enough data accumulates, or when the method
flushis invoked, the buffered data is written to the file all at once
– This is more efficient, since physical writes to a file can be slow
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Writing to a Text File Writing to a Text File
Th th d l i k th th d fl h
• The method close invokes the method flush, thus insuring that all the data is written to the file
– If a program relies on Java to close the file and theIf a program relies on Java to close the file, and the
program terminates abnormally, then any output that was buffered may not get written to the file
Also if a program rites to a file and later reopens it to – Also, if a program writes to a file and later reopens it to
read from the same file, it will have to be closed first anyway
– The sooner a file is closed after writing to it, the less likely it is that there will be a problem
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File Names File Names
h l f h fil h ld b f d
• The rules for how file names should be formed depend on a given operating system, not Java
– When a file name is given to a java constructor for a stream, it is just a string, not a Java identifier
( )
(e.g., "fileName.txt" )
– Any suffix used, such as .txt has no special meaning to a Java program
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A File Has Two Names A File Has Two Names
E i t fil d t t fil d b
• Every input file and every output file used by a program has two names:
1 The real file name used by the operating system 1. The real file name used by the operating system 2. The name of the stream that is connected to the file
• The actual file name is used to connect to the stream
• The stream name serves as a temporary name for
h fil d i h h i i il d
the file, and is the name that is primarily used within the program
IOException IOException
Wh f i fil I/O h i i i hi h
• When performing file I/O there are many situations in which an exception, such as FileNotFoundException, may be thrown
• Many of these exception classes are subclasses of the class IOException
– The class IOExceptionis the root class for a variety of exception classes having to do with input and/or output
• These exception classes are all checked exceptionsThese exception classes are all checked exceptions
– Therefore, they must be caught or declared in a throws clause
Unchecked Exceptions Unchecked Exceptions
• In contrast, the exception classes NoSuchElementException p , InputMismatchException , and IllegalStateException are all IllegalStateException are all unchecked exceptions
U h k d i i d b
– Unchecked exceptions are not required to be caught or declared in a throws clause
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Pitfall: a try Block is a Block Pitfall: a try Block is a Block
Si i fil l i i i h ld b
• Since opening a file can result in an exception, it should be placed inside a tryblock
• If the variable for aIf the variable for a PrintWriterPrintWriterobject needs to be usedobject needs to be used outside that block, then the variable must be declared outside the block
– Otherwise it would be local to the block, and could not be used elsewhere
– If it were declared in the block and referenced elsewhere, the compiler will generate a message indicating that it is an undefined identifier
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Appending to a Text File Appending to a Text File
• To create a PrintWriter object and connect it to a text file for appending, a second
argument set to true must be used in the argument, set to true, must be used in the constructor for the FileOutputStream object
outputStreamName = new PrintWriter(new FileOutputStream(FileName, true));
– After this statement, the methods print,println and/or printfcan be used to write to the file – The new text will be written after the old text in theThe new text will be written after the old text in the
file
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toString Helps with Text File Output toString Helps with Text File Output
• If a class has a suitable toString() method, and anObject is an object of that class, then
anObject can be used as an argument to System.out.println, and it will produce sensible output
• The same thing applies to the methods print and println of the class PrintWriter
outputStreamName.println(anObject);
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Some Methods of the Class PrintWriter (Part 1 of 3)
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Some Methods of the Class PrintWriter (Part 2 of 3)
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Some Methods of the Class PrintWriter
(Part 3 of 3) Reading From a Text File Using Scanner Reading From a Text File Using Scanner
Th l S b d f di f th
• The class Scannercan be used for reading from the keyboard as well as reading from a text file
– Simply replace the argument System.inS p y ep ace t e a gu e tSyste . (to the Scanner(to t eSca e constructor) with a suitable stream that is connected to the text file Scanner StreamObject =
new Scanner(new FileInputStream(FileName));
new Scanner(new FileInputStream(FileName));
• Methods of the Scannerclass for reading input behave the same whether reading from the keyboard or reading from a g y g text file
– For example, the nextIntand nextLinemethods
Reading Input from a Text File Using Scanner (Part 1 of 4)
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Reading Input from a Text File Using Scanner (Part 2 of 4)
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Reading Input from a Text File Using Scanner (Part 3 of 4)
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Reading Input from a Text File Using Scanner (Part 4 of 4)
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Testing for the End of a Text File with Scanner Testing for the End of a Text File with Scanner
• A program that tries to read beyond the end of a file using methods of the Scanner class will
i b h
cause an exception to be thrown
• However, instead of having to rely on an ti t i l th d f fil th exception to signal the end of a file, the Scanner class provides methods such as hasNextInt and hasNextLine
hasNextInt and hasNextLine
– These methods can also be used to check that the next token to be input is a suitable element of the next token to be input is a suitable element of the appropriate type
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Checking for the End of a Text File with hasNextLine (Part 1 of 4)
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Checking for the End of a Text File with hasNextLine (Part 2 of 4)
Checking for the End of a Text File with
hasNextLine (Part 3 of 4)
Checking for the End of a Text File with hasNextLine (Part 4 of 4)
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Checking for the End of a Text File with hasNextInt (Part 1 of 2)
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Checking for the End of a Text File with hasNextInt (Part 2 of 2)
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Methods in the Class Scanner (Part 1 of 11)
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Methods in the Class Scanner (Part 2 of 11)
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Methods in the Class Scanner (Part 3 of 11)
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Methods in the Class Scanner (Part 4 of 11)
Methods in the Class Scanner
(Part 5 of 11)
Methods in the Class Scanner (Part 6 of 11)
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Methods in the Class Scanner (Part 7 of 11)
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Methods in the Class Scanner (Part 8 of 11)
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Methods in the Class Scanner (Part 9 of 11)
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Methods in the Class Scanner (Part 10 of 11)
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Methods in the Class Scanner (Part 11 of 11)
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Reading From a Text File Using BufferedReader
Th l B ff dR d i t l th t b
• The class BufferedReaderis a stream class that can be used to read from a text file
– An object of the class BufferedReaderobject o t e c ass u e ed eade has the methods readas t e et ods eadand a d readLine
• A program using BufferedReader, like one using
PrintWriter will start with a set ofimportstatements:
PrintWriter, will start with a set of importstatements:
import java.io.BufferedReader;
import java.io.FileReader;p j
import java.io.FileNotFoundException;
import java.io.IOException;
Reading From a Text File Using BufferedReader Reading From a Text File Using BufferedReader
• Like the classes PrintWriterandScanner,
BufferedReaderhas no constructor that takes a file name as its argument
name as its argument
– It needs to use another class, FileReader, to convert the file name to an object that can be used as an argument to its (the BufferedReader) constructor
BufferedReader) constructor
• A stream of the class BufferedReaderis created and connected to a text file as follows:
BufferedReader readerObject;
readerObject = new BufferedReader(new FileReader(FileName)) FileReader(FileName));
Reading From a Text File Reading From a Text File
Aft th t t t th th d d d
• After these statements, the methods read and readLIne can be used to read from the file
– TheThe readLinereadLinemethod is the same method used to readmethod is the same method used to read from the keyboard, but in this case it would read from a file
The dmethod reads a single character and returns a – The readmethod reads a single character, and returns a
value (of type int) that corresponds to the character read – Since the read method does not return the character itself,
a type cast must be used:
char next = (char)(readerObject.read());
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Reading Input from a Text File Using BufferedReader (Part 1 of 3)
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Reading Input from a Text File Using BufferedReader (Part 2 of 3)
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Reading Input from a Text File Using BufferedReader (Part 3 of 3)
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Reading From a Text File Reading From a Text File
• A program using a BufferedReader object in this way may throw two kinds of exceptions
– An attempt to open the file may throw a
FileNotFoundException(which in this case has the expected meaning)
– An invocation of readLinemay throw an IOE ti
IOException
– Both of these exceptions should be handled
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Some Methods of the Class BufferedReader (Part 1 of 2)
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Some Methods of the Class BufferedReader
(Part 2 of 2) Reading Numbers Reading Numbers
• Unlike the Scannerclass, the class
BufferedReaderhas no methods to read a number from a text file
from a text file
– Instead, a number must be read in as a string, and then converted to a value of the appropriate numeric type using one of the wrapper classes
of the wrapper classes
– To read in a single number on a line by itself, first use the method readLine, and then use Integer.parseInt, D bl D bl t t t th t i i t Double.parseDouble, etc. to convert the string into a number
– If there are multiple numbers on a line, StringTokenizer
b d d h i i k d h h
can be used to decompose the string into tokens, and then the
Testing for the End of a Text File Testing for the End of a Text File
Th th d dLi f th l
• The method readLine of the class
BufferedReader returns null when it tries to read beyond the end of a text file
read beyond the end of a text file
– A program can test for the end of the file by testing for the value nullwhen using readLine
h h d f h l
• The method read of the class BufferedReader returns -1 when it tries to read beyond the end of a text file
text file
– A program can test for the end of the file by testing for the value -1when using read
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Path Names Path Names
• When a file name is used as an argument to a constructor for opening a file, it is assumed p g that the file is in the same directory or folder as the one in which the program is run
as the one in which the program is run
• If it is not in the same directory, the full or
l h b
relative path name must be given
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Path Names Path Names
h l i h f h
• A path name not only gives the name of the file, but also the directory or folder in which the file exists
• A full path name gives a complete path name, f p g p p , starting from the root directory
• A relative path name gives the path to the file A relative path name gives the path to the file, starting with the directory in which the
program is located program is located
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Path Names Path Names
• The way path names are specified depends on the operating system
– A typical UNIX path name that could be used as a file name argument is
"/user/sallyz/data/data.txt"
– A BufferedReaderinput stream connected to this
fil i d f ll
file is created as follows:
BufferedReader inputStream = new BufferedReader(new new BufferedReader(new
FileReader("/user/sallyz/data/data.txt"));
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Path Names Path Names
• The Windows operating system specifies path names in a different way
– A typical Windows path name is the following:
C:\dataFiles\goodData\data.txt
– AA BufferedReaderBufferedReaderinput stream connected to this file isinput stream connected to this file is created as follows:
BufferedReader inputStream = new B ff dR d ( Fil R d BufferedReader(new FileReader
("C:\\dataFiles\\goodData\\data.txt"));
– Note that in Windows \\must be used in place of \, since a single backslash denotes an the beginning of an escape sequence
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Path Names Path Names
A d bl b k l h (\\) t b d f Wi d
• A double backslash (\\) must be used for a Windows path name enclosed in a quoted string
– This problem does not occur with path names read in fromThis problem does not occur with path names read in from the keyboard
• Problems with escape characters can be avoided
l h b l h
altogether by always using UNIX conventions when writing a path name
A Java program will accept a path name written in either – A Java program will accept a path name written in either
Windows or Unix format regardless of the operating system on which it is run
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Nested Constructor Invocations Nested Constructor Invocations
h f h / lib l l
• Each of the Java I/O library classes serves only one function, or a small number of functions
– Normally two or more class constructors are combined to obtain full functionality
• Therefore, expressions with two constructors are common when dealing with Java I/O g / classes
Nested Constructor Invocations Nested Constructor Invocations
ff d d ( il d ( ff ))
new BufferedReader(new FileReader("stuff.txt"))
• Above, the anonymous FileReader object establishes a connection with thestuff txtfile
connection with the stuff.txtfile
– However, it provides only very primitive methods for input
• The constructor forThe constructor for BufferedReaderBufferedReadertakes thistakes this
FileReaderobject and adds a richer collection of input methods
– This transforms the inner object into an instance variable of the outer object
System.in , System.out, and System.err
Th t d d t S t i S t t d
• The standard streams System.in, System.out, and System.errare automatically available to every Java program
– System.outis used for normal screen output
– System.erris used to output error messages to the screen
• The Systemyste class provides three methods (setIn, setOut, p (set ,set ut, and setErr) for redirecting these standard streams:
public static void setIn(InputStream inStream) public static void setOut(PrintStream outStream) public static void setErr(PrintStream outStream)
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System.in , System.out, and System.err
U i th th d f th th t d d
• Using these methods, any of the three standard streams can be redirected
– For example instead of appearing on the screen errorFor example, instead of appearing on the screen, error messages could be redirected to a file
• In order to redirect a standard stream, a new stream
b d
object is created
– Like other streams created in a program, a stream object used for redirection must be closed after I/O is finished used for redirection must be closed after I/O is finished – Note, standard streams do not need to be closed
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System.in , System.out, and System.err
• Redirecting System.err:
public void getInput() public void getInput() {
. . .
PrintStream errStream = null;
PrintStream errStream = null;
try {
St P i tSt (
errStream = new PrintStream(new
FileOuptputStream("errMessages.txt"));
System.setErr(errStream);
. . . //Set up input stream and read }
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System.in , System.out, and System.err
catch(FileNotFoundException e) {
System.err.println("Input file not found");
}
finally {
. . .
errStream.close();();
} }
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The File Class The File Class
• The File class is like a wrapper class for file names
– The constructor for the class Filetakes a name, (known as the abstract name) as a string argument, and produces an object that represents the file with that name
Th Fil bj t d th d f th l Fil b
– The Fileobject and methods of the class Filecan be used to determine information about the file and its properties
properties
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Some Methods in the Class File (Part 1 of 5)
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Some Methods in the Class File (Part 2 of 5)
Some Methods in the Class File
(Part 3 of 5)
Some Methods in the Class File (Part 4 of 5)
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Some Methods in the Class File (Part 5 of 5)
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Binary Files Binary Files
Bi fil t d t i th f t d b
• Binary files store data in the same format used by computer memory to store the values of variables
– No conversion needs to be performed when a value isNo conversion needs to be performed when a value is stored or retrieved from a binary file
• Java binary files, unlike other binary language files, bl
are portable
– A binary file created by a Java program can be moved from one computer to another
one computer to another
– These files can then be read by a Java program, but only by a Java program
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Writing Simple Data to a Binary File Writing Simple Data to a Binary File
Th l Obj tO t tSt i t l th t
• The class ObjectOutputStreamis a stream class that can be used to write to a binary file
– An object of this class has methods to write strings, values ofAn object of this class has methods to write strings, values of primitive types, and objects to a binary file
• A program using ObjectOutputStreamneeds to import several classes from packagejava io:
several classes from package java.io:
import java.io.ObjectOutputStream;
import java.io.FileOutStream;p j import java.io.IOException;
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Opening a Binary File for Output Opening a Binary File for Output
A Obj tO t tSt bj t i t d d
• An ObjectOutputStream object is created and connected to a binary file as follows:
ObjectOutputStream outputStreamName = newbject utput t ea output t ea a e e ObjectOutputStream(new
FileOutputStream(FileName));
The constructor forFileOutputStreammay throw a – The constructor for FileOutputStreammay throw a
FileNotFoundException
– The constructor for ObjectOutputStreammay throw an IOException
– Each of these must be handled
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Opening a Binary File for Output Opening a Binary File for Output
Aft i th fil Obj tO t tSt th d
• After opening the file, ObjectOutputStreammethods can be used to write to the file
– Methods used to output primitive values include writeInt, et ods used to output p t e a ues c ude te t, writeDouble, writeChar, and writeBoolean
• UTF is an encoding scheme used to encode Unicode characters that favors the ASCII character set
characters that favors the ASCII character set
– The method writeUTFcan be used to output values of type String
• The stream should always be closed after writing
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Some Methods in the Class
ObjectOutputStream (Part 1 of 5)
Some Methods in the Class
ObjectOutputStream (Part 2 of 5)
Some Methods in the Class
ObjectOutputStream (Part 3 of 5)
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Some Methods in the Class
ObjectOutputStream (Part 4 of 5)
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Some Methods in the Class
ObjectOutputStream (Part 5 of 5)
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Reading Simple Data from a Binary File Reading Simple Data from a Binary File
Th l Obj tI tSt i t l th t
• The class ObjectInputStreamis a stream class that can be used to read from a binary file
– An object of this class has methods to read strings, values of primitiveAn object of this class has methods to read strings, values of primitive types, and objects from a binary file
• A program using ObjectInputStreamneeds to import several classes from packagejava io:
several classes from package java.io:
import java.io.ObjectInputStream;
import java.io.FileInputStream;p j p import java.io.IOException;
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Opening a Binary File for Reading Opening a Binary File for Reading
A bj bj i d d
• An ObjectInputStream object is created and connected to a binary file as follows:
ObjectInputStream inStreamName = new ObjectInputStream inStreamName = new
ObjectInputStream(new
FileInputStream(FileName));
– The constructor for FileInputStreammay throw a FileNotFoundException
Th f Obj S h
– The constructor for ObjectInputStreammay throw an IOException
– Each of these must be handledEach of these must be handled
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Opening a Binary File for Reading Opening a Binary File for Reading
Aft i th fil Obj tI tSt th d
• After opening the file, ObjectInputStreammethods can be used to read to the file
– Methods used to input primitive values include readInt, et ods used to put p t e a ues c ude ead t, readDouble, readChar, and readBoolean
– The method readUTFis used to input values of type String
• If the file contains multiple types each item type must be
• If the file contains multiple types, each item type must be read in exactly the same order it was written to the file
• The stream should be closed after readingThe stream should be closed after reading
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Some Methods in the Class ObjectInputStream (Part 1 of 5)
Some Methods in the Class ObjectInputStream
(Part 2 of 5)
Some Methods in the Class ObjectInputStream (Part 3 of 5)
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Some Methods in the Class ObjectInputStream (Part 4 of 5)
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Some Methods in the Class ObjectInputStream (Part 5 of 5)
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Checking for the End of a Binary File the Correct Way
All f th Obj tI tSt th d th t
• All of the ObjectInputStream methods that read from a binary file throw an EOFException when trying to read beyond the end of a file when trying to read beyond the end of a file
– This can be used to end a loop that reads all the data in a file
h d ff f l d h d h k f
• Note that different file‐reading methods check for the end of a file in different ways
Testing for the end of a file in the wrong way can cause a – Testing for the end of a file in the wrong way can cause a
program to go into an infinite loop or terminate abnormally
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Binary I/O of Objects Binary I/O of Objects
Obj l b i d f bi fil
• Objects can also be input and output from a binary file
– Use the writeObjectmethod of the class
ObjectOutputStreamto write an object to a binary file
– Use the readObjectmethod of the class ObjectInputStream to read an object from a binary file
– In order to use the value returned byIn order to use the value returned by readObjectreadObjectas an object of aas an object of a class, it must be type cast first:
SomeClass someObject =
(SomeClass)objectInp tStream readObject() (SomeClass)objectInputStream.readObject();
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Binary I/O of Objects Binary I/O of Objects
I i b h d f l l i fil
• It is best to store the data of only one class type in any one file
– Storing objects of multiple class types or objects of one class type mixed with primitives can lead to loss of data
• In addition, the class of the object being read or written must implement the Serializableinterface
Th S i li bl i f i d i
– The Serializableinterface is easy to use and requires no knowledge of interfaces
– A class that implements the Serializable interface is said to be a serializable class
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The Serializable Interface The Serializable Interface
• In order to make a class serializable, simply add implements Serializable to the
heading of the class definition
public class SomeClass implements Serializable
• When a serializable class has instance variables of a class type, then all those classes must be serializable also
– A class is not serializable unless the classes for all
Array Objects in Binary Files Array Objects in Binary Files
Si i bj l b d
• Since an array is an object, arrays can also be read and written to binary files using readObject and writeObject
writeObject
– If the base type is a class, then it must also be serializable, just like any other class type
just like any other class type
– Since readObjectreturns its value as type Object(like any other object), it must be type cast to the correct array type:
SomeClass[] someObject =
(SomeClass[])objectInputStream readObject();
(SomeClass[])objectInputStream.readObject();
Random Access to Binary Files Random Access to Binary Files
h f i l fil
• The streams for sequential access to files are the ones most commonly used for file access in Java
• However, some applications require very rapid , pp q y p access to records in very large databases
– These applications need to have random access to These applications need to have random access to particular parts of a file
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Reading and Writing to the Same File Reading and Writing to the Same File
Th t l R d A Fil hi h i i th
• The stream class RandomAccessFile, which is in the java.iopackage, provides both read and write random access to a file in Java
• A random access file consists of a sequence of numbered bytes
– There is a kind of marker called the file pointer that is always positioned at one of the bytes
– All reads and writes take place starting at the file pointer locationp g f p – The file pointer can be moved to a new location with the method
seek
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Reading and Writing to the Same File Reading and Writing to the Same File
Al h h d fil i b i d h
• Although a random access file is byte oriented, there are methods that allow for reading or writing values of the primitive types as well as string values to/from of the primitive types as well as string values to/from a random access file
– These includereadInt readDouble andreadUTF – These include readInt, readDouble, and readUTF
for input, and writeInt, writeDouble, and writeUTFfor output
– It does no have writeObjector readObject methods, however
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Opening a File Opening a File
Th t t f R d A Fil t k
• The constructor for RandomAccessFile takes either a string file name or an object of the class File as its first argument
File as its first argument
• The second argument must be one of four strings:
– "rw", meaning the code can both read and write to the , g file after it is open
– "r", meaning the code can read form the file, but not write to it
write to it
– "rws"or "rwd"(See Table of methods from RandomAccessFile)
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Pitfall: A Random Access File Need Not Start Empty
If th fil l d i t th h it i d th
• If the file already exists, then when it is opened, the length is not reset to 0, and the file pointer will be positioned at the start of the file
positioned at the start of the file
– This ensures that old data is not lost, and that the file pointer is set for the most likely position for reading (not writing)
writing)
• The length of the file can be changed with the setLength method
setLength method
– In particular, the setLengthmethod can be used to empty the file
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Some Methods of the Class RandomAccessFile (P t 1 f 7)
(Part 1 of 7)
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Some Methods of the Class RandomAccessFile (P t 2 f 7)
(Part 2 of 7)
Some Methods of the Class RandomAccessFile (P t 3 f 7)
(Part 3 of 7)
Some Methods of the Class RandomAccessFile (P t 4 f 7)
(Part 4 of 7)
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Some Methods of the Class RandomAccessFile (P t 5 f 7)
(Part 5 of 7)
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Some Methods of the Class RandomAccessFile (P t 6 f 7)
(Part 6 of 7)
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Some Methods of the Class RandomAccessFile (P t 7 f 7)
(Part 7 of 7)
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