1 泛型(Generic)1.1 说明增强了java的类型安全，可以在编译期间对容器内的对象进行类型检查，在运行期不必进行类型的转换。而在j2se5之前必须在运行期动态进行容器内对象的检查及转换减少含糊的容器，可以定义什么类型的数据放入容器ArrayList listOfIntegers; // is new to the syntaxInteger integerObject;listOfIntegers = new ArrayList(); // is new to the syntaxlistOfIntegers.add(new Integer(10)); // 只能是Integer类型integerObject = listOfIntegers.get(0); // 取出对象不需要转换1.2 用法声明及实例化泛型类：HashMap hm = new HashMap();//不能使用原始类型GenList nList = new GenList(); //编译错误J2SE 5.0目前不支持原始类型作为类型参数(type parameter)定义泛型接口：public interface GenInterface { void func(T t);}定义泛型类：public class ArrayList { ... }public class GenMap { ... }例1：public class MyList extends LinkedList{ public void swap(int i, int j) { Element temp = this.get(i); this.set(i, this.get(j)); this.set(j, temp); } public static void main(String[] args) { MyList list = new MyList(); list.add("hi"); list.add("andy"); System.out.println(list.get(0) + " " + list.get(1)); list.swap(0,1); System.out.println(list.get(0) + " " + list.get(1)); }}例2：public class GenList { private T[] elements; private int size = 0; private int length = 0; public GenList(int size) { elements = (T[])new Object[size]; this.size = size; } public T get(int i) { if (i < length) { return elements[i]; } return null; } public void add(T e) { if (length < size - 1) elements[length++] = e; }}泛型方法：public class TestGenerics{ public String getString(T obj) { //实现了一个泛型方法 return obj.toString(); } public static void main(String [] args){ TestGenerics t = new TestGenerics(); String s = "Hello"; Integer i = 100; System.out.println(t.getString(s)); System.out.println(t.getString(i)); }}1.3 受限泛型 受限泛型是指类型参数的取值范围是受到限制的. extends关键字不仅仅可以用来声明类的继承关系, 也可以用来声明类型参数(type parameter)的受限关系.例如, 我们只需要一个存放数字的列表, 包括整数(Long, Integer, Short), 实数(Double, Float), 不能用来存放其他类型, 例如字符串(String), 也就是说, 要把类型参数T的取值泛型限制在Number极其子类中.在这种情况下, 我们就可以使用extends关键字把类型参数(type parameter)限制为数字示例public class Limited { public static void main(String[] args) { Limited number; //正确 Limited str; //编译错误 }}1.4 泛型与异常类型参数在catch块中不允许出现，但是能用在方法的throws之后。例：import java.io.*;interface Executor { void execute() throws E;} public class GenericExceptionTest { public static void main(String args[]) { try { Executor e = new Executor() { public void execute() throws IOException{ // code here that may throw an // IOException or a subtype of // IOException } }; e.execute(); } catch(IOException ioe) { System.out.println("IOException: " + ioe); ioe.printStackTrace(); } }}1.5 泛型的通配符"?""?"可以用来代替任何类型, 例如使用通配符来实现print方法。public static void print(GenList list) {})1.6 泛型的一些局限型不能实例化泛型T t = new T(); //error不能实例化泛型类型的数组T[] ts= new T[10]; //编译错误不能实例化泛型参数数Pair[] table = new Pair(10); // ERROR类的静态变量不能声明为类型参数类型public class GenClass { private static T t; //编译错误}泛型类不能继承自Throwable以及其子类public GenExpection extends Exception{} //编译错误 不能用于基础类型int等Pair //errorPair //right2 增强循环(Enhanced for Loop)旧的循环LinkedList list = new LinkedList(); list.add("Hi");list.add("everyone!");list.add("Was");list.add("the");list.add("pizza");list.add("good?"); for (int i = 0; i < list.size(); i++) System.out.println((String) list.get(i));//或者用以下循环//for(Iterator iter = list.iterator(); iter.hasNext(); ) {//Integer stringObject = (String)iter.next();// ... more statements to use stringObject... //}新的循环LinkedList list = new LinkedList(); list.add("Hi");list.add("everyone!");list.add("Was");list.add("the");list.add("pizza");list.add("good?"); for (String s : list) System.out.println(s);很清晰、方便，一看便知其用法3 可变参数(Variable Arguments)实现了更灵活的方法参数传入方式，System.out.printf是个很好的例子用法：void test(Object … args)一个很容易理解的例子public static int add(int ... args){ int total = 0; for (int i = 0; i < args.length; i++) total += args[i]; return total;}public static void main(String[] args){ int a; a = Varargs.add(1, 2, 3, 4, 5, 6, 7, 8, 9, 10); System.out.println(a);}4 自动实现装箱和解箱操作(Boxing/Unboxing Conversions)说明：实现了基本类型与外覆类之间的隐式转换。基本类型至外覆类的转换称为装箱，外覆类至基本类型的转换为解箱。这些类包括Primitive Type Reference Typeboolean Booleanbyte Bytechar Charactershort Shortint Integerlong Longfloat Floatdouble Double例如，旧的实现方式Integer intObject;int intPrimitive;ArrayList arrayList = new ArrayList();intPrimitive = 11;intObject = new Integer(intPrimitive);arrayList.put(intObject); // 不能放入int类型，只能使Integer新的实现方式int intPrimitive;ArrayList arrayList = new ArrayList();intPrimitive = 11;//在这里intPrimitive被自动的转换为Integer类型arrayList.put(intPrimitive);5 静态导入(Static Imports)很简单的东西，看一个例子：没有静态导入Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));有了静态导入import static java.lang.Math.*;sqrt(pow(x, 2) + pow(y, 2)); 其中import static java.lang.Math.*;就是静态导入的语法，它的意思是导入Math类中的所有static方法和属性。这样我们在使用这些方法和属性时就不必写类名。需要注意的是默认包无法用静态导入，另外如果导入的类中有重复的方法和属性则需要写出类名，否则编译时无法通过。6 枚举类(Enumeration Classes)用法：public enum Name {types, ….}简单的例子：public enum Colors {Red, Yellow, Blue, Orange, Green, Purple, Brown, Black}public static void main(String[] args){ Colors myColor = Colors.Red; System.out.println(myColor);}又一个简单例子：import java.util.*;enum OperatingSystems {windows, unix, linux, macintosh}public class EnumExample1 { public static void main(String args[]) { OperatingSystems os; os = OperatingSystems.windows; switch(os) { case windows: System.out.println(“You chose Windows!”); break; case unix: System.out.println(“You chose Unix!”); break; case linux: System.out.println(“You chose Linux!”); break; case macintosh: System.out.println(“You chose Macintosh!”); break; default: System.out.println(“I don’t know your OS.”); break; } }}应运enum简写的例子：import java.util.*; public class EnumTest{ public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.print("Enter a size: (SMALL, MEDIUM, LARGE, EXTRA_LARGE) "); String input = in.next().toUpperCase(); Size size = Enum.valueOf(Size.class, input); System.out.println("size=" + size); System.out.println("abbreviation=" + size.getAbbreviation()); if (size == Size.EXTRA_LARGE) System.out.println("Good job--you paid attention to the _."); }} enum Size{ SMALL("S"), MEDIUM("M"), LARGE("L"), EXTRA_LARGE("XL"); private Size(String abbreviation) { this.abbreviation = abbreviation; } public String getAbbreviation() { return abbreviation; } private String abbreviation;}enum类中拥有方法的一个例子：enum ProgramFlags { showErrors(0x01), includeFileOutput(0x02), useAlternateProcessor(0x04); private int bit; ProgramFlags(int bitNumber) { bit = bitNumber; } public int getBitNumber() { return(bit); }}public class EnumBitmapExample { public static void main(String args[]) { ProgramFlags flag = ProgramFlags.showErrors; System.out.println(“Flag selected is: “ + flag.ordinal() + “ which is “ + flag.name()); }}7 元数据(Meta data)请参考http://www-900.ibm.com/developerWorks/cn/java/j-annotate1/http://www-900.ibm.com/developerworks/cn/java/j-annotate2.shtml8 Building Strings(StringBuilder类)在JDK5.0中引入了StringBuilder类，该类的方法不是同步(synchronized)的，这使得它比StringBuffer更加轻量级和有效。9 控制台输入(Console Input)在JDK5.0之前我们只能通过JOptionPane.showInputDialog进行输入，但在5.0中我们可以通过类Scanner在控制台进行输入操作 例如在1.4中的输入 String input = JOptionPane.showInputDialog(prompt);int n = Integer.parseInt(input);double x = Double.parseDouble(input);s = input;在5.0中我们可以Scanner in = new Scanner(System.in);System.out.print(prompt);int n = in.nextInt();double x = in.nextDouble();String s = in.nextLine();10 Covariant Return Types(不晓得怎么翻译，大概是 改变返回类型)JDK5之前我们覆盖一个方法时我们无法改变被方法的返回类型，但在JDK5中我们可以改变它例如1.4中我们只能public Object clone() { ... }...Employee cloned = (Employee) e.clone();但是在5.0中我们可以改变返回类型为Employeepublic Employee clone() { ... }...Employee cloned = e.clone();11 格式化I/O(Formatted I/O)增加了类似C的格式化输入输出，简单的例子：public class TestFormat{ public static void main(String[] args){ int a = 150000, b = 10; float c = 5.0101f, d = 3.14f; System.out.printf("%4d %4d%n", a, b); System.out.printf("%x %x%n", a, b); System.out.printf("%3.2f %1.1f%n", c, d); System.out.printf("%1.3e %1.3e%n", c, d*100); }}输出结果为：150000 10249f0 a5.01 3.15.010e+00 3.140e+02下面是一些格式化参数说明(摘自Core Java 2 Volume I - Fundamentals, Seventh Edition)Table 3-5. Conversions for printf Conversion Character Type Example d Decimal integer 159 x Hexadecimal integer 9f o Octal integer 237 f Fixed-point floating-point 15.9 e Exponential floating-point 1.59E+01 g General floating-point (the shorter of e and f) a Hexadecimal floating point 0x1.fccdp3 s String Hello c Character H b Boolean TRUE h Hash code 42628b2 tx Date and time See Table 3-7 % The percent symbol % n The platform-dependent line separator Table 3-7. Date and Time Conversion Characters Conversion Character Type Example C Complete date and time Mon Feb 09 18:05:19 PST 2004 F ISO 8601 date 2004-02-09 D U.S. formatted date (month/day/year) 02/09/2004 T 24-hour time 18:05:19 r 12-hour time 06:05:19 pm R 24-hour time, no seconds 18:05 Y Four-digit year (with leading zeroes) 2004 y Last two digits of the year (with leading zeroes) 04 C First two digits of the year (with leading zeroes) 20 B Full month name February b or h Abbreviated month name Feb m Two-digit month (with leading zeroes) 02 d Two-digit day (with leading zeroes) 09 e Two-digit day (without leading zeroes) 9 A Full weekday name Monday a Abbreviated weekday name Mon j Three-digit day of year (with leading zeroes), between 001 and 366 069 H Two-digit hour (with leading zeroes), between 00 and 23 18 k Two-digit hour (without leading zeroes), between 0 and 23 18 I Two-digit hour (with leading zeroes), between 01 and 12 06 l Two-digit hour (without leading zeroes), between 1 and 12 6 M Two-digit minutes (with leading zeroes) 05 S Two-digit seconds (with leading zeroes) 19 L Three-digit milliseconds (with leading zeroes) 047 N Nine-digit nanoseconds (with leading zeroes) 047000000 P Uppercase morning or afternoon marker PM p Lowercase morning or afternoon marker pm z RFC 822 numeric offset from GMT -0800 Z Time zone PST s Seconds since 1970-01-01 00:00:00 GMT 1078884319 E Milliseconds since 1970-01-01 00:00:00 GMT 1078884319047 Table 3-6. Flags for printf Flag Purpose Example + Prints sign for positive and negative numbers +3333.33 space Adds a space before positive numbers | 3333.33| 0 Adds leading zeroes 003333.33 - Left-justifies field |3333.33 | ( Encloses negative number in parentheses (3333.33) , Adds group separators 3,333.33 # (for f format) Always includes a decimal point 3,333. # (for x or o format) Adds 0x or 0 prefix 0xcafe ^ Converts to upper case 0XCAFE $ Specifies the index of the argument to be formatted; for example, %1$d %1$x prints the first argument in decimal and hexadecimal 159 9F < Formats the same value as the previous specification; for example, %d % |
