1. Telecooperation/RBG Technische Universität Darmstadt Copyrighted material; for TUD student use only Introduction to Computer Science I Topic 20: Streams and Input/Output Prof. Dr. Max Mühlhäuser Dr. Guido Rößling

2. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Outline Introduction to Input/Output Streams and Java Input/Output (I/O) Overview of Processing Streams Wrapping Streams and the Decorator Pattern User Defined Streams, StreamTokenizer and Random Access 2

3. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Input / Output: Motivation So far: –Program input data is either coded in the programs source code, or passed as parameters from the console define in Scheme Variable declaration and initialization in Java Actual arguments passed to functions in Scheme args parameters to main –Program output is shown on the display Problems: –Need to change the program in order to run it with different data –Closed world: No interactivity with the outside world No influence on the run is possible after the program starts –Results cannot be stored This lecture discusses other ways of input/output 3

4. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Input / Output (I/O) - Streams Other sources / sinks for program data / results –Keyboard entry (standard I/O), –Files on local machine ("file I/O") –Files/processes on the network ("network I/O") –Main memory ("memory I/O") To deal with all these sources/destinations uniformly, the concept of a data stream is introduced in Java –Input and output streams (I/O streams for short) Streams abstract from I/O devices –Hide details of the implementation and particularities of operating various I/O devices Keyboard, Files, Programs, Network, Memory, … –Provide unified interfaces for read/write data access –Java program talks to Java I/O stream objects 4

5. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 I/O-Streams In order to read data, an input stream is attached to a data source and the data is read element by element 5 In order to write data, an output stream is attached to a data sink and the data is written element by element

6. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Streams and delayed Lists The concept of streams is more general than just I/O We saw streams in Scheme –Delayed lists –We could create virtual lists –We could link transparently data with first and rest These ideas fit well with I/O –Input: on access to the next list item, read the input value –Output: on extension of the list, write the output value –Input stream delayed list without cons –Output stream delayed list without first/rest 6

7. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Predefined streams in java.io 7 Two orthogonal classifications: According to the type of data According to the structure of the stream

8. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Classification of streams According to the type of the data –character streams read / write char (16-bit Unicode character set) –byte streams read / write byte (8 bit): used to deal with binary data, e.g., images, sound, etc. According to stream structure –data streams: data is read/written directly from/to a concrete source/sink device –processing streams: data is read from/written to another stream Data is filtered, buffered, manipulated, etc. after reading, respectively before writing 8

9. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Hierarchy of Stream Classes in java.io 9 Character streams: –java.io.Reader/java.io.Writer provide the interface and a partial implementation for character input/output streams –Subclasses of Reader/Writer add/refine the implementation in Reader/Writer Byte streams: – java.io.InputStream/java.io.OutputStream provide the interface and a partial implementation for reading/writing bytes – All other byte streams are subclasses of InputStream/OutputStream

10. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Character Stream Classes (Examples) 10 Reader StringReader InputStreamReader FilterReader FileReader PushbackReader LineNumberReader BufferedReader PrintWriter StringWriter FilterWriter BufferedWriter Writer OutputStreamWriter FileWriter shadows indicate processing streams

11. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Byte Stream Classes (Examples) 11 InputStream OutputStream FileInputStream PipedInputStream FilterInputStream ByteArrayInputStream ObjectInputStream SequenceInputStream StringBufferInputStream LineNumberInputStream BufferedInputStream PushbackInputStream DataInputStream FileOutputStream PipedOutputStream ByteArrayOutputStream FilterOutputStream ObjectOutputStream PrintStream DataOutputStream BufferedOutputStream shadows indicate processing streams

12. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Data Streams: Overview 12 Type of DeviceCharacter StreamsByte Streams Main Memory Pipe File CharArrayReader, CharArrayWriter StringReader, StringWriter PipedReader, PipedWriter FileReader, FileWriter ByteArrayInputStream, ByteArrayOutputStream StringBufferInputStream PipedInputStream, PipedOutputStream FileInputStream, FileOutputStream CharArrayReader/CharArrayWriter ByteArrayInputStream/ByteArrayOutputStream – read from or write to an existing array in main memory StringReader / StringWriter, StringBufferInputStream – read from or write to Strings in main memory FileReader/FileWriter FileInputStream/FileOutputStream – read from or write to a file on disc

13. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Common patterns of I/O-streams Working with streams follows a pattern, independent of the data type, source, or sink Reading Open stream: implicit by creation Read data as long as data is needed and available Close stream After reading/writing, you should close() the stream! Writing Open stream Write data as long as required Close stream 13

14. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Common interfaces of I/O-streams 14 Use I/O Streams only via common Interface Information hiding via subtype polymorphism InputStream public int read() public int read(byte[] bbuf) public int read(byte[] bbuf, int offset, int len) Reader public int read() public int read(char[] cbuf) public int read(char[] cbuf, int offset, int len) OutputStream public int write(int b) public int write(byte[] bbuf) public int write(byte[] bbuf, int offset, int len) Writer public int write(int c) public int write(char[] cbuf) public int write(char[] cbuf, int offset, int len)

15. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 The java.io.Reader class public abstract int read(char[] c) throws IOException –reads the next Unicode characters into an array ( c ) –Returns the number of characters read, or -1 if the end of the stream has been reached ("EOF, End of File) –Any other problem causes an IOException, e.g., stream already closed, network connection lost,... 15 For more details, see the API specification http://java.sun.com/javase/6/docs/api/ public int read(char[] c) throws IOException public long skip(long n) throws IOException public void reset() throws IOException public int close() throws IOException...

16. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Java Documentation 16

17. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Java Documentation 17

18. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 File Streams File streams represent I/O streams from and to files in the file system: –FileReader for reading and FileWriter for writing character-wise from/to a file –Similarly FileInputStream, FileOutputStream for byte-wise reading/writing File streams are created by providing the source / destination file by means of: – A file name ( String ) – A file object ( java.io.File ) – A file descriptor ( java.io.FileDescriptor ) 18

19. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Class java.io.FileReader 19 Example: print file contents to screen (as int) and write 'a' to 'z' to a file import java.io.FileReader; import java.io.FileWriter; import java.io.IOException; public class ReadWriteFile { // constructor etc. public void readFrom(String fileName) throws IOException { FileReader in = new FileReader(fileName); int b; while ((b = in.read()) != -1) System.out.print(b); in.close(); } public void writeAToZ(String filename) throws IOException { FileWriter out = new FileWriter(filename); for (char c = 'a'; c <= 'z'; c++) out.write(c); out.close(); } // main() etc. }

20. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Outline Introduction to Input/Output Streams and Java Input/Output (I/O) Overview of Processing Streams Wrapping Streams and the Decorator Pattern User Defined Streams, Stream Tokenizer and Random Access 20

21. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Processing Streams A processing stream contains another (data or processing) stream –The latter is used as a data source or data sink, respectively Data might be transformed or functionality added –data buffering –counting lines –converting bytes and chars –compression – … 21 data drain processing stream data source device data stream

22. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Processing Streams: Overview 22 processcharacter streamsbyte streams filtering buffering byte / char conversion counting datatype handling undoing printing FilterReader, FilterWriter FilterInputStream, FilterOutputStream BufferedReader, BufferedWriter BufferedInputStream, BufferedInputStream InputStreamReader, OutputStreamWriter LineNumberReader LineNumberInputStream DataInputStream, DataOutputStream PushbackReader PushbackInputStream PrintWriter PrintStream byte char char byte

23. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Roots of processing streams FilterReader/-Writer and FilterInputStream/FilterOutputStream are general super-classes for every processing stream They encapsulate an internal stream in Default implementations of an operation op : pass on op() to the underlying stream concrete filter (sub-)classes refine this functionality 23

24. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Java Documentation 24

25. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Buffered Streams A buffered stream temporarily stores data from another stream in an internal buffer Reading from a buffered stream fills the buffer when it is empty –Further "read"-operations will access the buffer without reading from the attached stream –When the buffer is empty, additional data will be read from the underlying stream Writing to a buffered stream fills the buffer before the attached data sink is written to –The buffer is also emptied if the stream is flushed explicitly ( flush() ) or closed ( close() ) 25

26. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Buffered Streams BufferedReader and BufferedWriter –Subclasses of Reader/Writer, respectively BufferedInputStream and BufferedOutputStream –Subclasses of FilterInputStream / FilterOutputStream 26

27. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Buffered Streams 27 Data Sink Program BufferedWriter b write bbuf write bbuf write bbuf flush

28. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Buffered Streams: Performance 28 Reading a file of 2.5MB (x=buffer size, y=time [ms]) Buffer sizeTime[ms] 16433 23453 41935 8988 16471 32365 64363 12887 256327 51265 1024162 2048214 4096260 819261 16384166 32768159

29. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Data Streams 29 A data stream lets an application read/write primitive Java data types from/to an underlying I/O stream in a machine-independent way. An application uses a DataOutputStream to write data that can later be read by a DataInputStream. FilterInputStream DataInputStream readBoolean(): boolean readByte(): byte readShort() : short readChar() : char readInt() : int readFloat() : float FilterOutputStream DataOutputStream writeBoolean(boolean) : void writeByte(byte) : void writeShort(short) : void writeChar(char) : void writeInt(int) : void writeFloat(float) : void > DataInput > DataOutput

30. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Data Streams: Example 30 import java.io.FileOutputStream; import java.io.DataOutputStream; import java.io.IOException; public class DataStreamExample { String fileName = // some name; // some methods public void writeData() throws IOException { FileOutputStream fos = new FileOutputStream(filename); DataOutputStream out = new DataOutputStream(fos); out.writeInt(9); out.writeDouble(Math.PI); out.writeBoolean(true); out.close(); } // other methods public void readData() throws IOException { FileInputStream fis = new FileInputStream(fileName); DataInputStream in = new DataInputStream(fis); int i = in.readInt(); double d = in.readDouble(); boolean b = in.readBoolean(); in.close(); System.out.println("Read "+ i + ", " + d + ", and " + b+ "."); } }

31. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Standard-I/O Package java.lang contains the class System with the following class variables: static in standard input (keyboard) static out standard output (screen) static err standard error messages(screen) System.in yields an object of type InputStream –enables to read() characters (bytes) from the keyboard System.out, System.err are of type PrintStream –offer print(...) and println(...) to output data to the screen 31

32. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 PrintStream 32 FilterOutputStream PrintStream print(boolean) : void print(double) : void print(char) : void print(double) : void print(float) : void... println(Object): void println(String): void... Adds the ability to conveniently print user-readable representations of various data values Never throws an IOException –Exceptional situations set an internal flag that can be tested via the checkError method All characters printed by a PrintStream are converted into bytes using the platform's default character encoding –PrintWriter should be used in situations that require writing characters rather than bytes.

33. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Outline Introduction to Input/Output Streams and Java Input/Output (I/O) Overview of Processing Streams Wrapping Streams and the Decorator Pattern User Defined Streams, StreamTokenizer and Random Access 33

34. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 The Architecture of Java Streams Streams can be wrapped around each other –Abstraction layers, where underlying primitive streams are used by the enclosing (higher, more comfortable) streams 34 Data sink, e.g. file FileOutputStream BufferedOutputStream ZipOutputStream // create a buffered compressed output stream to a file OutputStream out = new FileOutputStream(filename); out = new BufferedOutputStream(out); out = new ZipOutputStream(out); //... more features can be dynamically added // the stream features are not visible for clients

35. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 The Architecture of Java Streams The technique that lets us combine streams at runtime is of more general interest. –A general way to extend objects with new features dynamically In software technology, such kind of techniques are documented as design patterns. –Reusable, documented design ideas –More in the lecture SE Design The technique underlying the architecture of Java I/O streams is known as the Decorator Pattern. 35

36. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Outline Introduction to Input/Output Streams and Java Input/Output (I/O) Overview of Processing Streams Wrapping Streams and the Decorator Pattern User Defined Streams, Stream Tokenizer and Random Access 36

37. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 User-Defined Processing Streams Sometimes it is useful to define streams that process (e.g., filtering, statistics, production of information chunks) data from other streams Best implemented by inheriting from FilterReader/FilterInputStream or FilterWriter/FilterOutputStream, respectively In the following example, a process stream filters out all lines which do not contain a certain substring Unix "grep" command 37

38. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 User-Defined Processing Streams 38 import java.io.BufferedReader; import java.io.FilterReader; import java.io.IOException; class GrepReader extends FilterReader { String substring; BufferedReader in; GrepReader(BufferedReader reader, String pattern) { super(reader); in = reader; substring = pattern; } // return the next line containing the search pattern String readLine() throws IOException { String line; while (((line = in.readLine()) != null) && (line.indexOf(substring) == -1)) ; return line; } }

39. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 User-Defined Processing Streams import java.io.*; public class Grep { public static void main(String[] args) { if ((args.length == 0) || (args.length > 2)) { System.out.println("Usage: java Grep [ ]"); System.exit(0); } try { Reader d; if (args.length == 2) d = new FileReader(args[1]); else d = new InputStreamReader(System.in); GrepReader g = new GrepReader(new BufferedReader(d), args[0]); String line; while ((line = g.readLine()) != null) System.out.println(line); g.close(); } catch (IOException e) { System.err.println(e); } } } 39 stream being searched pattern to look for

40. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Random Access Until now, we have only considered sequential streams that could only be read or written sequentially Sequential streams are a good fit for sequential storage media, e.g. magnetic tapes Random Access Files allow for non-sequential access (random access) to the contents of a file 40

41. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Motivating Example for Random Access How to extract one file? 41 Let us regard the file format of a ZIP archive: –ZIP archives contain files and are usually compressed to save disk space –Apart from the files, there is an additional entry at the end of the file, the so-called dir-entry (for directory) –The dir-entry is used to keep track of which files are contained and where – within the archive – they begin

42. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Motivating Example for Random Access Extracting a file using a sequential stream: –Search the whole archive until the file is found –Extract the file On average: half of the entries are searched… Extracting a file using a random access stream: –Jump to the directory & read the entry of the file –Jump to the recorded position of the file –Extract the file Only two entities (directory & file) need to be read This approach is far more efficient 42

43. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Random Access in Java 43 Realized by the class RandomAccessFile Can be used for reading and writingimplements the interfaces DataInput and DataOutput Similar to FileInputStream and FileOutputStream, you open a RandomAccessFile on a file and pass a file name or a File object as a parameter Additionally, you have to pass a parameter to specify whether the file is opened for read-only or also for writing –You have to be able to read a file to be able to write to it

44. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 The Class RandomAccessFile Creation a RandomAccessFile for reading file random.txt: –new RandomAccessFile("random.txt", "r"); Creating a RandomAccessFile for reading and writing on random.txt: –new RandomAccessFile("random.txt", "rw"); Afterwards, read and write operations may be used to read and write data from/to the file 44

45. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 The Class RandomAccessFile RandomAccessFile supports a file pointer: points to the current position in a file Upon creation, the pointer is 0, i.e., beginning of the file Calls to read and write operations automatically move the file pointer by the number of read, respectively written bytes 45 RandomAccessFile also supports positioning operations int skipBytes(int n) throws IOException moves read/write position by n bytes (relative positioning) native void seek(long pos) throws IOException positions read/write position right before pos (absolute positioning) native long getFilePointer() throws IOException returns the current read/write position

46. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Stream Tokenizer java.io.StreamTokenizer supports the generation of tokens out of character sequences A token is obtained by calling nextToken () –Ignores Whitespace User can define what is to be regarded as whitespace –The type of the token read in is stored in the attribute ttype The token types are explained on the next slide 46

47. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 StreamTokenizer : Token Types Basic token types: –StreamTokenizer.TT_NUMBER – number scanned; the number is stored in nval as a double –StreamTokenizer.TT_WORD – a composed word is recognized which is stored in sval –StreamTokenizer.TT_EOL – end of line, if the tokenizer is configured to recognize EOL as a token ( eolIsSignificant(true) ) –StreamTokenizer.TT_EOF – end of file –Any other value is an encoding of the character read in 47

48. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Customizing StreamTokenizer public void wordChars(int low, int hi) –Defines what is to be regarded as a word –All characters in the interval [low, hi] are "word parts public void whitespaceChars(int low, int hi) –Defines whitespace –All characters in the interval [low, hi] are "space" characters public void eolIsSignificant(boolean flag) –Defines whether the end of line character is of interest public void quoteChar(char c) –Defines quote character See the API documentation for more features… 48

49. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 StreamTokenizer : Example Read a list of tea data with the following format: –Each line contains data for one type of tea, i.e., the end of line token is of relevance –Data items per tea type (separated by spaces) Tea name in quotes Number of seconds required Number of recommended tea spoons per liter What would be a complex program without further support, is easily written with the help of StreamTokenizer 49 "Ali Baba's 40 scents" 180 5 "Asatsuyu" 90 7 "Generic Black Tea" 180 5 "Caramel" 120 6 "Ceylon Pekoe" 120 6 "China Jasmin" 120 6 "Chinese Love Dream" 150 5 "One For All" 540 1 "Cherry Cream" 120 6

50. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 StreamTokenizer Example void parseTeas(InputStream is) throws IOException { StreamTokenizer stok = new StreamTokenizer(is); stok.eolIsSignificant(true); // EOL is important! stok.quoteChar('\"'); // quote char int token = 0; String teaName = null; int nrSecs, nrSpoons; while ((token = stok.nextToken()) != StreamTokenizer.TT_EOF) { teaName = stok.sval; // 1. token (name) token = stok.nextToken(); nrSecs = (int)stok.nval; // 2. (secs) token = stok.nextToken(); nrSpoons = (int)stok.nval; // 3. (spoons) token = stok.nextToken(); // consume EOL System.out.println(teaName + "=>" + nrSpoons + " spoons, seconds: " + nrSecs); } } 50 Normally, you should check the type actually read in before assigning it!

51. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 Scanner Since Java 1.5, the Scanner class can be used for parsing A Scanner can be opened on a Stream, file, … –Typical use: Scanner sc = new Scanner(System.in); The class offers many helpful methods, such as…: –String next (); // returns the next token –x nextX (); // for x = byte, double, float, int, long, short, Line Throws InputMismatchException if the type is not found –boolean hasNextX (); // same x as above Checks if the next element would match type X –You can redefine the delimiter, even to words –Additionally, regular expressions can be used for matching Check the API documentation for more information 51

52. Dr. G. Rößling Prof. Dr. M. Mühlhäuser RBG / Telekooperation © Introduction to Computer Science I: T20 What else? Encrypted files, compressed files, files sent over internet connections,... Exceptions! All I/O involves exceptions! –See part T16 for information on handling exceptions 52 try { statements involving I/O } catch (IOException e) { e.printStackTrace(); }