Java Programming Course IO

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Types f Streams Stream class hierarchy Cntrl flw f an I/O peratin using Streams Byte

1 Java Prgramming Curse IO By Võ Văn Hải Faculty f Infrmatin Technlgies Industrial University f H Chi Minh City Sessin bjectives What is an I/O stream? Types f Streams Stream class hierarchy Cntrl flw f an I/O peratin using Streams Byte streams Character streams Buffered streams Standard I/O streams Data streams Object streams File class 2 1

2 I/O stream 3 I/O Streams An I/O Stream represents an input surce r an utput destinatin. A stream can represent many different kinds f surces and destinatins, including disk files, devices, ther prgrams, and memry arrays. Streams supprt many different kinds f data simple bytes, primitive data types, lcalized characters, and bjects. Sme streams simply pass n data; thers manipulate and transfrm the data in useful ways. 4 2

3 I/O Streams A stream is a sequence f data. A prgram uses an input stream t read data frm a surce, ne item at a time: A prgram uses an utput stream t write data t a destinatin, ne item at time: 5 Types f Streams 6 3

4 General Stream Types Character and Byte Streams Character vs. Byte Input and Output Streams Based n surce r destinatin Nde and Filter Streams Whether the data n a stream is manipulated r transfrmed r nt. 7 Character and Byte Streams Byte streams Fr binary data Rt classes fr byte streams: The InputStream Class The OutputStream Class Bth classes are abstract Character streams Fr Unicde characters Rt classes fr character streams: The Reader class The Writer class Bth classes are abstract 8 4

5 Input and Output Streams Input r surce streams Can read frm these streams Rt classes f all input streams: The InputStream Class The Reader Class Output r sink (destinatin) streams Can write t these streams Rt classes f all utput streams: The OutputStream Class The Writer Class 9 Nde and Filter Streams Nde streams (Data sink stream) Cntain the basic functinality f reading r writing frm a specific lcatin Types f nde streams include files, memry and pipes Filter streams (Prcessing stream) Layered nt nde streams between threads r prcesses Fr additinal functinality- altering r managing data in the stream Adding layers t a nde stream is called stream chaining 10 5

Java IO Stream hierarchy 17/01/2013 11 Cntrl Flw f an I/O peratin 1. Create a stream bject and assciate it with a datasurce (data-destinatin) 2.

6 Java IO Stream hierarchy 17/01/ Cntrl Flw f an I/O peratin 1. Create a stream bject and assciate it with a datasurce (data-destinatin) 2. Give the stream bject the desired functinality thrugh stream chaining 3. while (there is mre infrmatin) 4. read(write) next data frm(t) the stream 5. clse the stream 12 6

7 Byte Stream 13 Byte Stream Prgrams use byte streams t perfrm input and utput f 8-bit bytes All byte stream classes are descended frm InputStream and OutputStream There are many byte stream classes FileInputStream and FileOutputStream They are used in much the same way; they differ mainly in the way they are cnstructed 14 7

8 When Nt t Use Byte Streams? Byte Stream represents a kind f lw-level I/O that yu shuld avid If the data cntains character data, the best apprach is t use character streams There are als streams fr mre cmplicated data types Byte streams shuld nly be used fr the mst primitive I/O All ther streams are based n byte stream 15 Byte Stream example 16 8

9 Character Stream 17 Character Stream The Java platfrm stres character values using Unicde cnventins Character stream I/O autmatically translates this internal frmat t and frm the lcal character set. In Western lcales, the lcal character set is usually an 8-bit superset f ASCII. All character stream classes are descended frm Reader and Writer As with byte streams, there are character stream classes that specialize in file I/O: FileReader and FileWriter. 18 9

10 Character Stream Fr mst applicatins, I/O with character streams is n mre cmplicated than I/O with byte streams. Input and utput dne with stream classes autmatically translates t and frm the lcal character set. A prgram that uses character streams in place f byte streams autmatically adapts t the lcal character set and is ready fr internatinalizatin all withut extra effrt by the prgrammer. If internatinalizatin isn't a pririty, yu can simply use the character stream classes withut paying much attentin t character set issues. Later, if internatinalizatin becmes a pririty, yur prgram can be adapted withut extensive recding. 19 Character Stream example 20 10

11 Character Stream and Byte Stream Character streams are ften "wrappers" fr byte streams The character stream uses the byte stream t perfrm the physical I/O, while the character stream handles translatin between characters and bytes. FileReader, fr example, uses FileInputStream, while FileWriter uses FileOutputStream There are tw general-purpse byte-t-character "bridge" streams: InputStreamReader and OutputStreamWriter. Use them t create character streams when there are n prepackaged character stream classes that meet yur needs. 21 Line-Oriented I/O Character I/O usually ccurs in bigger units than single characters. One cmmn unit is the line: a string f characters with a line terminatr at the end. A line terminatr can be a carriage-return/line-feed sequence ("\r\n"), a single carriage-return ("\r"), r a single line-feed ("\n")

12 Line-Oriented I/O example 23 Buffered Stream 24 12

13 The necessary f Buffered Streams An unbuffered I/O means each read r write request is handled directly by the underlying OS This can make a prgram much less efficient, since each such request ften triggers disk access, netwrk activity, r sme ther peratin that is relatively expensive. T reduce this kind f verhead, the Java platfrm implements buffered I/O streams Buffered input streams read data frm a memry area knwn as a buffer; the native input API is called nly when the buffer is empty Similarly, buffered utput streams write data t a buffer, and the native utput API is called nly when the buffer is full. 25 Buffered Stream Classes There are fur buffered stream classes used t wrap unbuffered streams: BufferedInputStream and BufferedOutputStream create buffered byte streams. BufferedReader and BufferedWriter create buffered character streams

15 Flushing Buffered Streams It ften makes sense t write ut a buffer at critical pints, withut waiting fr it t fill. This is knwn as flushing the buffer. Sme buffered utput classes supprt autflush, specified by an ptinal cnstructr argument. When autflush is enabled, certain key events cause the buffer t be flushed Fr example, an autflush PrintWriter bject flushes the buffer n every invcatin f println r frmat. T flush a stream manually, invke its flush methd The flush methd is valid n any utput stream, but has n effect unless the stream is buffered. 29 Standard Streams 30 15

16 Standard Streams n Java Platfrm Three standard streams Standard Input, accessed thrugh System.in Standard Output, accessed thrugh System.ut Standard Errr, accessed thrugh System.err These bjects are defined autmatically and d nt need t be pened System.ut and System.err are defined as PrintStream bjects 31 Data Streams 32 16

17 Data Streams Data streams supprt binary I/O f primitive data type values (blean, char, byte, shrt, int, lng, flat, and duble) as well as String values All data streams implement either the DataInput interface r the DataOutput interface DataInputStream and DataOutputStream are mst widely-used implementatins f these interfaces 33 The DataOutputStream class A data utput stream lets an applicatin write primitive Java data types t an utput stream in a prtable way. An applicatin can then use a data input stream t read the data back in. Cnstructr: DataOutputStream(OutputStream ut) Creates a new data utput stream t write data t the specified underlying utput stream

An applicatin uses a data utput stream t write data that can later be read by a data input stream.

18 DataOutputStream dem 35 The DataInputStream class A data input stream lets an applicatin read primitive Java data types frm an underlying input stream in a machine-independent way. An applicatin uses a data utput stream t write data that can later be read by a data input stream. Cnstructr : DataInputStream(InputStream in) Creates a DataInputStream that uses the specified underlying InputStream

19 The DataInputStream exemple 37 Object Streams 38 19

20 Object Serializatin Persistenceis the cncept that an bject can exist separate frm the executing prgram that creates it Java cntains a mechanism called bject serializatin fr creating persistent bjects executing prgram that creates it. When an bject is serialized, it is transfrmed int a sequence f bytes; this sequence is raw binary representatin f the bject. Later, this representatin can be restred t the riginal bject. Once serialized, the bject can be stred in a file fr later use. 39 Object Streams Object streams supprt I/O f bjects Like Data streams supprt I/O f primitive data types The bject has t be Serializable type The bject stream classes are ObjectInputStream and ObjectOutputStream These classes implement ObjectInput and ObjectOutput, which are subinterfaces f DataInput and DataOutput An bject stream can cntain a mixture f primitive and bject values 40 20

Object Serializatin Any bject we want t serialize must implement the Serializable interface. T serialize an bject, we invke the writeobject methd f an ObjectOutputStream.

21 Object Serializatin Any bject we want t serialize must implement the Serializable interface. T serialize an bject, we invke the writeobject methd f an ObjectOutputStream. T deserialize the bject, we invke the readobject methd f an ObjectInputStream. The actual data streams t which the serialized bject is written can represent a file, netwrk cmmunicatin, r sme ther type f stream. 41 Object serializatin dem 42 21

22 Output and Input f Cmplex Objects The writeobject and readobject methds are simple t use, but they cntain sme very sphisticated bject management lgic This isn't imprtant fr a class like Calendar, which just encapsulates primitive values. But many bjects cntain references t ther bjects. If readobject is t recnstitute an bject frm a stream, it has t be able t recnstitute all f the bjects the riginal bject referred t. These additinal bjects might have their wn references, and s n. 43 I/O f multiple referred-t bjects Object a cntains references t bjects b and c, while b cntains references t d and e Invking writeobject(a) writes nt just a, but all the bjects necessary t recnstitute a, s the ther fur bjects in this web are written als When a is read back by readobject, the ther fur bjects are read back as well, and all the riginal bject references are preserved

23 NIO - New I/O 45 NIO.2 - New I/O versin 2 Is aimed at simplifying I/O in Java A new file system and path abstractin Based n java.ni.file.path Bulk access t file attributes File system specific supprt (e.g. Symblic links) Extra prviders (e.g. zip files treated as nrmal files) Asynchrnus (nn-blcking) I/O Fr sckets and files Mainly utilises java.util.cncurrent.future Scket/Channel cnstruct Binding, ptins and multicast 46 23

24 NIO.2 - Path java.ni2.file.path interface Typically represents a file n a file system nrmalize() methd remves cnstructs such as. and.. in a Path relativize(path) cnstructs relative Path between this path and the ne given Lts f ther expected methds defined in the interface Dealing with real path, abslute path etc Can cnvert java.i.file bjects via the public Path tpath() methd java.ni2.file.paths Helper class, prvides get(uri) methd t return yu a Path Uses FileSystems helper class under the hd Uses the default file system unless yu specify therwise 47 NIO.2 - Files java.ni2.file.files helper class Day t day class fr perfrming simple file I/O Many cmmn methds cpy delete mve createdirectry/file/link write readalllines walkfiletree (recurse ver a directry) newinputstream/outputstream Cmbined with try-with-resurces makes cde cncise 48 24

25 URLStream t file - Java 6 vs. Java 7 49 NIO.2 - Asynchrnus I/O The ability t perfrm read/write peratins in the backgrund Withut having t write yur wn java.util.cncurrent cde Is available fr: file I/O (AsynchrnusFileChannel) netwrking I/O (AsynchrnusScketChannel) And AsynchrnusServerScketChannel Can wrk in tw styles Future based (rder cffee, d smething else, cllect cffee) Callbacks (rder cffee, d smething else, have cffee thrwn at yu) 50 25

26 NIO.2 - Future based file I/O example 51 NIO.2 - Callback based file I/O example 52 26

27 Summary What is an I/O stream? Types f Streams Stream class hierarchy Cntrl flw f an I/O peratin using Streams Byte streams Character streams Buffered streams Standard I/O streams Data streams Object streams File class

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