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>();

file://不能使用原始类型

GenList<int> nList = new GenList<int>(); file://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> { ... }

例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));

}

例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) { file://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; file://正确

Limited<String> str; file://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(); file://error

不能实例化泛型类型的数组

T[] ts= new T[10]; file://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; file://编译错误

}

泛型类不能继承自Throwable以及其子类

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

不能用于基础类型int等

Pair<double> file://error

Pair<Double> file://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));

file://或者用以下循环

file://for(Iterator iter = list.iterator(); iter.hasNext(); ) {

file://Integer stringObject = (String)iter.next();

// ... more statements to use stringObject...

file://}

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;

file://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

Decimal integer

159

Hexadecimal integer

Octal integer

237

Fixed-point floating-point

15.9

Exponential floating-point

1.59E+01

General floating-point (the shorter of e and f)

Hexadecimal floating point

0x1.fccdp3

String

Hello

Character

Boolean

TRUE

Hash code

42628b2

Date and time

See Table 3-7

The percent symbol

The platform-dependent line separator

Table 3-7. Date and Time Conversion Characters

Conversion Character

Type

Example

Complete date and time

Mon Feb 09 18:05:19 PST 2004

ISO 8601 date

2004-02-09

U.S. formatted date (month/day/year)

02/09/2004

24-hour time

18:05:19

12-hour time

06:05:19 pm

24-hour time, no seconds

18:05

Four-digit year (with leading zeroes)

2004

Last two digits of the year (with leading zeroes)

First two digits of the year (with leading zeroes)

Full month name

February

b or h

Abbreviated month name

Feb

Two-digit month (with leading zeroes)

Two-digit day (with leading zeroes)

Two-digit day (without leading zeroes)

Full weekday name

Monday

Abbreviated weekday name

Mon

Three-digit day of year (with leading zeroes), between 001 and 366

069

Two-digit hour (with leading zeroes), between 00 and 23

Two-digit hour (without leading zeroes), between 0 and 23

Two-digit hour (with leading zeroes), between 01 and 12

Two-digit hour (without leading zeroes), between 1 and 12

Two-digit minutes (with leading zeroes)

Two-digit seconds (with leading zeroes)

Three-digit milliseconds (with leading zeroes)

047

Nine-digit nanoseconds (with leading zeroes)

047000000

Uppercase morning or afternoon marker

Lowercase morning or afternoon marker

RFC 822 numeric offset from GMT

-0800

Time zone

PST

Seconds since 1970-01-01 00:00:00 GMT

1078884319

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|

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.