Session9 IO

8/11/2019 Session9 IO

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Session 9I/O


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I/O Java does strong, flexible support for I/O as it

relates to files and networks. Javas I/O system is cohesive and consistent. Java programs perform I/O through streams. Most real applications are either graphically

oriented programs that rely on Javas AbstractWindow ToolKit (AWT) or Swing for userinteraction, or Web applications.


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Streams A stream is an abstraction that either produces or

consumes information. That is, it is an object thateither delivers data to its destination (screen, file,

etc.) or that takes data from a source (keyboard,file, etc.)

It acts as a buffer between the data source anddestination.

Java implements streams within class hierarchiesdefined in the java.io package.


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A stream is linked to a physical device bythe Java I/O system.

All streams behave in the same manner, even ifthe actual physical devices to which they arelinked differ.

Thus, the same I/O classes and methods can beapplied to any type of device. Failure to explicitly close a stream can result in

memory leaks because of unused resourcesremaining allocated, i.e. resource starvation.

Java defines 2 types of streams: byte-stream andcharacter-stream.


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Byte Streams Byte streams provide a convenient means for

handling input and output of bytes. They areused when reading or writing binary data.

Byte streams are defined by using 2 classhierarchies. At the top are 2 abstract classes:InputStream and OutputStream.

Each of these abstract classes has severalconcrete subclasses that handle the differencesamong various devices.


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INPUTSTREAM CLASSES OUTPUTSTREAM CLASSES

BufferedInputStream BufferedOutputStream

ByteArrayInputStream ByteArrayOutputStream

DataInputStream DataOutputStream

FileInputStream FileOutputStream

FilterInputStream FilterOutputStream

InputStream OutputStream

ObjectInputStream ObjectOutputStream

PipedInputStream PipedOutputStream

PushbackInputStream PrintStream

SequenceInputStream

Byte Stream Classes in java.io


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These abstract classes define several keymethods that the other stream classes

implement. Two of the most important are read() and write(),

which, respectively, read and write bytes of data. Each has forms that are abstract and must be

overridden by derived stream classes.

Console input can be performed using the bytestream by reading from System.in, which is anobject of type InputStream and refers to the

standard input, which is the keyboard by default.


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System.out is an object of type

PrintStream and refers to the standardoutput stream. By default, this is the console.

System.err is an object of type PrintStream and

refers to the standard error stream, which also isthe console by default.

These are byte streams, even though they aretypically used to read and write characters fromand to the console.


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Character Streams Character streams provide a convenient means

for handling input and output of characters. Theyuse Unicode, and hence can be internationalized.

Character streams are defined by using 2 classhierarchies. At the top are 2 abstract classes:Reader and Writer.

These abstract classes handle Unicode characterstreams. Java has several concrete subclasses ofeach of these.


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INPUTSTREAM CLASSES OUTPUTSTREAM CLASSES

BufferedReader BufferedWriter

CharArrayIReader CharArrayWriter

StringReader StringWriter

FileReader FileWriter

FilterReader FilterWriter

InputStreamReader OutputStreamWriter

Reader Writer

PipedReader PipedWriter

PushbackReader PrintWriter

LineNumberReader

Character Stream Classes in java.io


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These abstract classes define several key

methods that the other stream classesimplement. Two of the most important are read()and write(), which read and write characters of

data, respectively. Each has forms that are abstract and must be

overridden by derived stream classes. The byte streams System.in, System.out and

System.err can be wrapped within character-

based streams.


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Putting it all together creates aBufferedReader that is connected to thekeyboard.

BufferedReader br=new BufferedReader(new

InputStreamReader(System.in)); Now, br is a character-based stream that is linked

to the console through System.in. To read a character from a BufferedReader, read()

is used.

int read() throws IOException


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Each time that read() is called, it reads acharacter from the input stream andreturn it as an integer value.

It returns -1 when the end of the stream is

encountered. It can throw IOExceptions, which are simply

thrown out of main(). To read a string from the keyboard, the

BufferedReader class member readLine() is used.

String readLine() throws IOException


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File I/O Advantages:

permanent copy

output from one program can be inputto another

input can be automated (rather than

entered manually


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Reading & Writing Files FileInputStream and FileOutputStream are 2

classes that create byte streams linked to files. To open a file, an object of one of these classes is

created, specifying the name of the file as anargument to the constructor.

FileInputStream(String filename or path) throwsFileNotFoundException

If the file does not exist when an input stream is

created, a FileNotFoundException is thrown.


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To read from a file, the read() defined

within the FileInputStream is used:int read() throws IOException

Each time the read() is called, a single byte is read

from the file and returns the byte as an integer.read() returns -1 when the end of file isencountered. It can throw an IOException.

FileOutputStream(String filename) throwsFileNotFoundException


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If the file cannot be opened or createdwhen an output stream is created, aFileNotFoundException is thrown.

When an output file is opened, any preexisting

file by the same name is destroyed. The FileNotFoundException is a subclass of

IOException. To write to a file, write() defined by

FileOutputStream is used.

void write(int byteval) throws IOException


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This method writes the byte specified bybyteval to the file. Only the low order 8bits are written to the file.

If an error occurs during writing, an IOException is

thrown. At the end, the file must be closed using close().

void close() throws IOException Closing a file releases the system resources

allocated to the file, allowing them to be used byanother file.


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The try-with-resources statement can beused only with those resources thatimplement the AutoCloseable interface definedby java.lang.

This interface defines the close() method.AutoCloseable is inherited by the Closeableinterface in java.io. Both interfaces areimplemented by the stream classes.

Thus, try-with-resources can be used when

working with streams, including file streams.


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try(resource-specification) {

//use the resource } Resource-specification is a statement that

declares and initializes a resource, such a file

stream. This resource is implicitly final. It consists of a variable declaration in which the

variable is initialized with a reference to the

object being managed. This variable is local to the try block, being

created when the try is entered.


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Also, more then one resource can be managedwithin a single try statement, byseparating each resource specification with asemicolon.

However, when the try block ends, the resource isautomatically released, i.e the stream associatedwith the variable is automatically closed by animplicit call to close().

This form of try block can also include catch and

finally clauses

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Session9 IO