COMP 213. Advanced Object-oriented Programming. Lecture 19. Input/Output

Transcription

1 COMP 213 Advanced Object-oriented Programming Lecture 19 Input/Output

2 Input and Output A program that read no input and produced no output would be a very uninteresting and useless thing. Forms of input/output in Java: Command-line arguments: main(string[] args) GUI I/O: TextFields, TextAreas, mouse-clicks, etc. System.in and System.out The last item gives two examples of input and output streams.

3 System.in and System.out In java.lang.system: public static InputStream in; public static PrintStream out; System fields In the java.io.inputstream API: public abstract class InputStream This abstract class is the superclass of all classes representing an input stream of bytes.

4 System.in and System.out In java.lang.system: public static InputStream in; public static PrintStream out; System fields In the java.io.inputstream API: public abstract class InputStream This abstract class is the superclass of all classes representing an input stream of bytes.

5 I/O: What? Input/Output is a general term that covers many ways that a program can recieve information (input), or send information (output). Most programs receive information from one or more sources, and deliver information to one ore more destinations. Information might flow from/to: files (e.g., a compiler reads source-code files; writes to executable files) a TextArea, or other GUI components

6 I/O: What? Input/Output is a general term that covers many ways that a program can recieve information (input), or send information (output). Most programs receive information from one or more sources, and deliver information to one ore more destinations. Information might flow from/to: files (e.g., a compiler reads source-code files; writes to executable files) a TextArea, or other GUI components

7 I/O: Where? Information may also flow in particular directions; e.g., from a keyboard (standard input) to a terminal (standard output) from a web server (if the program is a client) to a remote client (if the program is a server) When there is a particular source (or destination) for information, we talk of an input (or output) stream.

8 I/O: Where? Information may also flow in particular directions; e.g., from a keyboard (standard input) to a terminal (standard output) from a web server (if the program is a client) to a remote client (if the program is a server) When there is a particular source (or destination) for information, we talk of an input (or output) stream.

9 Bits and Bytes In computing, the basic unit of information is a bit. Eight of these are packaged together in a byte. One byte can represent an ASCII character. (there are no more than 2 8 ASCII characters) Several bytes are needed to represent an integer. (4 for 32-bit ints; 8 for 64-bit ints) At the lowest level of abstraction (concrete implementation), information has to be sent bit-by-bit. (At higher levels of abstraction, we think of sending characters, strings, files, pictures, web-pages, ebay bids, hotel-bookings....)

10 Bits and Bytes In computing, the basic unit of information is a bit. Eight of these are packaged together in a byte. One byte can represent an ASCII character. (there are no more than 2 8 ASCII characters) Several bytes are needed to represent an integer. (4 for 32-bit ints; 8 for 64-bit ints) At the lowest level of abstraction (concrete implementation), information has to be sent bit-by-bit. (At higher levels of abstraction, we think of sending characters, strings, files, pictures, web-pages, ebay bids, hotel-bookings....)

11 Protocols Sending information typically involves a protocol. (I.e., a set of conventions about how data is sent.) For example, the hypertext-transfer protocol (http) requires the first line of data to describe the content-type. Protocols are also used to indicate what kind of data is being sent. E.g., if 32-bit integers are being sent, the receiver needs to read four bytes at a time.

12 Byte-Oriented Streams Because bytes are fundamental to most data representations, most protocols specify that bits are sent eight at a time: in bytes. Any process that sends or receives a number of bytes, one after the other, is thought of as an input or output stream. Such processes include: keyboards (keyboard input) hard-disk busses (reading or writing to memory).

13 Byte-Oriented Input Streams In the package java.io: abstract class InputStream has subclasses: BufferedInputStream FileInputStream PipedInputStream DataInputStream that all offer ways of working with input streams, for example, reading from files.

14 Byte-Oriented Output Streams In the package java.io: abstract class OutputStream has subclasses: BufferedOutputStream FileOutputStream PipedOutputStream DataOutputStream that all offer ways of working with output streams, for example writing to files.

15 Writing Bytes In the java.io.outputstream API: OutputStream#write(int) API public abstract void write(int b) throws IOException Writes the specified byte to this output stream. The general contract for write is that one byte is written to the output stream. The byte to be written is the eight low-order bits of the argument b. The 24 high-order bits of b are ignored. I.e., the specification for implementations in concrete subclasses.

16 Writing Bytes In the java.io.outputstream API: OutputStream#write(int) API public abstract void write(int b) throws IOException Writes the specified byte to this output stream. The general contract for write is that one byte is written to the output stream. The byte to be written is the eight low-order bits of the argument b. The 24 high-order bits of b are ignored. I.e., the specification for implementations in concrete subclasses.

17 In the java.io.inputstream API: public abstract int read() throws IOException Reading Bytes InputStream#read() API Reads the next byte of data from the input stream. The value byte is returned as an int in the range 0 to 255. If no byte is available because the end of the stream has been reached, the value -1 is returned. This method blocks until input data is available, the end of the stream is detected, or an exception is thrown. I.e., a program will wait until input is available. A return value of -1 indicates the end of the stream: e.g., the end of a file, or a closed connection a bit like ending a telephone call.

18 In the java.io.inputstream API: public abstract int read() throws IOException Reading Bytes InputStream#read() API Reads the next byte of data from the input stream. The value byte is returned as an int in the range 0 to 255. If no byte is available because the end of the stream has been reached, the value -1 is returned. This method blocks until input data is available, the end of the stream is detected, or an exception is thrown. I.e., a program will wait until input is available. A return value of -1 indicates the end of the stream: e.g., the end of a file, or a closed connection a bit like ending a telephone call.

19 In the java.io.inputstream API: public abstract int read() throws IOException Reading Bytes InputStream#read() API Reads the next byte of data from the input stream. The value byte is returned as an int in the range 0 to 255. If no byte is available because the end of the stream has been reached, the value -1 is returned. This method blocks until input data is available, the end of the stream is detected, or an exception is thrown. I.e., a program will wait until input is available. A return value of -1 indicates the end of the stream: e.g., the end of a file, or a closed connection a bit like ending a telephone call.

20 Reading to the End int b = 0; while (b >= 0) // or b!= -1 { } b = theinputstream.read(); // do something with b some sample code If, for example, theinputstream is connected to keyboard input (System.in), each call of read() will cause the Java interpreter to pause until inout is available (i.e., until the user hits a key).

21 Reading Input Stream Value Returned 97 read() read() read() 121 close read() -1

22 Reading Input Stream Value Returned 97 read() read() read() 121 close read() -1

23 Reading Input Stream Value Returned 97 read() read() read() 121 close read() -1

24 Reading Input Stream Value Returned 97 read() read() read() 121 close read() -1

25 Reading Input Stream Value Returned 97 read() read() read() 121 close read() -1

26 Reading Input Stream Value Returned 97 read() read() read() 121 close read() -1

27 Reading Input Stream Value Returned 97 read() read() read() 121 close read() -1

28 Reading Input Stream Value Returned 97 read() read() read() 121 close read() -1

29 Reading Input Stream Value Returned 97 read() read() read() 121 close read() -1

30 Reading Input Stream Value Returned 97 read() read() read() 121 close read() -1

31 Reading Input Stream Value Returned 97 read() read() read() 121 close read() -1

32 Creating Streams Most constructors for the InputStream subclasses take an InputStream as argument. BufferedInputStream constructor public BufferedInputStream(InputStream in) { }... These give us what are known as wrapper classes. They wrap extra functionality/features around the input stream.

33 Wrapper Classes A wrapper class adds extra functionality to a stream. For example, we have the InputStream System.in. We can add a buffer to store keyed characters until they are read by wrapping System.in in a BufferedInputStream: BufferedInputStream bis = new BufferedInputStream(System.in); sample code

34 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

35 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

36 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

37 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

38 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

39 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

40 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

41 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

42 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

43 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

44 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

45 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

46 Reading Buffered Input Stream Value Read read() read() read() 98-1 read() -1

47 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

48 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

49 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

50 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

51 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

52 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

53 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

54 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

55 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

56 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

57 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

58 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

59 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

60 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

61 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

62 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

63 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

64 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

65 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

66 Program BufferedOutputStream write(12) 12 write(0) 0 12 write(174) Writing Value Read (at other end) write(8) write(62) 62 8 write(202) flush()

67 Pass the Parcel Wrapper classes can be nested to incrementally add functionality. For example, the DataInputStream class provides methods to read characters, integers, etc. Its constructor takes an InputStream as parameter (so it will accept any subclass of InputStream as parameter): DataInputStream dis = new DataInputStream( new BufferedInputStream(System.in) ); sample code

68 Pass the Parcel Wrapper classes can be nested to incrementally add functionality. For example, the DataInputStream class provides methods to read characters, integers, etc. Its constructor takes an InputStream as parameter (so it will accept any subclass of InputStream as parameter): DataInputStream dis = new DataInputStream( new BufferedInputStream(System.in) ); sample code

69 For example: ASCII using DataInputStream public static void main(final String[] args) throws IOException { DataInputStream dis = new DataInputStream( BufferedInputStream(System.in) ); byte i = 0; while (i!= 32) // ASCII for space { i = dis.readbyte(); System.out.println("read " + i); } }

70 Example: Piped I/O The PipedInputStream and PipedOutputStream classes allow output from one thread to be passed as input to another thread. Instances of these classes have to be connected: either by calling the connect() method (in either class) or by passing one as a parameter to the constructor of the other. Our example uses the latter approach. We ll cover threads properly in Lectures 22 25; for the moment, we re just going to concentrate on I/O.

71 Example: Piped I/O The PipedInputStream and PipedOutputStream classes allow output from one thread to be passed as input to another thread. Instances of these classes have to be connected: either by calling the connect() method (in either class) or by passing one as a parameter to the constructor of the other. Our example uses the latter approach. We ll cover threads properly in Lectures 22 25; for the moment, we re just going to concentrate on I/O.

72 Example: Piped I/O The PipedInputStream and PipedOutputStream classes allow output from one thread to be passed as input to another thread. Instances of these classes have to be connected: either by calling the connect() method (in either class) or by passing one as a parameter to the constructor of the other. Our example uses the latter approach. We ll cover threads properly in Lectures 22 25; for the moment, we re just going to concentrate on I/O.

73 The Receiver public class PipeIn extends PipedInputStream implements Runnable { PipeIn(PipedOutputStream source) throws IOException { super(source); // connects the streams } class PipeIn because the PipedInputStream constructor throws IOException

74 The Receiver public class PipeIn extends PipedInputStream implements Runnable { PipeIn(PipedOutputStream source) throws IOException { super(source); // connects the streams } class PipeIn because the PipedInputStream constructor throws IOException

75 The Receiver public class PipeIn extends PipedInputStream implements Runnable { PipeIn(PipedOutputStream source) throws IOException { super(source); // connects the streams } class PipeIn because the PipedInputStream constructor throws IOException

76 public void run() { int b = 0; while (b >= 0) { try { b = read(); if (b!= -1) { System.out.print((char)b); } } catch(ioexception e) { } } class PipeIn, contd. Because read() (inherited from PipedInputStream) throws IOException.

77 public void run() { int b = 0; while (b >= 0) { try { b = read(); if (b!= -1) { System.out.print((char)b); } } catch(ioexception e) { } } class PipeIn, contd. Because read() (inherited from PipedInputStream) throws IOException.

78 Closing the Stream try { close(); } catch(ioexception e) { } } } end of run() and class PipeIn It s good programming practice to close any input/output streams once we re finished with them. because close() (inherited from PipedInputStream) throws IOException

79 Closing the Stream try { close(); } catch(ioexception e) { } } } end of run() and class PipeIn It s good programming practice to close any input/output streams once we re finished with them. because close() (inherited from PipedInputStream) throws IOException

80 Hang on! If read() does throw an IOException, that means there is a fault with the connection, and we should exit the while loop: try { while(b >= 0) { try { b = read();... } } } catch (IOException ioe) { try { close(); } catch (IOException ioe1) { } }

81 Hang on! If read() does throw an IOException, that means there is a fault with the connection, and we should exit the while loop: try { while(b >= 0) { try { b = read();... } } } catch (IOException ioe) { try { close(); } catch (IOException ioe1) { } }

82 Hang on! If read() does throw an IOException, that means there is a fault with the connection, and we should exit the while loop: try { while(b >= 0) { try { b = read();... } } } catch (IOException ioe) { try { close(); } catch (IOException ioe1) { } }

83 Hang on! But now we have: try { while(b >= 0) { b = read();... } } catch (IOException ioe) { try { close(); } catch (IOException ioe1) { } } try { close(); } catch (IOException ioe) { }

84 Hang on! It would be simpler to write: try { while(b >= 0) { b = read();... } } catch (IOException ioe) { } finally { try { close(); } catch (IOException ioe) { } } The code in the finally-clause is executed whether an exception is thrown or not.

85 Hang on! It would be simpler to write: try { while(b >= 0) { b = read();... } } catch (IOException ioe) { } finally { try { close(); } catch (IOException ioe) { } } The code in the finally-clause is executed whether an exception is thrown or not.

86 or even: Hang on! try { while(b >= 0) { b = read();... } } catch (IOException ioe) { System.err.println("I/O failure: " + "unable to read all data"); } finally { try { close(); } catch (IOException ioe) { } }

87 The Sender public class PipeOut extends PipedOutputStream implements Runnable { public void run() { class PipeOut // the data to be sent byte[] bs = new String("hello, world").getbytes(); This string will be sent byte-by-byte on this output stream

88 The Sender public class PipeOut extends PipedOutputStream implements Runnable { public void run() { class PipeOut // the data to be sent byte[] bs = new String("hello, world").getbytes(); This string will be sent byte-by-byte on this output stream

89 method PipeOut#run(), contd. try { for (int i=0; i < bs.length; i++) { write(bs[i]); } } catch (IOException ioe) { } finally { try { close(); } catch(ioexception e) { } } }

90 Putting it all together class PipeOut, contd public static void main(string[] args) { try { PipeOut po = new PipeOut(); PipeIn pi = new PipeIn(po); Thread to = new Thread(po); Thread ti = new Thread(pi); to.start(); ti.start(); } catch(ioexception e) { } } }

91 Character-Oriented Streams Input and Output Streams are useful for efficient coding of low-level data transfer (byte-by-byte). Very often, it is desirable to read and write strings of characters. The java.io package provides several classes with methods for reading and writing characters or lines of text. For example, BufferedReader.readLine().

92 Character-Oriented Streams In the package java.io: abstract class Reader has subclasses: BufferedReader FileReader PipedReader

93 Character-Oriented Streams In the package java.io: abstract class Writer has subclasses: BufferedWriter FileWriter PipedWriter PrintWriter Class PrintWriter has methods print(string) and println(string). PrintWriter is also the type of System.out.

94 Character-Oriented Streams In the package java.io: abstract class Writer has subclasses: BufferedWriter FileWriter PipedWriter PrintWriter Class PrintWriter has methods print(string) and println(string). PrintWriter is also the type of System.out.

95 More Wrapper Classes Recall that classes BufferedInputStream, etc., had constructors that took InputStreams as arguments, and wrapped extra functionality about them. Classes BufferedReader, etc., have constructors that take a Reader as argument. Classes BufferedWriter, etc., have constructors that take a Writer as argument. Abstract classes Reader and Writer are the abstract superclasses of character-oriented input and output streams.

96 How to get a Reader The InputStreamReader class is a bridge between byte-oriented streams and character-oriented streams. It reads bytes and translates them into characters. It has a constructor that takes an InputStream as argument. BufferedReader br = new BufferedReader (new InputStreamReader (System.in) );

97 For Example BufferedReader br = new BufferedReader (new InputStreamReader (System.in) ); String input; while(!(input = br.readline()).equals("q")) { System.out.println("you typed: " + input) } readline() blocks until input is available: in this case, the program waits until the user hits enter

98 For Example BufferedReader br = new BufferedReader (new InputStreamReader (System.in) ); String input; while(!(input = br.readline()).equals("q")) { System.out.println("you typed: " + input) } readline() blocks until input is available: in this case, the program waits until the user hits enter

99 How to Get a Writer Class OutputStreamWriter is a concrete subclass of Writer, and has a constructor that takes an OutputStream as argument. This gives the bridge between byte-oriented output streams and character-oriented output streams.

100 Summary Byte-oriented streams Character-oriented streams Wrapper classes Next: Network connections