JDK5.0的11个主要新特征

2009-12-22 00:43| 发布者: admin| 查看: 71| 评论: 0|原作者: 冰淇淋

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 syntax
Integer integerObject;
listOfIntegers = new ArrayList(); // 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 hm = new HashMap();
file://不能使用原始类型
GenList nList = new GenList(); 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 {

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) { 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 {

public static void main(String[] args) {
Limited number; file://正确
Limited 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 {
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(); 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[] 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; file://编译错误
}
　　泛型类不能继承自Throwable以及其子类
public GenExpection extends Exception{} file://编译错误

　　不能用于基础类型int等
Pair file://error

Pair 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 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 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 CharacterTypeExample
dDecimal integer159
xHexadecimal integer9f
oOctal integer237
fFixed-point floating-point15.9
eExponential floating-point1.59E+01
gGeneral floating-point (the shorter of e and f)
aHexadecimal floating point0x1.fccdp3
sStringHello
cCharacterH
bBooleanTRUE
hHash code42628b2
txDate and timeSee Table 3-7
%The percent symbol%
nThe platform-dependent line separator

Table 3-7. Date and Time Conversion Characters
Conversion CharacterTypeExample
CComplete date and timeMon Feb 09 18:05:19 PST 2004
FISO 8601 date2004-02-09
DU.S. formatted date (month/day/year)02/09/2004
T24-hour time18:05:19
r12-hour time06:05:19 pm
R24-hour time, no seconds18:05
YFour-digit year (with leading zeroes)2004
yLast two digits of the year (with leading zeroes)04
CFirst two digits of the year (with leading zeroes)20
BFull month nameFebruary
b or hAbbreviated month nameFeb
mTwo-digit month (with leading zeroes)02
dTwo-digit day (with leading zeroes)09
eTwo-digit day (without leading zeroes)9
AFull weekday nameMonday
aAbbreviated weekday nameMon
jThree-digit day of year (with leading zeroes), between 001 and 366069
HTwo-digit hour (with leading zeroes), between 00 and 2318
kTwo-digit hour (without leading zeroes), between 0 and 2318
ITwo-digit hour (with leading zeroes), between 01 and 1206
lTwo-digit hour (without leading zeroes), between 1 and 126
MTwo-digit minutes (with leading zeroes)05
STwo-digit seconds (with leading zeroes)19
LThree-digit milliseconds (with leading zeroes)047
NNine-digit nanoseconds (with leading zeroes)047000000
PUppercase morning or afternoon markerPM
pLowercase morning or afternoon markerpm
zRFC 822 numeric offset from GMT-0800
ZTime zonePST
sSeconds since 1970-01-01 00:00:00 GMT1078884319
EMilliseconds since 1970-01-01 00:00:00 GMT1078884319047

Table 3-6. Flags for printf
FlagPurposeExample
+Prints sign for positive and negative numbers+3333.33
spaceAdds a space before positive numbers| 3333.33|
0Adds leading zeroes003333.33
-Left-justifies field|3333.33 |
(Encloses negative number in parentheses(3333.33)
,Adds group separators3,333.33
# (for f format)Always includes a decimal point3,333.
# (for x or o format)Adds 0x or 0 prefix0xcafe
^Converts to upper case0XCAFE
$Specifies the index of the argument to be formatted; for example, %1$d %1$x prints the first argument in decimal and hexadecimal159 9F
< Formats the same value as the previous specification; for example, %d %
　　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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JDK5.0的11个主要新特征

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