System.out.format("The square root of %d is %f.%n", i, r);

Transcription

1 1 Input/Output in Java Vedi anche: 2 Formattazione public class Root2 { public static void main(string[] args) { int i = 2; double r = Math.sqrt(i); System.out.format("The square root of %d is %f.%n", i, r); System.out.format("%f, %< f %n", Math.PI); ,

2 3 Formattazione d formats an integer value as a decimal value; f formats a floating point value as a decimal value; n outputs a locale-specific line terminator. x formats an integer as a hexadecimal value; s formats any value as a string; tb formats an integer as a locale-specific month name. 4 Streams To bring in information, a program opens a stream on an information source (a file, memory, a socket) and reads the information serially, like this: 2

3 5 Streams Similarly, a program can send information to an external destination by opening a stream to a destination and writing the information out serially, like this: 6 Streams No matter where the information is coming from or going to and no matter what type of data is being read or written, the algorithms for reading and writing data is pretty much always the same. Reading open a stream while more information read information close the stream Writing open a stream while more information write information close the stream 3

4 7 Streams The java.io package contains a collection of stream classes that support these algorithms for reading and writing. These classes are divided into two class hierarchies based on the data type (either characters or bytes) on which they operate. 8 Raw IO from stdin,stdout import java.io.*; public class Simple { public static void main(string a[]){ new Simple(); public Simple() { String s=null; byte buffer[]=new byte[100]; String s=null; System.out.println("Dammi una stringa"); try { int letti=system.in.read(buffer); s=new String(buffer,0,letti); catch (IOException e) { e.printstacktrace(); System.out.println(s.length()+": +s.indexof('\n')+":"+s); 4

5 9 Simple IO from stdin,stdout import java.io.*; public class Simple { public static void main(string a[]){ new Simple(); public Simple() { BufferedReader String s=null; is=new BufferedReader( BufferedReader new InputStreamReader(System.in)); is=new BufferedReader( System.out.println("Dammi new InputStreamReader(System.in)); una stringa"); try System.out.println("Dammi { una stringa"); try s=is.readline(); { catch s=is.readline(); (IOException e) { e.printstacktrace(); System.out.println(s.length()+": catch (IOException e) { e.printstacktrace(); System.out.println(s.length()+": +s.indexof('\n')+":"+s); +s.indexof('\n')+":"+s); 10 Reading an int from stdin import java.io.*; public class Simple { public static void main(string a[]){ new Simple(); public Simple() { String s=null; BufferedReader is=new BufferedReader(new InputStreamReader(System.in)); int i=0; System.out.println("Dammi un int"); try { s=is.readline(); catch (IOException e) { e.printstacktrace(); try{ i=integer.parseint(s); System.out.println("Hai scritto "+i); catch (NumberFormatException e) { System.out.println("Input non valido"); 5

6 11 package IO; import java.awt.*; import java.io.*; public class Application1 { public static void main(string[] args) { new Application1(); public Application1() { Frame fr=new Frame(); FileDialog fd=new FileDialog(fr); fd.show(); String filename=fd.getfile(); String pathname=fd.getdirectory(); System.out.println(pathname+" "+filename); File file=new File(pathname,filename); try { FileWriter fw=new FileWriter(file); fw.write("hello World"); fw.close(); catch (IOException e) { 12 However, it's often more convenient to group the classes based on their purpose rather than on the data type they read and write. Thus, we can cross-group the streams by whether they read from and write to data "sinks" or process the information as its being read or written. 6

7 13 Character Streams Reader and Writer are the abstract superclasses for character streams in java.io. Reader provides the API and partial implementation for readers--streams that read 16-bit characters--and Writer provides the API and partial implementation for writers-- streams that write 16-bit characters. Subclasses of Reader and Writer implement specialized streams and are divided into two categories: those that read from or write to data sinks (shown in gray in the following figures) and those that perform some sort of processing (shown in white). 14 Character Streams 7

8 15 Programs should use the byte streams, descendants of InputStream and OutputStream, to read and write 8-bit bytes. InputStream and OutputStream provide the API and some implementation for input streams (streams that read 8-bit bytes) and output streams (streams that write 8-bit bytes). These streams are typically used to read and write binary data such as images and sounds. As with Reader and Writer, subclasses of InputStream and OutputStream provide specialized I/O that falls into two categories: data sink streams and processing streams. Byte Streams 16 IO superclasses - reading Understanding the I/O Superclasses Reader and InputStream define similar APIs but for different data types. For example, Reader contains these methods for reading characters and arrays of characters: int read() int read(char cbuf[]) int read(char cbuf[], int offset, int length) InputStream defines the same methods but for reading bytes and arrays of bytes: int read() int read(byte cbuf[]) int read(byte cbuf[], int offset, int length) Also, both Reader and InputStream provide methods for marking a location in the stream, skipping input, and resetting the current position. 8

9 17 IO superclasses - writing Understanding the I/O Superclasses Writer and OutputStream are similarly parallel. Writer defines these methods for writing characters and arrays of characters: int write(int c) int write(char cbuf[ ]) int write(char cbuf[ ], int offset, int length) And OutputStream defines the same methods but for bytes: int write(int c) int write(byte cbuf[ ]) int write(byte cbuf[ ], int offset, int length) All of the streams--readers, writers, input streams, and output streams--are automatically opened when created. You can close any stream explicitly by calling its close method. Or the garbage collector can implicitly close it, which occurs when the object is no longer referenced. 18 Standard IO The Standard I/O Streams The concept of standard input and output streams is a C library concept that has been assimilated into the Java environment. There are three standard streams, all of which are managed by the java.lang.system class: Standard input--referenced by System.in (instance of InputStream) Used for program input, typically reads input entered by the user. Standard output--referenced by System.out (instance of PrintStream) Used for program output, typically displays information to the user. Standard error--referenced by System.err (instance of PrintStream) Used to display error messages to the user. 9

10 19 Data sink streams Sink Type Memory Pipe File Character Streams CharArrayReader, CharArrayWriter StringReader, StringWriter PipedReader, PipedWriter FileReader, FileWriter Byte Streams ByteArrayInputStream, ByteArrayOutputStream StringBufferInputStream PipedInputStream, PipedOutputStream FileInputStream, FileOutputStream 20 Processing streams Sink Type Buffering Filtering Converting Bytes/Chars Character Streams BufferedReader, BufferedWriter FilterReader, FilterWriter InputStreamReader, OutputStreamWriter Byte Streams BufferedInputStream, BufferedOutputStream FilterInputStream, FilterOutputStream Concatenation SequenceInputStream 10

11 21 Processing streams Sink Type Object Serialization Data Conversion Counting Character Streams LineNumberReader Byte Streams ObjectInputStream, ObjectOutputStream DataInputStream, DataOutputStream LineNumberInputStream Peeking Ahead PushbackReader PushbackInputStream Printing PrintWriter PrintStream 22 Classi accessorie File Represents a file on the native file system. You can create a File object for a file on the native file system and then query the object for information about that file (such as its full pathname). FileDescriptor Represents a file handle (or descriptor) to an open file or an open socket. You will not typically use this class. StreamTokenizer Breaks the contents of a stream into tokens. Tokens are the smallest unit recognized by a text-parsing algorithm (such as words, symbols, and so on). A StreamTokenizer object can be used to parse any text file. For example, you could use it to parse a Java source file into variable names, operators, and so on, or to parse an HTML file into HTML tags. FilenameFilter Used by the list method in the File class to determine which files in a directory to list. The FilenameFilter accepts or rejects files based on their names. You could use FilenameFilter to implement simple regular expression style file search patterns such as foo*. 11

12 23 Classi accessorie CheckedInputStream and CheckedOutputStream An input and output stream pair that maintains a checksum as the data is being read or written. DeflaterOutputStreamand InflaterInputStream Compresses or uncompresses the data as it is being read or written. GZIPInputStream and GZIPOutputStream Reads and writes compressed data in the GZIP format. ZipInputStream and ZipOutputStream Reads and writes compressed data in the ZIP format. 24 Esempio 1 Input/Output 12

13 25 Esempio: : copy - 1 import java.io.*; public class Copy { public static void main(string[] args) throws IOException { File inputfile = new File( originale.txt"); File outputfile = new File( copia.txt"); FileReader in = new FileReader(inputFile); FileWriter out = new FileWriter(outputFile); int c; while ((c = in.read())!= -1) out.write(c); in.close(); out.close(); Copy 26 Esempio: : copy - 2 Remember that FileReader and FileWriter read and write 16-bit characters. However, most native file systems are based on 8-bit bytes. These streams encode the characters as they operate according to the default character-encoding scheme. You can find out the default character-encoding by using System.getProperty("file.encoding"). To specify an encoding other than the default, you should construct an OutputStreamWriter on a FileOutputStream and specify it. 13

14 27 Esempio: : copy - 3 public class CopyBytes { public static void main(string[] args) throws IOException{ File inputfile = new File( originale.txt"); File outputfile = new File( copia.txt"); FileInputStream in = new FileInputStream(inputFile); FileOutputStream out = new FileOutputStream(outputFile); int c; while ((c = in.read())!= -1) out.write(c); in.close() out.close(); CopyBytes Java Serialization 14

15 Il problema della persistenza Persistenza dei dati quando l applicazione non è running: salvataggio della struttura interna di un oggetto. E un problema ricorsivo! Serializzazione: trasformazione di oggetti in stream di dati (seriali). Il JDK prevede delle API per la serializzazione, e la maggior parte delle sue classi sono serializzabili. persistenza selettiva Le variabili dichiarate transient non vengono serializzate. 15

16 Supporto di JDK alla persistenza ObjectOutputStream: converte oggetti dalla rappresentazione in memoria a quella serializzata. ObjectInputStream: converte oggetti dalla rappresentazione serializzata a quella in memoria. Serializable: interfaccia senza metodi che marca una classe come seralizzabile. Externalizable: interfaccia con metodi (sottoclasse di Serializable) che fornisce maggior controllo sul processo di serializzazione. ObjectInputValidation: interfaccia per verificare la corretta decodifica di un oggetto. Principali Metodi di ObjectOutputstream ObjectOutputStream(OutputStream) costruttore writeobject(object) serializza Object close() flush() reset() writeint(int) writefloat(float) writedouble(double) 16

17 Principali Metodi di ObjectInputstream ObjectInputStream(InputStream) costruttore Object readobject() deserializza Object available() dice quanti bytes ci sono da leggere int readint() float writefloat() double writedouble() package serialdemo; import java.io.*; import java.util.*; public class Writer { Esempio Writer 1/2 HashMap hm=new HashMap(); public Writer() { System.out.println("WRITER "); for (int k=1;k<=10;k+=2) { String key=new String("a"+k); Integer ik=new Integer(k); hm.put(key,ik); System.out.println(hm.size()); Iterator i=hm.keyset().iterator(); while (i.hasnext()) { Object o=i.next(); System.out.println(o.toString()+"#"+ hm.get(o).tostring()); 17

18 Esempio Writer 2/2 FileOutputStream ofs=null; ObjectOutputStream oos=null; try { ofs=new FileOutputStream("serializedStream.txt"); oos=new ObjectOutputStream(ofs); oos.writeobject(hm); catch (IOException ex) { ex.printstacktrace(); // public static void main(string[] args) { Writer writer = new Writer(); package serialdemo; import java.io.*; import java.util.*; public class Reader { HashMap hm=null; // public static void main(string[] args) { Reader reader = new Reader(); Esempio Reader 1/2 18

19 Esempio Reader 2/2 public Reader() { System.out.println("READER "); FileInputStream ofs=null; ObjectInputStream oos=null; try { ofs=new FileInputStream("serialiazidStream.txt"); oos=new ObjectInputStream(ofs); hm=(hashmap)oos.readobject(); catch (ClassNotFoundException ex) { ex.printstacktrace(); catch (IOException ex) { ex.printstacktrace(); System.out.println(hm.size()); Iterator i=hm.keyset().iterator(); while (i.hasnext()) { Object o=i.next(); System.out.println(o.toString()+" # +hm.get(o).tostring()); 19