JDK5.0的11个主要新特征

文章来源：csdn 作者：kmlzkma

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> { ... }

　　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) { //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等

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
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 %<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.