Java Input/Output

Reading standard input Surprisingly complicated (GUI focus) Old-fashioned way: BufferedReader r = new BufferedReader(new InputStreamReader(System.in)); String line = null; while ((line = r.readLine()) != null) { // process line … }

Two modern ways Scanner s = new Scanner(System.in); while (s.hasNextLine()) { String line = s.nextLine(); // process line … } Console r = System.console(); String line = null; while ((line = r.readLine()) != null) { // process line … }

Utility methods for Strings “the quick brown fox”.split(“ ”); => {“the”,“quick”,”brown”,”fox”} String.valueOf(13) => “13” String.valueOf(x) => x.toString() Integer.parseInt(“37”) => 37 (int) new Integer(“37”) => 37 (Integer object) similar for other primitives types …

RandomAccessFile makes files available “like a big array of bytes”, allows random read/write access anywhere inside a file only supports primitive types + Strings new RandomAccessFile(String fileName, String mode), where mode is “r”,”rw”,”rws”,”rwd” void seek(long pos) long getFilePointer() double readDouble() and similar long length() void setLength(long newLength)

RandomAccessFile example RandomAccessFile rf = new RandomAccessFile(“doubles”,”rw”); // read third double: go to 2 * 8 (size of one) rf.seek(8*2); double x = rf.readDouble(); // overwrite first double value rf.seek(0); rf.writeDouble(x); rf.close();

Streams vs. Readers/Writers Confusing number of alternatives: –Streams: for binary 8bit (byte) quantities –Readers/Writers: for String and 16bit Unicode characters Streams: always 8bit quantities at the lowest level, but may provide higher-level abstractions as well, e.g. –readInt() [in class DataInputStream] will process four 8bit values to read a single 32bit int value

InputStream hierarchy InputStream ByteArray File Filter Piped Object Sequence InputStream InputStream InputStream Data Buffered LineNumber PushBack InputStream InputStream InputStream InputStream Physical: ByteArrayInputStream, FileInputStream, PipedInputStream Virtual: all others

Class InputStream abstract class InputStream { int read() throws IOException; int read(byte[] buffer) throws IOException; long skip(long n) throws IOException; int available() throws IOException; void close() throws IOException; } IOException: e.g. file cannot be opened, read operation was interrupted, …

Physical input streams Some constructors: –ByteArrayInputStream(byte[] buffer); –ByteArrayInputStream(byte[] buffer, int offset, int count) –FileInputStream(File f); –FileInputStream(String fileName); –PipedInputStream(PipedOutputStream p); Why use File instead of String as argument to the FileInputStream constructor?

Virtual input streams Build on top of physical ones using composition, add additional behaviour, e.g.: –SequenceInputStream: appends a couple of InputStreams giving the impression/illusion of just a single stream –FilterInputStream: an instance of the “Wrapper design pattern” Client read() Filter read() InputStream

DataInputStream class DataInputStream extends FilterInputStream implements DataInput { … public boolean readBoolean() throws IOException public byte readByte() throws IOException public char readChar() throws IOException public double readDouble() throws IOException public int readInt() throws IOException public long readLong() throws IOException public short readShort() throws IOException public int readUnsignedByte() throws IOException public int readUnsignedShort() throws IOException } Read bytes (binary representation, not textual), return them as primitive data types Possibly generated by DataOutputStream Same interface also used by RandomAccessFile

Stream Tokenizer Useful to break up text into tokens (words, numbers, others): public class TokenTest { public static void main(String[] args) { StreamTokenizer tok = new StreamTokenizer(System.in); try { while (tok.nextToken() != tok.TT_EOF) { switch(tok.ttype) { case tok.TT_NUMBER: System.out.println(“number “ + tok.val); break; … case default: … }} } catch (IOException e) {} }} “23-skidoo, kid!” yields: number 23.0 token – word skidoo …

Output stream hierachy OutputStream ByteArray File Filter Piped Object OutputStream OutputStream OutputStream OutputStream OutputStream Data Buffered PrintStream OutputStream OutputStream Similar to InputStream hierarchy: physical: ByteArrayOutputStream, FileOutputStream, PipedOutputStream virtual: all others

OutputStream public abstract class OutputStream { public abstract void write(int b) throws IOException public void write(byte[] buffer) throws IOException public void flush() throws IOException public void close() throws IOException } Quite a few output streams use buffering (for efficiency reasons), flush() explicitly forces completion of pending output operations

PrintStream public class PrintStream extends FilterOutputStream { … public void print(boolean bool) public void print(int inum) public void print(float fnum) public void print(double dnum) public void print(String str) public void print(Object o) { print(o.toString()); } … } Generates textual representations print(Object o) is purely polymorphic

Object Serialization Transform instances into 8bit units can be send over networks, stored in filesystems Allows for object persistence, i.e. save state across program invocations Interface Serializable, classes ObjectInputStream and ObjectOutputStream Would be tricky: need to get shared structure right!

Object Serialization example import java.io.Serializable; class Pair implements Serializable { public Object first; public Object second; } class Holder implements Serializable { public Object value; } Actually empty interface, so “implements Serializable” suffices

Object Serialization example contd. import java.util.Date; … Date today = new Date(); Holder a = new Holder(); a.value = today; Holder b = new Holder(); b.value = today; Pair c = new Pair(); c.first = a; c.second = b; c a b today

Object Serialization: write to file try { FileOutputStream f = new FileOutputStream(“saveState”); ObjectOutputStream s = new ObjectOutputStream(f); s.writeObject(“The value of c is”); s.writeObject(c); s.flush(); } catch (IOException e) System.out.println(“received error “ + e); }

Object Serialization: read back try { FileInputStream f = new FileInputStream(“saveState”); ObjectInputStream s = new ObjectInputStream(f); String tag = (String) s.readObject(); Pair c = (Pair) s.readObject(); } catch (IOException e) { System.out.println(“received IO exception “ + e); } catch (ClassNotFoundException e) { System.out.println(“received class exception “ + e); }

Object Serialization: check == … Holder a = (Holder) c.first; Holder b = (Holder) c.second; if( a.value == b.value ) System.out.println(“it’s the same object”); …

Piped IO Producer/consumer paradigm Multiple threads and pipes A pipe: buffered data area used for both reading and writing May suspend on IO operations: –Read, because empty –Write, because full In Java: matched pair of streams: PipedInputStream in = new PipedInputStream(); PipedOutputStream out = new PipedOutputStream(in);

Piped IO Example: Fibonacci + Prime class FibMaker extends Thread { private DataOutputStream out; public FibMaker(DataOutputStream o) {out = o;} public void run() { int n = 0; int m = 1; try { out.writeInt(m); while(m<100000) { int newValue = n+m; n=m; m=newValue; out.writeInt(newValue); } out.close(); } catch (IOException e) {return; }}}

Piped IO Example contd. class PipeTest { static public void main(String[] args) {PipeTest w = new PipeTest(); w.show(System.out);} private PipeTest(PrintStream out) { DataInputStream fibs = makeFibs(); DataInputStream primes = makePrimes(); try { int x = fibs.readInt(); int y = primes.readInt(); while (x < ) { if (x == y) { out.println(x + “ is both prime and fib”); x = fibs.readInt(); y = primes.readInt(); } else if (x<y) x = fibs.readInt(); else y = primes.readInt(); } } catch (IOException e) { System.exit(0);}}

Piped IO Example contd. private DataInputStream makeFibs() { try { PipedInputStream in = new PipedInputStream(); PipedOutputStream out = new PipedOutputStream(in); Thread fibThread = new FibMaker (new DataOutputStream(out)); fibThread.start(); return new DataInputStream(in); } catch (IOException e) { return null;} }

Readers and Writers hierarchy Reader Buffered CharArray InputStream Filter Piped String Reader Reader Reader LineNumber File Pushback Reader Reader Reader Writer Buffered CharArray OutputStream Filter Piped Print String Writer Writer Writer Writer Writer Writer Writer FileWriter

Readers/Writers Purely textual, but similar to IO streams try { FileReader f = new FileReader(“filename”); BufferedReader b = new BufferedReader(b); … } catch (…){ … } … Also wrap around IO streams: FileInputStream f = new FileInputStream(“f”); InputStreamReader r = new InputStreamReader(f, “MacCyrillic”); BufferedReader b = new BufferedReader(r); String text = r.readLine();

String.split(String Regex) even simpler than StreamTokenizer "boo:and:foo”.split(“:”) -> {“boo”,”and”,”foo”} "boo:and:foo”.split(“o”) -> {“b”,””,”:and:f”} Typical loop to process file input: BufferedReader reader = new BufferedReader( new FileReader(fileName)); String line = null; while ((line = reader.readLine()) != null) { String[] tokens = line.split(“ “); // process tokens … }