Object-Oriented Programming (Java)

1. Object-Oriented Programming (Java)

2. Topics Covered Today • Unit 1.1 Java Applications • 1.1.3 Beginning with the Java API • 1.1.4 Console I/O

3. Java API • API stands for Application Programming Interface(应用程序接口). • Java API 是Java提供给应用程序的类库，这些库经过仔细的编写，严格地与广泛地测试。 • Most programs use both features from the Java API and essential language features. • http://java.sun.com/j2se/1.5.0/docs/api/

4. Package • The classes in the Java API are grouped into packages. • Java用包来管理类名空间，为了解决同名的类有可能发生冲突的问题，包实际提供了一种命名机制和可见性限制机制. • A package is simply a collection of related classes • 所有的图形界面的类都放在java.awt这个包中， • 与网络功能有关的类都放到java.net这个包中。 • The fully qualified name of a class that is part of a package is the package name and the class name separated by a dot. • java.awt.Color

5. import Statement • 如果在源程序中用到了除java.lang这个包以外的类，无论是系统的类还是自己定义的包中的类，都必须用import语句标识，以通知编译器在编译时找到相应的类文件。 • 如果要从一个包中引入多个类则在包名后加上“.*”表示, 如 • import java.awt.Color; • import java.awt.*; • Classes java.lang.* are automatically imported

6. Qualified names Simple name Example import java.lang.String; import java.io.FileWriter; import java.io.IOException; public class test { public static void main(String[] args) { try { FileWriter fw = new FileWriter("hello.txt"); String h = "Hello"; String w = "World"; fw.write(h+ " " + w); fw.close (); } catch (IOException e) { System.out.println("Error writing to file:" + e); } } }

7. The java.lang.String Class (1) • Java中没有处理字符串的基本类型，但是有一个String类可以用来存储和处理字符串。 • String类的成员函数： • String(). Constructs a new String object that represents an empty character sequence. • String(char[] value). Constructs a new String object that represents the sequence of characters contained in the character array. • String(String original). Constructs a new String object that represents the same sequence of characters as the argument. • int size(). Obtains the number of characters in the String. • char charAt(int index). Returns the character at the specified index.

8. The java.lang.String Class (2) • boolean equals(Object anObject). Returns true if the specified Object represents a String with the same sequence of characters. • int indexOf(int ch). Returns the index of the first occurrence of the character. • int indexOf(String str). Returns the index of the first occurrence of the String. • boolean startsWith(String prefix). Checks if the String has the specified prefix. • String substring(int beginIndex, int endIndex). Returns a substring.

9. The java.lang.String Class (3) • String类的操作： • String中提供字符串的比较的方法：equals( )和equalsIgnoreCase( ) • 它们与运算符‘= =’实现的比较是不同的。 • 运算符‘= =’比较两个对象是否引用同一个实例， • 而equals( )和equalsIgnoreCase( )则比较两个字符串中对应的每个字符值是否相同。 • 字符串的转化 • java.lang.Object中提供了方法toString( )把对象转化为字符串。 • 字符串"+"操作 • 运算符‘+’可用来实现字符串的连接：String s = “He is ”+age+“ years old.”; • 注意：除了对运算符"+"进行了重载外，java不支持其它运算符的重载。

10. The java.util.StringTokenizer Class (1) • Tokenizing is the process of breaking a string into smaller pieces called tokens. • 例：下面字符串用空格化分，可分为几个字符串？ "This string has five tokens" • Popular delimiters(分割符) include the white space, underscore ( _ ) and the comma ( , ).

11. The java.util.StringTokenizer Class (2) • StringTokenizer类在java.util 包中，常用的方法有： • StringTokenizer(String str). Constructs a string tokenizer. The tokenizer uses the default delimiter set, white space. • StringTokenizer(String str, String delim). Constructs a string tokenizer. The argument delim contains the character delimiters for separating tokens. • boolean hasMoreTokens(). Tests if there are more tokens to extract. • String nextToken(String delim). Returns the next token in the string. • int countTokens(). Obtains the number of tokens left to be extracted, not the number of tokens in the string.

12. The java.util.StringTokenizer Class (3) import java.util.*; public class ProductInfo { public static void main(String[] args) { String data = "Mini Discs 74 Minute (10-Pack)_5_9.00"; StringTokenizer tknzr = new StringTokenizer(data, "_"); String name = tknzr.nextToken(); String quantity = tknzr.nextToken(); String price = tknzr.nextToken(); System.out.println("Name: " + name); System.out.println("Quantity: " + quantity); System.out.println("Price: " + price); } }

13. Java’s Primitive Types • 整型： • byte：8-bit • short：16-bit • int：32-bit • long：64-bit • 浮点型： • float：32-bit • double：64-bit • 字符型：char 16-bit • 布尔型：boolean

14. The Wrapper Classes (1) • 为了保证处理数据的一致性，Java将基本数据类型也封装成了类，这些类统称为wrapped classes。 • java.lang.Byte • java.lang.Short • java.lang.Integer • java.lang.Long • java.lang.Character • java.lang.Float • java.lang.Double • java.lang.Boolean

15. The Wrapper Classes (2) public class WrapperConversion { public static void main(String[] args) { Integer objectValue = new Integer(100); int intValue = objectValue.intValue(); long longValue = objectValue.longValue(); double doubleValue = objectValue.doubleValue(); String stringValue = objectValue.toString(); System.out.println("objectValue: " + objectValue); System.out.println("intValue: " + intValue); System.out.println("longValue: " + longValue); System.out.println("doubleValue: " + doubleValue); System.out.println("stringValue: " + stringValue); } }

16. The Wrapper Classes (3) import java.util.*; public class ProductInfo { public static void main(String[] args) { String data = "Mini Discs 74 Minute (10-Pack)_5_9.00"; StringTokenizer tknzr = new StringTokenizer(data, "_"); String name = tknzr.nextToken(); int quantity = Integer.parseInt(tknzr.nextToken()); double price = Double.parseDouble(tknzr.nextToken()); System.out.println("Name: " + name); System.out.println("Quantity: " + quantity); System.out.println("Price: " + price); } }

17. Primitives and Wrappers • Primitives in Java aren't objects at all. • Wrapper classesare object versions of the primitive types. • Things get annoying when you have to go back and forth between the two - converting a primitive to its wrapper, using it, then converting the object's value back to a primitive. • Happily, Tiger finally takes care of this issue.

18. New Java Language Feature • Java 5.0 introduced new language features • http://java.sun.com/j2se/1.5.0/docs/guide/language/ • Autoboxing(自动装箱) /unboxing(自动拆箱) • Automatically converts primitives (such as int) to wrapper classes (such as Integer) • Data value  Object (of matching class)

19. Autoboxing/Unboxing Example(Classical Approach) • Traditional “boxing” example: List ssnList = new ArrayList(); . . . int ssn = getSocSecNum(); . . . Integer ssnInteger = new Integer(ssn); ssnList.add(ssnInteger); • Why do I have to convert from int to Integer?

20. Autoboxing/Unboxing Example(New Approach) • “Autoboxing” example: List ssnList = new ArrayList(); . . . int ssn = getSocSecNum(); . . . ssnList.add(ssn); • No need for an explicit conversion to Integer.

21. Unboxing(Converting Wrapper Types to Primitives) • “Autoboxing/unboxing” example: // Boxing int foo = 0; Integer integer = foo; // Simple Unboxing int bar = integer;

22. Autoboxing / Unboxing • The need to explicitly convert between primitive types and wrapper objects. • Such as primitive type int, and wrapper object Integer. • Can’t put primitives into Collections. • Autoboxing and Unboxing • Automatic conversion by the compiler • Eliminates casts • Compiler performs these conversions for us.

23. Unboxing(Converting Wrapper Types to Primitives) • Null value assignment: Integer i = null; int j = i; • i is assigned null (which is legal), and then i is unboxed into j. However, null isn't a legal value for a primitive, so this code throws a NullPointerException.

24. Incrementing and Decrementing Wrapper Types • Every operation available to a primitive should be available to its wrapper-type counterpart, and vice versa: Integer counter = 1; while (true) { System.out.printf("Iteration %d%n", counter++); if (counter > 1000) break; } • Remember that counter is an Integer. So the value in counter was first auto-unboxed into an int, as that's the type required for the ++ operator. Once the value is unboxed, it is incremented. Then, the new value has to be stored back in counter, which requires a boxing operation.

25. Levels of Java API • Each JDK defines a standard set of API classes and methods • Sun provides standard extensions to the standard API • Others can and do provide Java APIs

26. Console I/O

27. d w o l o r l l e h read( ) Filter InputStream InputStream e.g. from disk file Stream • 在程序中提供一种将数据源连接到应用程序的方法，这样的方式叫做流(stream)。流是数据的真正流动.

28. Stream Input/Output • Stream • A connection carrying a sequence of data • Bytes  InputStream, OutputStream • Byte streams are used for data-based I/O, called input streams and output streams • Characters  FileReader, PrintWriter • Character streams are used for text-based I/O, called readers and writers • From a source to a destination • Keyboard • File • Network • Memory

29. Standard Input/Output • Standard I/O • Provided in System class in java.lang • System.in • An instance of InputStream • To input bytes from keyboard • System.out • An instance of PrintStream • To allow output to the screen • System.err • An instance of PrintStream • To allow error messages to be sent to screen

30. java.io package • Class java.io.BufferedReader • public BufferedReader(Reader in) • Create a buffering character-input stream that uses a default-sized input buffer. • readline(): Read a line of text. • read(): Read a single character. • Class java.io.PrintWriter • public PrintWriter(OutputStream out, boolean autoFlush) • Create a new PrintWriter from an existing OutputStream. • Print formatted representations of objects to a text-output stream. • print(String s): Print a string • println(String s):Print a string and then terminate the line

31. Template of a class using Console I/O import java.io.*; public class AnyClassUsingIO { private static BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in)); private static PrintWriter stdOut = new PrintWriter(System.out, true); private static PrintWriter stdErr = new PrintWriter(System.err, true); /* other variables */ /* methods */ }

32. 4 examples in unit 1.1.4 • PrintlnDemo.java • PrintDemo.java • Hello.java • ReadThreeIntegers.java

33. Using the printf( ) Convenience Method • Tiger gives us the ability to type printf( ). • import java.io.*; • public class PrintTester { • public static void main(String[] args) { • String filename = args[0]; • try { • File file = new File(filename); • FileReader fileReader = new FileReader(file); • BufferedReader reader = new BufferedReader(fileReader); • String line; • int i = 1; • while ((line = reader.readLine( )) != null) { • System.out.printf("Line %d: %s%n", i++, line); • } • } catch (Exception e) { • System.err.printf("Unable to open file named '%s': %s", • filename, e.getMessage( )); • } } }