J2SE5.0 新特性（11个特性）

J2SE5.0 新特性
（11个特性）
内容概要
   泛型(Generic)
   增强循环(Enhanced for Loop)
   可变参数(Variable Arguments)
   自动实现装箱和解箱操作(Boxing/Unboxing Conversions)
   静态导入(Static Imports)
枚举类(Enumeration Classes)
   元数据(Meta data)
轻量级字符串类(StringBuilder)
   控制台输入(Console Input)
   改变覆盖方法返回类型（Covariant Return Types）
   格式化I/O(Formatted I/O)
  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。
import java.util.*;
class TestFX
{
public static void main(String[] args)
{
ArrayList<String> al=new ArrayList<String>();
al.add("afds");
al.add("afds");
al.add("afds");
print(al);
ArrayList<Integer> al2=new ArrayList<Integer>();
al2.add(new Integer(3));
al2.add(new Integer(5));
al2.add(new Integer(8));
print(al2);
}

//下面的方法等价于 把参数改成(ArrayList list)
public static void print(ArrayList<?> list) 
{
       Iterator ite=list.iterator();
   while(ite.hasNext())
   System.out.print(ite.next()+"  ");
   System.out.println();
}
}
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
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
  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);
}
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);
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.po-‘)w(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。
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());
}
}
元数据(Meta data)
描述数据的数据，在JAVA中，主要是对注释加了些新的用法，由于对功能没任何关系，我们就不多说了。
轻量级字符串类(StringBuilder)
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。

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();
改变覆盖方法返回类型（Covariant Return Types）
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();
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





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