天空是蓝色的

做好软件为中国 #gcc -c helloworld.c -o helloworld.o //编译目标文件 #gcc helloworld.o -o helloworld //编译成可执行exe #helloworld //运行exe
数据加载中……
[转载]JDK5.0的11个主要新特征

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<Integer> listOfIntegers; // <TYPE_NAME> is new to the syntax

Integer integerObject;

listOfIntegers = new ArrayList<Integer>(); // <TYPE_NAME> is new to the syntax

listOfIntegers.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<String,Float> hm = new HashMap<String,Float>();

//不能使用原始类型

GenList<int> nList = new GenList<int>();  //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<T> {

    void func(T t);

}

定义泛型类:

 

public class ArrayList<ItemType> { ... }

public class GenMap<T, V> { ... }

1

public class MyList<Element> extends LinkedList<Element>

{

       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<String> list = new MyList<String>();

              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));

       }

}

2

public class GenList <T>{

       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 <T> 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<T extends Number> {

       public static void main(String[] args) {

              Limited<Integer> number;   //正确

              Limited<String> 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<E extends Exception> {

       void execute() throws E;

}

 

public class GenericExceptionTest {

       public static void main(String args[]) {

              try {

                     Executor<IOException> e = new Executor<IOException>() {

                            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<String>[] table = new Pair<String>(10); // ERROR

 

类的静态变量不能声明为类型参数类型

 

public class GenClass<T> {

 

     private static T t;    //编译错误

 

}

泛型类不能继承自Throwable以及其子类

public GenExpection<T> extends Exception{}    //编译错误

不能用于基础类型int

 

Pair<double> //error

 

Pair<Double> //right

 

2           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<String> list = new LinkedList<String>();         

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 Type

 

boolean           Boolean

byte              Byte

char              Character

short             Short

int               Integer

long              Long

float              Float

double            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.shtml

 

8          Building Strings(StringBuilder)

 

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)

 

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;

 

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Employee

 

public 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   10

 

249f0 a

 

5.01 3.1

 

5.010e+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

 

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 %<x prints the same number in decimal and hexadecimal

 

 

 

这里是一些简单的介绍,更详细的说明请参考:

 

Core Java 2 Volume I - Fundamentals, Seventh Edition

 

Core Java 2 Volume II - Advanced Features, Seventh Edition

 

里面都有一些很精彩的描述,中文名称就是《Java核心技术》。只有第七版才有J2SE5.0的介绍,但是第七版好像还没有中文版。本文还参考了Professional Java JDK - 5th Edition.

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posted on 2006-02-10 10:07 bluesky 阅读(502) 评论(0)  编辑  收藏 所属分类: 工作总结


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