Advanced Programming in Java Peyman Dodangeh Sharif University

Advanced Programming in Java Peyman Dodangeh Sharif University of Technology Fall 2014

Agenda �Java I/O �Java Files �Streams �Reader/Writer �Serialization Fall 2014 Sharif University of Technology 2

Files and Streams �What is a file? �A collection of bytes stored in secondary storage �Why we need files? �Persistent storage �How can a program use a file? �Open, read/write, close �What are the file types? �Binary, Text �What is a data stream? �Network, file, devices, … Fall 2014 Sharif University of Technology 3

File Types �Text file �Building blocks: characters �txt files �Xml files �Binary file �Building blocks: bytes �Exe files �Zip files �PDF Fall 2014 Sharif University of Technology 4

Data Hierarchy �Bit �Byte �Character �Word Fall 2014 Sharif University of Technology 5

Java Characters �A Java character has two bytes �Java supports Unicode character set standard �ASCII �Java uses UTF-16 encoding �Other unicode encodings: �UTF-8 �UTF-16 �Other non-unicode encodings �Windows-1256 Fall 2014 Sharif University of Technology 6

Unicode �For more information: �http: //www. joelonsoftware. com/articles/Unicode. html �http: //java. sun. com/developer/technical. Articles/Intl/S upplementary/ �http: //en. wikipedia. org/wiki/Unicode Fall 2014 Sharif University of Technology 7

Java Special Characters String s = "Salam!n. I am St. A"; System. out. println(s); s = "\ ' ""; System. out. println(s); Salam! I am S '" Fall 2014 A Sharif University of Technology 8

Streams �Data flowing into and out of a program (I/O) is called a stream �Streams are either �binary: byte-based �text: character-based (unicode) �The java. io library provides classes to handle a wide variety of I/O situations Fall 2014 Sharif University of Technology 9

Streams Input Stream Output Stream Fall 2014 Sharif University of Technology 10

Java I/O Classes �Text I/O �Stream of characters (Unicode format) �Support provided by Reader and Writer classes �Binary I/O �Stream of bytes (raw format) �Support provided by Input. Stream and Output. Stream classes Fall 2014 Sharif University of Technology 11

Text Files �A text file is a common way to organize a file as a sequence of lines. �Each line is a sequence of characters �Each OS's file system has its own way to mark the ends of lines �java. io abstracts this in a consistent way �Information from text files must be parsed to identify meaningful components �The Scanner class helps with parsing Fall 2014 Sharif University of Technology 12

Binary Files �The term binary file is used for every other type of file organization �Interpreting binary files requires knowledge of how the bytes are to be grouped and interpreted � Text files are also binary files; � but the bytes have predefined meanings (character and line data) �Binary files provide highly efficient storage �Java allows entire objects to be serialized as byte sequences for this purpose Fall 2014 Sharif University of Technology 13

File. Reader / File. Writer �File. Reader extends �Input. Stream. Reader extends � Reader extends Object �fr = new File. Reader(location of a file); �Connects to and opens the file for character input �File. Writer extends �Output. Stream. Writer extends � Writer extends Object �fw = new File. Writer(location of a file); �Creates and opens the file for character output �If the file exists, it is erased Fall 2014 Sharif University of Technology 14

File. Reader Example File. Reader inf = new File. Reader("filename"); int ch. Code; while(-1 != (ch. Code=inf. read()) ) System. out. println( "Next char: "+(char)ch. Code); inf. close(); Fall 2014 Sharif University of Technology 15

Returned int �Why does Reader. read() return int, not char ? �Because you may read an eof �which is -1 �and you'd have no way to distinguish between eof and a valid char value otherwise Fall 2014 Sharif University of Technology 16

Other Reader Methods �Reader. read() is not commonly used �Some other methods are (usually) better �int read(char[] cbuf, int off, int len) �int read(char[] cbuf) �int read(Char. Buffer target) Fall 2014 Sharif University of Technology 17

File. Writer outf = new File. Writer("filename"); outf. write('A'); outf. write('n'); outf. write("Strings too!n"); outf. close(); Fall 2014 Sharif University of Technology 18

Reader & Writers �File. Readers and File. Writers provide only very basic IO capabilities �The read and write methods are also overloaded to read and write an array of characters �File. Writer has a constructor with a boolean parameter �It can be used for appending the file �File. Writer(String file. Name, boolean append) Fall 2014 Sharif University of Technology 19

File. Input. Stream/File. Output. Stream �File. Input. Stream extends �Input. Stream extends Object �fr = new File. Input. Stream(location of a file); �Connects to and opens the file for byte-oriented input �File. Output. Stream extends �Output. Stream extends Object �fw = new File. Output. Stream(location of a file); �Creates and opens the file for byte-oriented output �If the file exists, it is erased Fall 2014 Sharif University of Technology 20

File. Input. Stream inf = new File. Input. Stream("filename"); int b. Code; while(-1 != (b. Code=inf. read()) ) System. out. println( "Next byte: "+(byte)b. Code); inf. close(); Fall 2014 Sharif University of Technology 21

�Some other Input. Stream methods: �int read(byte b[]) �int read(byte b[], int off, int len) Fall 2014 Sharif University of Technology 22

File. Output. Stream outf = new File. Output. Stream("filename"); byte[] out = {52, 99, 13, 10}; outf. write(out); outf. close(); Fall 2014 Sharif University of Technology 23

Input. Stream/Output. Stream �File. Input. Stream and File. Output. Stream provides the same basic IO capabilities �Transfer is in bytes rather than characters. �There are no "lines" in these files. �How to append to a file �File. Output. Stream(String name, boolean append) Fall 2014 Sharif University of Technology 24

Paths and Filenames �Microsoft chose to use the backslash character in path names �new File. Reader("c: textfilesnewfile. txt"); �What is wrong with this file name? �In Java the backslash character in a String literal is an escape character �"c: {tab}extfiles{newline}ewfile. txt" �Either type double backslashes in String literals, or use the forward slash �"c: \textfiles\newfile. txt" �"c: /textfiles/newfile. txt" Fall 2014 Sharif University of Technology 25

Random. Access. File �This class is not a reader/writer �nor a inputstream/outputstream �You can use file as binary or text file �Used to access desired location of file �For read or write �It has a file pointer �The place where you read from/write into the file �You can move file pointer using seek(long) method �It has different methods for reading and writing Fall 2014 Sharif University of Technology 26

Random. Access. File raf = new Random. Access. File("c: /1. txt", "rw"); byte ch = raf. read. Byte(); System. out. println("first character : " + (char)ch); ch = raf. read. Byte(); System. out. println("second character : " + (char)ch); String line = raf. read. Line(); System. out. println("Read a line: " + line); raf. seek(5); float fl = raf. read. Float(); System. out. println("Read a float from index 5: " + fl); raf. seek(26); raf. write('r'); raf. write('n'); raf. write. Double(1. 2); raf. write. Bytes("This will complete the Demo"); raf. close(); Fall 2014 Sharif University of Technology 27

File Class �The java. io. File class abstracts the connection to and properties of a file or folder (directory) �It does not offer read/write operations �File f = new File("c: /data/sample. txt"); �Sample methods: f. delete(); f. length(); f. is. File(); … �File d = new File("c: /"); �This object represents a folder, not a file Fall 2014 Sharif University of Technology 28

File Methods � boolean can. Read(); � boolean can. Write(); � boolean can. Execute(); � boolean exists(); � boolean is. File() ; � boolean is. Directory() ; � boolean is. Absolute() ; //constructed by ” 1” or “c: /test/1” � String get. Name(); � String get. Path(); // “ 1” � String get. Absolute. Path() ; // “c: /test/1” � String get. Parent(); � long length() ; //zero for folders � long last. Modified() ; � String[] list() ; Fall 2014 Sharif University of Technology 29

Scanner �The Scanner class is not technically an I/O class �It is found in java. util �You can use a Scanner wrapped around any Input. Stream object to provide sophisticated tokenoriented input methods �new Scanner(System. in); �new Scanner(new File. Input. Stream("t. txt")); �scanner = new Scanner(new File("sample. txt)); �scanner. next. Double() �scanner. next() Fall 2014 Sharif University of Technology 30

Formatter �Also found in java. util �Used to format output to text files �Formatter f = new Formatter("afile. txt"); �Formatter g = new Formatter(a. File. Object); �The format method is the most important �f. format("x=%d; s=%sn", 23, "skidoo"); �similar to printf in C++ �The stream can be closed using… �g. close(); Fall 2014 Sharif University of Technology 31

Serialization �Most Objects in Java are serializable �Can turn themselves into a stream of bytes �Can reconstruct themselves from a stream of bytes �A serialized object includes all instance variables �Unless marked as transient �Members that are Object references are also serialized �Serializable is an interface �The serialized file is a binary file �Not a text file Fall 2014 Sharif University of Technology 32

public class Student implements Serializable{ private String name; private String student. ID; private double[] grades ; private transient double average = 17. 27; public Student(String name, String student. ID, double[] grades) { this. name = name; this. student. ID = student. ID; this. grades = grades; } public double get. Average() { double sum = 0; if(grades==null) return -1; for (double grade : grades) { sum+=grade; } return sum/grades. length; } //setters and getters for name, student. ID and grades } Fall 2014 Sharif University of Technology 33

Object Serialization Object. Output. Stream output = new Object. Output. Stream( new File. Output. Stream("c: /1. txt")); Student student = new Student("Ali Alavi", "88305489", new double[]{17. 2, 18. 9, 20, 13}); output. write. Object(student); output. close(); Fall 2014 Sharif University of Technology 34

Object Deserialization Object. Input. Stream stream = new Object. Input. Stream( new File. Input. Stream("c: /1. txt")); Student student = (Student) stream. read. Object(); System. out. println(student. get. Name()); System. out. println(student. get. Average()); stream. close(); Fall 2014 Sharif University of Technology 35

java. net. Socket �This class implements client sockets �also called just "sockets" �A socket is an endpoint for communication between two machines. �A stream of data is communicated between two nodes �Very similar to local I/O operations Fall 2014 Sharif University of Technology 36

Writing into Socket socket = new Socket(“ 192. 168. 10. 21", 8888); Output. Stream output. Stream = socket. get. Output. Stream(); Formatter formatter = new Formatter(output. Stream); formatter. format("Salam!n"); formatter. flush(); formatter. format("Chetori? n"); formatter. flush(); formatter. format("exit"); formatter. flush(); socket. close(); System. out. println("finished"); Fall 2014 Sharif University of Technology 37

Reading from a Socket Input. Stream input. Stream = socket. get. Input. Stream(); Scanner scanner = new Scanner(input. Stream); while(true){ String next = scanner. next(); if(next. contains("exit")) break; System. out. println("Server : " + next); System. out. flush(); } socket. close(); Fall 2014 Sharif University of Technology 38

Server. Socket �How to listen to other sockets? �What do yahoo and google do? Server. Socket server. Socket = new Server. Socket(8888); Socket socket = server. Socket. accept(); Fall 2014 Sharif University of Technology 39

Binary or Text? �You can use a socket as a binary or text stream Fall 2014 Sharif University of Technology 40

Summary �Streams access sequences of bytes �Readers and Writers access sequences of characters �File. Reader, File. Writer, File. Input. Stream, File. Output. Stream are the 4 major file access classes �Scanner provides sophisticated input parsing �Formatter provides sophisticated output formatting Fall 2014 Sharif University of Technology 41

Summary �Most objects can be serialized for storage in a file �The File class encapsulates files and paths of a file system Fall 2014 Sharif University of Technology 42

Further Reading �Other java I/O classes �Buffered input and output �Decorator pattern in java. io classes �java. nio �Socket Programming �Object serialization applications �RMI Fall 2014 Sharif University of Technology 43

Fall 2014 Sharif University of Technology 44