Streams. Overview. The notion of stream Java I/O streamhierarchy Files, and file access using streams Serialization Sockets

Transcription

1 Streams Bruce Eckel, Thikig i Java, 4th editio, PreticeHall, New Jersey, cf. jvo@ualg.pt José Valete de Oliveira 16-1 Overview The otio of stream Java I/O streamhierarchy Files, ad file access usig streams Serializatio Sockets jvo@ualg.pt José Valete de Oliveira

2 Check poit Why the read method of IputStream class returs a it ad ot a char or byte? jvo@ualg.pt José Valete de Oliveira 17-3 Streams for accessig the stadard I/O classe System provides 3 static fields for accessig the stadard I/O: System.i System.out System.err jvo@ualg.pt José Valete de Oliveira

3 Streams for accessig the stadard I/O Stadard I/O streams ca be redirected to aother stream Thus trasferrig data to/from aother data source This ca be doe with methods: System.setI() System.setOut() System.setErr() José Valete de Oliveira 16-5 Streams for accessig the stadard I/O Example redirect OutputStream to a file: OutputStream output = ew FileOutputStream("c:\\data\\system.out.txt"); PritStream pritout = ew PritStream(output); System.setOut(pritOut); Note: Actually Stadard output is a kid of PritStream jvo@ualg.pt José Valete de Oliveira

4 Character stream subhierarchy Example 1 Number of 1 i a file 1. import java.io.*; 2. public class No1 { 3. public static void mai(strig [] args) throws IOExceptio { 4. FileReader f; 5. if (args.legth == 0) 6. f = System.i; 7. else 8. f = ew FileReader(args[0]); 9. it c, us=0; 10. while(c = f.read()!= -1) 11. if ((char)c == 1 ) us++; 12. System.out.pritl(us + us! ); 13. f.close(); 14. } 15. } jvo@ualg.pt José Valete de Oliveira

5 MyScaer: A applicatio for streams 1. import java.io.*; 2. public class MyScaer { 3. private IputStream f; 4. public MyScaer(IputStream fr) { 5. f = fr; 6. } 7. public void close() { 8. try { 9. f.close(); 10. } catch(ioexceptio e) { 11. e.getstacktrace(); 12. } } 13. public void fialize() { close(); } MyScaer: A applicatio for streams 1. public Strig extlie() { 2. Strig s = ""; 3. try { 4. BufferedReader i = ew 5. BufferedReader(ew IputStreamReader(f), 10); 6. s = i.readlie(); 8. } catch (IOExceptio e) { 9. e.getstacktrace(); 10. } 11. retur s; 12. } 5

6 MyScaer: A applicatio for streams 1. public it extit() { 2. retur Iteger.parseIt(extLie().trim()); 3. } 1. public double extdouble() { 2. retur Double.parseDouble(extLie().trim()); 3. } MyScaer 1. public char extchar() { 2. char c=' '; 3. try { 4. BufferedReader i = ew 5. BufferedReader(ew IputStreamReader(f), 10); 8. c = (char) i.read(); 9. } catch (IOExceptio e) { 10. e.getstacktrace(); 11. } 12. retur c; 13. } 14. } 6

7 Usig MyScaer 1. public class Mai { 2. public static void mai(strig[] args) { 3. MyScaer s = ew MyScaer (System.i); 4. System.out.prit(s.extIt()); 5. s.close(); 6. } 7. } Example1, revisited Number of 1 i a file 1. import java.io.*; 2. public class No1 { 3. public static void mai(strig [] args) throws IOExceptio { 4. FileReader f; 5. if (args.legth == 0) 6. f = System.i; 7. else 8. f = ew FileReader(args[0]); 9. it c, us=0; 10. while(c = f.read()!= -1) 11. if ((char)c == 1 ) us++; 12. System.out.pritl(us + us! ); 13. f.close(); 14. } 15. } jvo@ualg.pt José Valete de Oliveira

8 Example1, revisited Number of 1 i a file 1. public class No1 { 2. public static void mai(strig[] args) 3. MyScaer sc; 4. if (args.legth == 0) 5. sc = ew MyScaer(System.i); 6. else 7. sc = ew MyScaer(ew File(args[0])); 8. it us=0; 9. while(sc.hasnextit()) 10. if (sc.extit() == 1 ) us++; 11. System.out.pritl(us + us! ); 12. sc.close(); 13. } 14. } jvo@ualg.pt José Valete de Oliveira Radom Access Radom access allows access at arbitrary locatios i the file

9 The class RadomAccessFile The class RadomAccessFile views a file as a seuetial list of bytes ad implemets both DataIput ad DataOutput iterfaces We ca ope a file for either Readig oly ("r") or Readig ad writig ("rw") RadomAccessFile f = ew RadomAcessFile( ame.ext","rw"); jvo@ualg.pt José Valete de Oliveira iterface DataIput iterface DataIput{ public abstract void readfully(byte b[]) throws IOExceptio public abstract it skipbytes(it ) throws IOExceptio public abstract boolea readboolea() throws IOExceptio public abstract byte readbyte() throws IOExceptio public abstract it readusigedbyte() throws IOExceptio public abstract char readchar() throws IOExceptio //... } jvo@ualg.pt José Valete de Oliveira

10 iterface DataOutput iterface DataOutput { } public abstract void write(byte b[]) throws IOExceptio public abstract void writeboolea(boolea v) throws IOExceptio public abstract void writebyte(it v) public abstract void writechar(it v) throws IOExceptio throws IOExceptio public abstract void writeit(it v) throws IOExceptio //... jvo@ualg.pt José Valete de Oliveira RadomAccessFile To move the file poiter to a specific byte f.seek(); To get the curret positio of the file poiter log = f.getfilepoiter(); To skip a k umber of byte it m = f.skipbytes(k); jvo@ualg.pt José Valete de Oliveira

11 RadomAccessFile, example 1. File afile = ew File( test.txt ); 2. RadomAccessFile f = ew RadomAccessFile(afile, rw ); 3. for(it i=65;i<91;i++) // escreve o alfabeto 4. f.write(i); // em MAIÚSCULAS 5. byte[] ler = ew byte[10]; 6. f.seek(4); // posicioa-se o uarto byte 7. f.read(ler, 0, 5); // lê as próximas 5 posições 8. for(it i=0;i<5;i++) 9. System.out.prit((char) ler[i] + : ); 10. f.seek(3); // posicioa-se o terceiro byte 11. System.out.pritl((char)f.read()); 12. f.close(); File class File provides methods for file (ad directory) ames hadlig Some available methods caread() cawrite() exists() isfile() isdirectory() isabsolute() ishidde() legth() list() jvo@ualg.pt José Valete de Oliveira

12 Ex: Listigthe cotets ofthe curret dir import java.io.*; public class DirList { public static void mai(strig[] args) { File dir = ew File(System.getProperty("user.dir")); System.out.pritl(dir); Strig[] list = dir.list(); for(it i=0; i<list.legth; i++) System.out.pritl(" "+list[i]); } } jvo@ualg.pt José Valete de Oliveira File: a ote o code portability // Caledar today = Caledar.getIstace(); it year = today.get(caledar.year); Strig pathame = "poo"+file.separator+year; System.out.pritl(pathame); // I Widows poo\2009 // I Liux ad Mac OS poo/2009 jvo@ualg.pt José Valete de Oliveira

13 Serializatio José Valete de Oliveira Serializatio Serializatio is the process of writig a imemory object ito a byte stream Deserializatio restores the object from a byte stream Serializatio allows for: object persistece object trasmissio memory swappig jvo@ualg.pt José Valete de Oliveira

14 The byte stream subhierarchy, agai Example: serializig a collectio public class SerialMap { public static void mai(strig [] args) throws IOExceptio { Map<Strig, Poit> hm = ew HashMap<Strig, Poit>(); Poit p = ew Poit(0,0); hm.put("0@0", p); hm.put("origi", p); hm.put("1@2", ew Poit(1, 2)); FileOutputStream fos = ew FileOutputStream("hm.out"); ObjectOutputStream oos = ew ObjectOutputStream(fos); oos.writeobject(hm); oos.close(); fos.close(); for (Strig key: hm.keyset()) System.out.pritl(key+"->"+hm.get(key)); } } jvo@ualg.pt José Valete de Oliveira

15 Deserializig the same collectio import java.io.*; import java.util.*; public class DeSerialMap { public static void mai(strig [] args) throws IOExceptio, ClassNotFoudExceptio { ObjectIputStream ois = ew ObjectIputStream(ew FileIputStream("hm.out")); Map<Strig, Poit> hm = (HashMap<Strig, Poit>) ois.readobject(); ois.close(); for (Strig key: hm.keyset()) System.out.pritl(key+"->"+hm.get(key)); } } jvo@ualg.pt José Valete de Oliveira Serializatio Serializatio preserves the itegrity of i memory object graph I the serializatio of a object, all related objects are automatically serialized; By default, each related object is serialized oly oce. If a related object is ot serializable java.io.notserializableexceptio is throw jvo@ualg.pt José Valete de Oliveira

16 Ad hoc represetatio of the object hm i memory hm class HashMap p 0@0 origi 1@2 0 0 class Poit 1 2 jvo@ualg.pt José Valete de Oliveira Summary The otio of stream Java I/O stream hierarchy Files, ad file accesss usig streams Serializatio (1/2) jvo@ualg.pt José Valete de Oliveira