I/O Streams. Object-oriented programming

Transcription

1 I/O Streams Object-oriented programming

2 Outline Concepts of Data Streams Streams and Files File class Text file Binary file (primitive data, object) Readings: GT, Ch. 12 I/O Streams 2

3 Data streams Ultimately, data are always stored as a sequence of bytes But, it is often more convenient to think of data as having some higher-level structure such as being a sequence of characters or objects Stream: a sequence of data that is read from a source or written to sink Source: file, memory, keyboard, network connection,. Sink: file, memory, screen, network connection,. Java programs send and receive data via objects of some data stream type. I/O Streams 3

4 Streams Water analogy Think of streams as pipes for water Do you know whether the water that comes out of your tap is coming from? the ocean? some river? a water tank? Idea: The stream may be connected to a file on a floppy, a file on a hard disk, a network connection or may even just be in memory! You abstract away what the stream is connected to and perform all your I/O operations on the stream I/O Streams 4

5 Hierarchy of Streams Java provides a hierarchy of streams Think of this as different filters you can add on to your water pipe Some may compress/decompress data Some may provide buffers Common Use Scenario Streams are used by layering them together to form the type of pipe we eventually want client code read() requests BufferedReader GZIPInputStream FileInputStream file in filesystem data I/O Streams 5

6 Byte and character streams Byte streams: manipulate data in bytes InputStream OutputStream Unicode text streams Reader Writer Basic methods are the same I/O Streams 6

7 InputStream / OutputStream int read() int read(byte buf[]) int read(byte buf[], int offset, int length) void close() int write(int c) int write(byte buf[]) int write(byte buf[], int offset, int length) void close() void flush() I/O Streams 7

8 Reader / Writer int read() int read(char buf[]) int read(char buf[], int offset, int length) void close() int write(int c) int write(char buf[]) int write(char buf[], int offset, int length) void close() void flush() I/O Streams 8

9 InputStream hierarchy (incomplete) I/O Streams 9

10 OutputStream hierarchy (incomplete) I/O Streams 10

11 Reader hierarchy I/O Streams 11

12 Writer hierarchy I/O Streams 12

13 Important Types of Streams InputStream / OutputStream Base class streams with few features read() and write() FileInputStream / FileOutputStream Specifically for connecting to files ByteArrayInputStream / ByteArrayOutputStream Use an in-memory array of bytes for storage! BufferedInputStream / BufferedOutputStream Improve performance by adding buffers Should almost always use buffers BufferedReader / BufferedWriter Convert bytes to unicode Char and String data Probably most useful for what we need I/O Streams 13

14 Input/output stream object to read or write data, we need a stream object The I/O stream object of the appropriate stream type must be attached to a data source or sink BufferedReader in = new BufferedReader(new FileReader(fname)); I/O Streams 14

15 Use of buffered streams Buffering is a technique to improve I/O performance Read and write in blocks Reduce number of accesses to I/O devices The program writes to buffer instead of the output device When the buffer is full, data in buffer is pushed to device in blocks We can force data to be pushed by calling flush() The program reads from buffer instead of input device When the buffer is empty, data is retrieved from input device in blocks I/O Streams 15

16 Standard I/O streams System.out & System.err are PrintStream objects System.in is an InputStream object System.in cannot be used directly, instead: InputStreamReader: character stream BufferedReader: stream with buffer I/O Streams 16

17 The File class In java.io Provides basic operations on files and directories Create, open files, query directory information Files are not streams I/O Streams 17

18 Create a File object File myfile; myfile = new File( data.txt ); myfile = new File( mydocs, data.txt ); Directories are treated the same as files File mydir = new File( mydocs ); File myfile = new File(myDir, data.txt ); There're also specific methods to deal with directories I/O Streams 18

19 File's methods File/directory name String getname() String getpath() String getabsolutepath() String getparent() boolean renameto(file newname) File/directory status boolean exists() boolean canwrite() boolean canread() boolean isfile() boolean isdirectory() I/O Streams 19

20 File's methods (continued) status long lastmodified() long length() boolean delete() directory boolean mkdir() String[] list() I/O Streams 20

21 Manipulate text files Read from files FileReader: read characters BufferedReader: buffered, read in lines (readline()) Write to files FileWriter: write characters PrintWriter: write in lines (print() and println()) I/O Streams 21

22 Reading Example public void readlines(string fname) { try { // Build a reader on the fname, (also works with File object) BufferedReader in = new BufferedReader(new FileReader(fname)); String line; while ((line = in.readline())!= null) { // do something with 'line' System.out.println(line); in.close(); catch (IOException e) { e.printstacktrace(); I/O Streams 22

23 Writing Example public void writelines(string fname) { try { // Build a writer on the fname (also works on File objects) PrintWriter out = new PrintWriter(new FileWriter(fname)); // Send out.print(), out.println() to write chars for (int i=0; i<data.size(); i++) { out.println(... ith data string... ); out.close(); // polite catch (IOException e) { e.printstacktrace(); I/O Streams 23

24 Binary files Read FileInputStream: read data from files DataInputStream: read primitive data ObjectInputStream: read objects Write FileOutputStream: write data to files DataOutputStream: write primitive data ObjectOutputStream: write objects I/O Streams 24

25 DataInputStream/DataOutputStream DataInputStream: read primitive data readboolean, readbyte, readchar, readshort, readint, readlong, readfloat, readdouble DataOutputStream: write primitive data writeboolean, writebyte, writechar, writeshort, writeint, writelong, writefloat, writedouble I/O Streams 25

26 Write primitive data import java.io.*; public class TestDataOutputStream { public static void main(string args[]) { int a[] = {2, 3, 5, 7, 11; try { FileOutputStream fout = new FileOutputStream(args[0]); DataOutputStream dout = new DataOutputStream(fout); for (int i=0; i<a.length; i++) dout.writeint(a[i]); dout.close(); catch (IOException e) { e.printstacktrace(); I/O Streams 26

27 Read primitive data import java.io.*; public class TestDataInputStream { public static void main(string args[]) { try { FileInputStream fin = new FileInputStream(args[0]); DataInputStream din = new DataInputStream(fin); while (true) { System.out.println(din.readInt()); catch (EOFException e) { catch (IOException e) { e.printstacktrace(); I/O Streams 27

28 File of objects Objects can be stored Data classes must implement interface Serializable import java.io.serializable; class Record implements Serializable { private String name; private float score; public Record(String s, float sc) { name = s; score = sc; public String tostring() { return "Name: " + name + ", score: " + score; I/O Streams 28

29 import java.io.*; File of objects public class TestObjectOutputStream { public static void main(string args[]) { Record r[] = { new Record("john", 5.0F), Objects can be new stored Record("mary", 5.5F), new Record("bob", 4.5F) ; Data classes must implement imterface Serializable try { FileOutputStream fout = new FileOutputStream(args[0]); ObjectOutputStream out = new ObjectOutputStream(fout); for (int i=0; i<r.length; i++) out.writeobject(r[i]); out.close(); catch (IOException e) { e.printstacktrace(); I/O Streams 29

30 public class TestObjectInputStream { public static void main(string args[]) { Record r; try { FileInputStream fin = new FileInputStream(args[0]); ObjectInputStream in = new ObjectInputStream(fin); while (true) { r = (Record) in.readobject(); System.out.println(r); catch (EOFException e) { System.out.println("No more records"); catch (ClassNotFoundException e) { System.out.println("Unable to create object"); catch (IOException e) { e.printstacktrace(); I/O Streams 30

31 The class RandomAccessFile Read and write data in an unordered Implements DataInput and DataOutput Record size must be fixed I/O Streams 31

32 import java.io.*; public class WriteRandomFile { public static void main(string args[]) { int a[] = { 2, 3, 5, 7, 11, 13 ; Objects can be stored Data try classes { must implement imterface Serializable File fout = new File(args[0]); RandomAccessFile out; out = new RandomAccessFile(fout, "rw"); for (int i=0; i<a.length; i++) out.writeint(a[i]); out.close(); catch (IOException e) { e.printstacktrace(); I/O Streams 32

33 import java.io.*; public class ReadRandomFile { public static void main(string args[]) { Objects can be stored try { Data classes File fin must = new implement File(args[0]); imterface Serializable RandomAccessFile in = new RandomAccessFile(fin, "r"); int recordnum = (int) (in.length() / 4); for (int i=recordnum-1; i>=0; i--) { in.seek(i*4); System.out.println(in.readInt()); catch (IOException e) { e.printstacktrace(); I/O Streams 33

34 Networking as streams of data Client/server stream sockets Reading from http network connections example I/O Streams 34

35 HTTP public static void dumpurl(string urlstring) { try { URL url = new URL(urlString); URLConnection conn = url.openconnection(); InputStream stream = conn.getinputstream(); public static void dumpurl(string urlstring) { try { URL url BufferedReader = new URL(urlString); in = URLConnection conn = url.openconnection(); InputStream stream = conn.getinputstream(); BufferedReader in = new BufferedReader( new InputStreamReader(stream)); new BufferedReader( new InputStreamReader(stream)); String line; while ( (line = in.readline())!= null) { System.out.println(line); String line; while ( (line = in.readline())!= null) { System.out.println(line); in.close(); in.close(); catch (MalformedURLException e) { e.printstacktrace(); catch (IOException e) { e.printstacktrace(); catch (MalformedURLException e) { e.printstacktrace(); catch (IOException e) { e.printstacktrace(); I/O Streams 35