Streams. Programação Orientada por Objetos (POO) Centro de Cálculo Instituto Superior de Engenharia de Lisboa

Transcription

1 Streams Programação Orientada por Objetos (POO) Centro de Cálculo Instituto Superior de Engenharia de Lisboa Pedro Alexandre Pereira

2 4 hieraquias de streams em Java Escrita Leitura Byte OutputStream InputStream Char Writer Reader Stream Escrita??? Leitura 2

3 Output Stream Escreve bytes num recurso OutputStream {abstract} void write(int b) void write(byte[] b) void write(byte[] b, int off, int len) void flush() void close() ByteArrayOutputStream Garante a escrita dos bytes ainda em buffer FileOutputStream Faz flush() e termina o stream Pode existir bufferização de bytes para minimizar as escritas. ByteArrayOutputStream() void write(int b) byte[] tobytearray() FileOutputStream(String name, boolean app) void write(int b) PipedOutputStream

4 Escrever num ficheiro FileOutputStream f; try { byte[] bytes = {65,66,67}; f = new FileOutputStream("abc.dat"); f.write(bytes); f.write(8); f.close(); } catch (IOException e) { System.out.println("Error:"+e); } Pode lançar FileNotFoundException Podem lançar IOException C:\POO>dir abc.dat 4 abc.dat 1 File(s) - 4 bytes C:\POO>type abc.dat ABC C:\POO>_ 4

5 Input Stream Lê bytes de um recurso InputStream {abstract} Ignora os próximos bytes Marca a posição corrente Retrocede à posição marcada int read() int read(byte[] b) int read(byte[] b, int off, int len) long skip(long n) int available() void mark(int readlimit) void reset() void close() Quantos bytes podem ser lidos sem bloquear A leitura pode bloquear à espera de informação. ByteArrayInputStream FileInputStream Os métodos read( ) retornam -1 se o stream terminar. ByteArrayInputStream(byte[] array) int read() PipedInputStream FileInputStream(String name) int read()

6 Copiar ficheiros binários public static void main(string[] args) { OutputStream fout = null; InputStream fin = null; try { fout = new FileOutputStream(args[1]); fin = new FileInputStream(args[0]); int b, cntr; for(cntr=0 ; (b = fin.read())!= -1 ; ++cntr) fout.write(b); System.out.println(cntr+" bytes copied."); } catch (IOException e) { System.out.println("Error:"+e); } catch (IndexOutOfBoundsException e) { } Copy.java System.err.println("Use: copy <source> <destination>"); } finally { try { if (fin!=null) fin.close(); if (fout!=null) fout.close(); } catch (IOException e) { e.printstacktrace(); } } C:\POO>dir 25 in.dat C:\POO>java Copy Use: copy <source> <destination> C:\POO>java Copy in.dat out.dat 25 bytes copied. C:\POO>dir 25 in.dat 25 out.dat C:\POO>_ 6

7 byte Streams OutputStream InputStream * FilterOutputStream BufferedOutputStream DataOutputStream PrintStream FilterInputStream BufferedInputStream DataInputStream PushbackInputStream SequenceInputStream FileOutputStream ByteArrayOutputStream PipedOutputStream ObjectOutputStream FileInputStream ByteArrayInputStream PipedInputStream ObjectInputStream

8 Writer Escreve carateres num recurso Writer {abstract} Program a char Stream b c d A B C D void write(int c) void write(char[] c [, int off, int len] ) void write(string s [, int off, int len] ) Writer append(char c) Writer append(charsequence s [, int start, int end] ) void flush() void close() OutputStream StringWriter OutputStreamWriter StringWriter( [ int initsize ] ) StringBuffer getbuffer() String tostring() OutputStreamWriter(OutputStream out [, Charset tab] ) OutputStreamWriter(OutputStream out, String setname) PipedWriter FileWriter FileWriter(String name, boolean app)

9 Reader a char Stream Program b c d A B C D InputStream Lê carateres de um recurso Reader {abstract} int read() int read(char[] c [, int off, int len] ) int read(charbuffer c) boolean ready() long skip(long n) void mark(int limit) void reset() void close() InputStreamReader StringReader InputStreamReader(InputStream in [, Charset tab] ) InputStreamReader(InputStream in, String setname) StringReader( String s ) FileReader PipedReader FileReader(String name) 9

10 Copiar ficheiros de texto try ( Reader in = new FileReader("abc.txt"); Writer out = new FileWriter("xpto.txt") ){ int c, cntr; for(cntr=0 ; (c = in.read())!=-1 ; ++cntr) out.write(c); System.out.println(cntr+" chars copied."); } catch (IOException e) { System.out.println("Error:"+e); } Falta fazer close() de cada stream? Que sintaxe é esta de bloco try? new FileReader("abc.txt") é equivalente a new InputStreamReader(new FileInputStream("abc.txt")) A versão 1.7 passou a permitir blocos: try ( recursos ) { } em que o finally implícito chama o método close() dos recursos que implementem a interface AutoCloseable. Todos os streams passaram a implementar AutoCloseable. 10

11 Leitura e escrita de linhas de texto try ( BufferedReader in = new BufferedReader( new FileReader("abc.txt")); PrintWriter out = new PrintWriter( new FileWriter("xpto.txt")) ){ String line; int cntr; for(cntr=0 ; (line = in.readline())!=null ; ++cntr) out.println(line); System.out.println(cntr+" lines copied."); } catch (IOException e) { System.out.println("Error:"+e); } Dependendo do SO, o terminador de linha é "\r\n", '\n' ou '\r'. As classes BufferedReader, BufferedWriter além de fazerem bufferização, fazem este mapeamento. A classe PrintWriter, semelhante a PrintStream, também é usada para a escrita de linhas. 11

12 Escrita formatada As classes PrintWriter e PrintStream têm os métodos: - print e println para escrita formatada de valores individuais - format e printf para escrita formatada de vários valores. int i = 2; double r = Math.sqrt(i); PrintStream prt = System.out; prt.print("the square root of "); prt.print(i); prt.print(" is "); prt.out.print(r); prt.println("."); prt.println("the square root of " + i + " is " + f + "."); prt.format("the square root of %d is %f.%n", i, r); The square root of 2 is prt.format("%f, %1$ f %n", Math.PI); , %[argument_index$][flags][width]conversion 12

13 char Streams Writer OutputStream Reader InputStream FilterWriter FilterReader PushbackReader OutputStreamWriter FileWriter BufferedWriter CharArrayWriter PipedWriter PrintWriter StringWriter InputStreamReader FileReader BufferedReader LineNumberReader CharArrayReader PipedReader StringReader

14 Streams byte streams char streams Input Output Input Output Base InputStream OutputStream Reader Writer Converter InputStreamReader OutputStreamWriter Arrays ByteArrayInputStream ByteArrayOutputStream CharArrayReader CharArrayWriter Files FileInputStream RandomAccessFile FileOutputStream RandomAccessFile FileReader FileWriter Pipes PipedInputStream PipedOutputStream PipedReader PipedWriter Buffering BufferedInputStream BufferedOutputStream BufferedReader BufferedWriter Filtering FilterInputStream FilterOutputStream FilterReader FilterWriter Parsing PushbackInputStream StreamTokenizer PushbackReader LineNumberReader Scanner Strings StringReader StringWriter Data DataInputStream DataOutputStream Formatted PrintStream PrintWriter Objects ObjectInputStream ObjectOutputStream Utilities SequenceInputStream