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CS 1312

CS 1312. Introduction to Object Oriented Programming Lecture 10 Java’s String class Java File I/O. class java.lang.String. Recall that the type String is provided by the class String! Namely: java.lang.String

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CS 1312

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  1. CS 1312 Introduction to Object Oriented Programming Lecture 10 Java’s String classJava File I/O

  2. class java.lang.String • Recall that the type String is provided by the class String! • Namely: java.lang.String • Strings are so common, Java makes some special allowances that make them seem somewhat like primitives. • String str = “Catfood”; • String strDitto = new String(“Catfood”); • Class String has many handy String manipulation methods allowing: • pattern matching • substring operations • conversion of case

  3. String concatenation revisited • recall concatenation operator + • beware:System.out.println(“three plus five is “ + 3 + 5);output: three plus five is 35 • correction:System.out.println(“three plus five is “ + (3 + 5));output: three plus five is 8

  4. Other String methods revisited • .length()returns the length of a String • .charAt( int )returns as a proper char, the single character indexed by int • .substring( int )returns the substring starting at index int and continuing until the end of the String • .substring( int1, int2 )returns the substring starting at index int1 (inclusive) and ending at index int2 (exclusive)

  5. substring methods revisited 0 1 2 3 4 S a s h a Example: String str = “Sasha”; String strSub = str.substring(1); System.out.println(strSub); strSub = str.substring(2,4); System.out.println(strSub); System.out.println(str.substring(1,1)); Output: asha sh <blank line>

  6. other methods revisited 0 1 2 3 4 S a s h a Example: String str = “Sasha”; String strSmall = str.substring(4); selects “a” strSmall = str.substring(5); selects “” strSmall = str.substring(6); throws java.lang.StringIndexOutOfBoundsException strSmall = str.substring(2,1);  throws java.lang.StringIndexOutOfBoundsException

  7. other methods revisited 0 1 2 3 4 S a s h a Example: String str = “Sasha”; String emptyOne = new String(); System.out.println(str.length()); System.out.println( str.substring(2,4).length()); System.out.println( str.substring(1,1).length()); System.out.println(“”.length()); System.out.println(emptyOne.length()); Output: 5 2 0 0 0

  8. String methods having to do with case • .equalsIgnoreCase()can test for equality between two strings while ignoring case • .toUpperCase()creates an uppercase version of a string • .toLowerCase()creates a lowercase version of a string Note: none of these actually modify the original String in any way.

  9. .toUpperCase() String str = “Dorfmeister”; String strBig = str.toUpperCase(); System.out.println(str); System.out.println(strBig); System.out.println(str.toUpperCase()); System.out.println(str); Output: Dorfmeister DORFMEISTER DORFMEISTER Dorfmeister

  10. .toLowerCase() String str = “Paul Oakenfold”;String strSmall = str.toLowerCase();System.out.println(str);System.out.println(strSmall);System.out.println(str.toLowerCase());System.out.println(str); Output: Paul Oakenfold paul oakenfold paul oakenfold Paul Oakenfold

  11. String comparisons String method .equals()versus String method .equalsIgnoreCase()String str1 = “HeLLo”;String str2 = “hello”;System.out.println( str1.equals(str2));System.out.println( str1.equalsIgnoreCase(str2)); Output: false true

  12. String comparisons (cont) String method .compareTo(String) Usage:String str1, str2;. . .if (str1.compareTo(str2) < 0) { // str1 is alphabetically 1st} else if (str1.compareTo(str2)==0) { // str1 equals str2} else { // implies str1 > str2 // str1 is alphabetically 2nd}

  13. What? String has a compareTo method?? Yes! Hmm….Does that have anything to do with the Comparable interface? Yes! It’s not just a coincidence!! The class String implements and satisfies the Comparable interface. Straight from the API: public final class String implements java.io.Serializable, Comparable { // blah blah… }

  14. A word about the API(application programmer interface) Straight from the API: public final class String implements java.io.Serializable, Comparable { // blah blah… } Notice how that looks exactly like code for a class you might write? Don’t be afraid of the API…it’s your friend!

  15. More tricks with Strings • .replace(char, char)replaces all occurrences of the first char with the second char specified • .trim()removes white space (spaces, newlines, tabs, etc) from both ends of the String Note: again, none of these actually modify the original String in any way.

  16. Examples using.replace(char, char) String str = “Carl Cartwright”; String strTwo; strTwo = str.replace(‘C’,‘K’); System.out.println(strTwo); str = “\tthis has two tabs\t” strTwo = str.replace(‘\t’, ‘\n’); System.out.println(strTwo); str = “take out spaces”.replace(‘ ‘, ‘’); Output: Karl Kartwright <blank line> this has two tabs <new line> Error!Cannot havean empty char

  17. Examples using.trim() String str = “ \n\n\tStuff \n ”; String strTwo; strTwo = str.trim(); System.out.println(strTwo); strTwo = “ this has blanks “.trim(); System.out.println(strTwo); .trim() doesn’t remove interior whitespace! Output: Stuff this has blanks

  18. Finding patterns with .indexOf(String) catfood 0123456 String str = “catfood”; int location = str.indexOf(“food”); System.out.println(“pattern food begins at ” + location); System.out.println( “pattern dog begins at ” + str.indexOf(“dog”)); -1 is returned when the pattern is not found! Output: pattern food begins at 3 pattern dog begins at -1

  19. Finding patterns with . .lastIndexOf(String) catfood 0123456 String str = “catfood”; int location = str.indexOf(“o”); System.out.println(“last o begins at ” + location); System.out.println( “last dog begins at ” + str.lastIndexOf(“dog”)); Output: last o begins at 5 last dog begins at -1 -1 is returned when the pattern is not found!

  20. Finding a pattern with .indexOf(String, int) Can specify the starting index for the pattern search. Useful if we want to find ALL occurrences! String str = “abracadabra abracadabra”; int index = str.indexOf(“abra”); while (index != -1) { System.out.println( “found at “ + index); index = str.indexOf(“abra”, index + 1); } // note the final –1 is not printed Output: found at 0 found at 7 found at 12 found at 19 again, -1 is returned when the pattern is not found!

  21. Testing if a string ends with the given pattern .endsWith() The method .endsWith(String) returns a boolean String str = “somebody@cc.gatech.edu”; if (str.endsWith(“.edu”)) System.out.println( str + “ is a school address”); else System.out.println( str + “ is not a school address”); Output: somebody@cc.gatech.edu is a school address

  22. Integer.parseInt(String) Integer.parseInt(String) is used to take a String representation and create from it an actual int. Exceptionthrown! String str = “125”; int j = Integer.parseInt(str); int k = Integer.parserInt(“-1020”); int wrong = Integer.parserInt(“-1020.55”); In each case, the String must actually have the proper form of an int, otherwise it throws java.lang.NumberFormatExceptionas shown by the third example above.

  23. Now on to the next topic: File I/O

  24. Java I/O: Fun with Streams • In general, there are streams inherent in any Java process: • System.in • System.out • System.err • You are already familiar with most of these streams. E.g., • System.out.println (“This is the out stream”);

  25. Java IO: The Big Picture • The System.in, System.out, and System.err streams are building blocks for more complex IO objects • Java’s JDK 1.02 contained particularly weak IO design. Many classes just didn’t work. • Instead of scrapping JDK 1.02, the Sun engineers extended the 1.02 library with JDK 1.1 • In general, the JDK 1.1 classes provide adequate IO. (In some instances, one still must use JDK 1.02.)

  26. Java IO -- Basic Divisions (1.0) InputStream: through inheritance, all derivatives of InputStream have the basic method “read()” to access a byte or an array of bytes OutputStream through inheritance, all derivatives of OutputStream have the basic method “write()” to write a single byte Java I/O is divided based on directional flow: Conceptually, the two are separate

  27. Types of InputStreams (1.0) Java’s InputStreams have six general flavors: Watch this class

  28. Decorator Classes • Java IO uses ‘decorator’ objects to provide layers of functionality to IO classes Concept: A ‘decorator pattern’ wraps the inner object, all using the same interface devices. Pro/Con: Flexibility with the cost of complexity Example: notice how many IO classes feature the “readLine()” method.

  29. FilterInputStream Class • The FilterInputStream offers a ‘grab bag’ of methods. • Sun: this is the base class for “enhancing input stream functionality” • Eckel: “[they] couldn’t figure out where else to put this stuff, but it seemed like it belonged together” TIJ (9th ed.) • Avoid it--use InputStreamReader (JDK 1.1)

  30. Keyboard Access: Use InputStreamReader (1.1) • For reading keystrokes, use an InputStreamReader • Wrap with a BufferedReader decorator: public IOHelper(boolean DEBUG) { this.DEBUG = DEBUG; /* For reading data from standard in (stdin)*/ iStreamReader = new InputStreamReader(System.in); keyboard = new BufferedReader(iStreamReader); } // constructor

  31. Hierarchy of BufferedReader InputStreamReader System.in BufferedReader

  32. BufferedReader in ActionReading from the Keyboard public String readLine() { String strInput =""; try { strInput = keyboard.readLine(); } catch (Exception e) { System.err.println (”Keyboard error:\n\t" + e.toString()); e.printStackTrace(); System.exit(0); } return strInput; } // readLine()

  33. Writing to a File import java.io.*; public class DoFileStuff { private PrintWriter outFile; public static void main(String[] args) { try { outFile = new PrintWriter( new FileWriter(“testout.txt”,true)); outFile.println(“this prints to file ”); outFile.close(); } catch (IOException e) { System.out.println(“problem with file”); } } // DoFileStuff

  34. Reading from a File import java.io.*; public class DoFileStuff { private BufferedReader inFile; public static void main(String[] args) { String line; try { inFile = new BufferedReader( new FileReader(“data.txt”)); while((line = inFile.readLine()) != null){ System.out.println(line); } // end while inFile.close(); } // end try catch (IOException e) { System.out.println(“problem with file”); } // end catch } // end class DoFileStuff

  35. BufferedReader • Notice how the BufferedReader created for reading from a file is basically the same as the one we created for reading from the keyboard! • That’s the benefit of using the same decorator objectnamely, BufferedReader, to wrap the inner object that does differ whether we have • input from the keyboard new InputStreamReader(System.in)) or • input from a filenew FileReader(“data.txt”)))

  36. The File Class (1.0) • Another important object in Java’s IO library is the File Class • Misnomer: the File class does not necessarily refer to a single file. It can represent the name of a single file, or the names of a set of files. • The method list() returns a String[] array of file names • Therefore, ‘filepath’ might have been a better name • There is no JDK 1.1 equivalent

  37. Using File to Obtain A Directory Listing public class Lister { public static void main (String args[]){ try{ File path = new File (“.”); String[] listing = path.list(); for(int i=0;i<=listing.length;i++) System.out.println(listing[i]); } //try catch(Exception bummer) { bummer.printStackTrace(); } } //main } //class

  38. A FilenameFilter Example public void printDirListing(){ final String[] list; /* MUST be final */ final File path = new File("."); /* MUST be final */ list = path.list( new FilenameFilter(){ public boolean accept (File dir, String n){ String f = new File(n).getName().toLowerCase(); /* don't list .java files */ return f.indexOf(".class") == -1 && f.indexOf(".java") == -1; } // accept } //FilenameFilter ); // anonymous inner class for (int i=0; i< list.length; i++) System.out.println ("\t"+list[i]); } // printDirListing()

  39. Object Serialization--eh? • Normally, objects live only during the life of the program. Stop the program, and the object disappears • JDK 1.1 offers the Serializable interface to create ‘lightweight persistence’. • Persistent objects live beyond the life of the program, usually in byte form on disk or on a network.

  40. Object Serialization • You may have heard about ‘object persistence’ as a new Java buzzword. • The Serializable interface provides this capability for lightweight persistence. • The Serializable interface is lightweight because one must manually create/load persistent objects. • This technique is necessary for remote method invocation (RMI), where objects live on another machine

  41. Object Serialization (cont) • Serialization is also necessary for Java Bean design, where functional components have state information created and saved during design, not runtime. • How does one serialize an object? Just have your class implement the Serializable interface and save appropriately

  42. Saving Serialized Objects • Simple: create an OutputStream, and wrap it with an ObjectOutputStream. • Twin methods: • writeObject(); • readObject(); /* requires InputStream wrapped by ObjectInputStream */ • Restoring objects requires a .class file! • Therefore, don’t modify your .java files!

  43. Externalization of Objects • JDK 1.1 also has the java.io.Externalizable class. • Externalization is different from serialization. • The Externalizable interface extends Serializable and adds two methods one must implement: public abstract void readExternal(ObjectInput in) throws IOException, ClassNotFoundException public abstract void writeExternal(ObjectOutput out) throws IOException

  44. Externalized Objects (cont) • Externalized Objects are similar to serialized objects, except that the objects themselves control the field creation during reading and writing. • The readExternal and writeExternal are called with readObject() and writeObject() • This produces greater security, and offers the possibility of selective serialization • Note Bene: Constructors for externalized objects must be public

  45. Serialization & the transient State • There may be a time when we want to serialize an object, but omit certain key features, like passwords or other sensitive data. • Even objects and data identified as private get serialized. How does one keep data out of externalized objects? • Solution: use the transient keyword: private transient String password = “pssst.”; • Data and methods identified as transient are not serialized.

  46. Networking Basics • This class is not about networking, but in order to use the java.net.* package and classes, you’ll have to be familiar with some networking concepts. • The following slides cover simple networking terminology.

  47. Basic Client-Server Relations User Process Application Transport Protocol stack in kernel Network Link • At a fundamental level, all networked applications divide into either client or server services. Client and server applications communicate with each other through some type of communications link--often an agreed protocol for sharing information. • It is often helpful to conceive of the network as a series of ‘layers’, representing different levels of abstraction. • At the top level, we have applications--web browsers, ftp programs, telnet applications. • This top-level software utilizes a ‘transport layer’ (TCP protocol) • Which in turn uses a network layer (IP protocol: e.g., IPv4) • Which in turn uses a link layer (ethernet protocol)

  48. The User-Process Level • For the most part, we will be utilizing the user-process level, and rely on TCP/IP protocols. • Defined: The Transport Control Protocol (TCP) is a connection-based protocol that provides reliable flow of data between two computers. • If time permits, we may take a look at UDP (user datagram protocol) communications as well. (In short: UDP has no guarantees that information will arrive, but it is considerably faster.)

  49. Ports & Sockets: What? • Generally, computers possess only a single physical connection to a network. All information arrives and departs through this connection. For many different applications to use this same connection, computers create logical groupings, or ports. • Together with an IP address--a unique number assigned to a machine on a network--a port allows one to identify a specific process on a specific machine. • A socket is a software abstraction that provides a terminus to a connection between machines; it’s a point of abstraction that allows for users to address a connection between machines.

  50. URLs: How To An essential part of any socket is the URL. A URL has two main components: Other components include: Host Name -- the name of the machine ‘hosting’ the resource Filename -- the pathname to the file on the host Port Number -- the port number to which to connect (typically optional). Reference -- a reference to a named anchor within a resource that usually identifies a specific location within a file (typically optional). URL = uniform resource locator

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