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Java session08
 

Java session08

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    Java session08 Java session08 Presentation Transcript

    • Java Programming LanguageObjectives In this session, you will learn to: Write a program that uses command-line arguments and system properties Write a program that reads from standard input Describe the C-type formatted input and output Write a program that can create, read, and write files Describe the basic hierarchy of collections Write a program that uses sets and lists Write a program to iterate over a collection Write a program that uses generic collections Ver. 1.0 Session 8 Slide 1 of 20
    • Java Programming LanguageCommand-Line Arguments • Command-line arguments are the parameters passed to a Java application at run time. • Each command-line argument is placed in the args array that is passed to the static main method. For example: public static void main(String[] args) Ver. 1.0 Session 8 Slide 2 of 20
    • Java Programming LanguageSystem Properties System properties are a feature that replaces the concept of environment variables (which are platform-specific). System properties include information about the current user, the current version of the Java runtime, and the character used to separate components of a file path name. The System.getProperties() method returns a Properties object. – The System.getProperty(String) method returns a String representing the value of the named property. – The System.getProperty(String, String) method enables you to supply a default string value (second parameter), which is returned if the named property does not exist. Ver. 1.0 Session 8 Slide 3 of 20
    • Java Programming LanguageConsole I/O • Applications interact with the user using console I/O. • Java 2 SDK supports console I/O with three public variables in the java.lang.System class: – The variable System.out enables you to write to standard output. It is an object of type PrintStream. – The variable System.in enables you to read from standard input. It is an object of type InputStream. – The variable System.err enables you to write to standard error. It is an object of type PrintStream. Ver. 1.0 Session 8 Slide 4 of 20
    • Java Programming LanguageWriting to Standard Output • The println() method print the argument and a newline character (n). • The print() method print the argument without a newline character. • The print() and println() methods are overloaded for most primitive types (boolean, char, int, long, float, and double) and for char[], Object, and String. • The print(Object) and println(Object) methods call the toString() method on the argument. Ver. 1.0 Session 8 Slide 5 of 20
    • Java Programming LanguageReading from Standard Input The application program can use the following methods of the java.io package to read from the standard input: Read characters from the keyboard and convert the raw bytes into Unicode characters: InputStreamReader ir=new InputStreamReader(system.in); Create a buffered reader to read each line from the keyboard: BufferedReader in = new BufferedReader(ir); – The BufferedReader(in) provides a readLine() method to read from standard input one line at a time: s=in.readLine(); • The Scanner class of java.util package provides formatted input functionality. Ver. 1.0 Session 8 Slide 6 of 20
    • Java Programming LanguageFiles and File I/O The java.io package enables you to do the following: Create File objects Manipulate File objects Read and write to file streams Ver. 1.0 Session 8 Slide 7 of 20
    • Java Programming LanguageFiles and File I/O (Contd.) Creating a new File Object: File myFile; The File class provides several utilities: myFile = new File("myfile.txt"); myFile = new File("MyDocs", "myfile.txt"); Directories are treated just like files in Java; the File class supports methods for retrieving an array of files in the directory, as follows: File myDir = new File("MyDocs"); myFile = new File(myDir, "myfile.txt"); Ver. 1.0 Session 8 Slide 8 of 20
    • Java Programming LanguageFiles and File I/O (Contd.) For file input: – Use the FileReader class to read characters. – Use the BufferedReader class to use the readLine() method. For file output: – Use the FileWriter class to write characters. – Use the PrintWriter class to use the print() and println() methods. Ver. 1.0 Session 8 Slide 9 of 20
    • Java Programming LanguageFiles and File I/O (Contd.) The application program can use the following methods of the java.io package to read input lines from the keyboard and write each line to a file: Create file File file = new File(args[0]); Create a buffered reader to read each line from the keyboard InputStreamReader isr=new InputStreamReader(System.in); BufferedReader in = new BufferedReader(isr); Create a print writer on this file PrintWriter out = new PrintWriter(new FileWriter(file)); Ver. 1.0 Session 8 Slide 10 of 20
    • Java Programming LanguageFiles and File I/O (Contd.) Read each line from the input stream and print to a file one line at a time: s = in.readLine(); out.println(s); The application program can use the following methods of the java.io package to read from a text file and display each line on the standard output. Create file: File file = new File(args[0]); Create a buffered reader to read each line from the keyboard: BufferedReader in = new BufferedReader(new FileReader(file)); Ver. 1.0 Session 8 Slide 11 of 20
    • Java Programming LanguageFiles and File I/O (Contd.) Read each line from the file and displays it on the standard output: s = in.readLine(); System.out.println(s); Ver. 1.0 Session 8 Slide 12 of 20
    • Java Programming LanguageDemonstration Lets see how to read data from a file and display the output on the standard output device. This demo also shows how to run a program with user provided command line arguments. Ver. 1.0 Session 8 Slide 13 of 20
    • Java Programming LanguageThe Collections API A collection is a single object representing a group of objects known as its elements. The Collections API contains interfaces that group objects as one of the following: – Collection: A group of objects called elements; any specific ordering (or lack of) and allowance of duplicates is specified by each implementation. – Set: An unordered collection; no duplicates are permitted. – List: An ordered collection; duplicates are permitted. Ver. 1.0 Session 8 Slide 14 of 20
    • Java Programming LanguageGenerics Generics are described as follows: Provides compile-time type safety Eliminates the need for casts Example of before Generics code: ArrayList list = new ArrayList(); list.add(0, new Integer(42)); int total = ((Integer)list.get(0)).intValue(); Example of after Generics code: ArrayList<Integer> list = new ArrayList<Integer>(); list.add(0, new Integer(42)); int total = list.get(0).intValue(); Ver. 1.0 Session 8 Slide 15 of 20
    • Java Programming LanguageDemonstration Lets see how to use the Collection API and generics in a Java program. Ver. 1.0 Session 8 Slide 16 of 20
    • Java Programming LanguageIterators Iteration is the process of retrieving every element in a collection. An iterator of a set is unordered. A ListIterator of a List can be scanned forwards (using the next method) or backwards (using the previous method). List list = new ArrayList(); // add some elements Iterator elements = list.iterator(); while ( elements.hasNext() ) { System.out.println(elements.next()); } Ver. 1.0 Session 8 Slide 17 of 20
    • Java Programming LanguageEnhanced for Loop The enhanced for loop has the following characteristics: Simplified iteration over collections Much shorter, clearer, and safer Effective for arrays Simpler when using nested loops Iterator disadvantages removed Iterators are error prone: Iterator variables occur three times per loop. This provides the opportunity for code to go wrong. Ver. 1.0 Session 8 Slide 18 of 20
    • Java Programming LanguageSummary In this session, you learned that: – A program can be parameterized by command-line arguments and system properties. – Applications interaction with the user is accomplished using text input and output to the console. – The Scanner class provides formatted input functionality. It is a part of the java.util package. – Java.io package enables you to create file objects, manipulate them, and read and write to the file streams. – The J2SE platform supports file input in two forms: • The FileReader class to read characters. • The BufferedReader class to use the readLine method. The J2SE platform supports file output in two forms: • The FileWriter class to write characters. • The PrintWriter class to use the print and println methods. Ver. 1.0 Session 8 Slide 19 of 20
    • Java Programming LanguageSummary (Contd.) A collection is a single object representing a group of objects. These objects are known as elements. There are two types of collections: • Set: It is an unordered collection, where no duplicates are permitted. • List: It is an ordered collection, where duplicates are permitted. – The Iterator interface enables you to scan forward through any collection. The enhanced for loop can be used to iterate through a collection. – Generics provide compile-time type safety and eliminate the need for casts. Ver. 1.0 Session 8 Slide 20 of 20