JDK5.0的11個主要新特征 。

1.2 用法

聲明及實例化泛型類：

HashMap hm = new HashMap();

//不能使用原始類型

GenList nList = new GenList(); //編譯錯誤

J2SE 5.0目前不支持原始類型作為類型參數(type parameter)

定義泛型接口：

public interface GenInterface {

void func(T t);

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));

}

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;

}

public class TestGenerics{

public String getString(T obj) {
//實現了一個泛型方法

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));

}

受限泛型是指類型參數的取值范圍是受到限制的.
extends關鍵字不僅僅可以用來聲明類的繼承關系,
也可以用來聲明類型參數(type
parameter)的受限關系.例如,
我們只需要一個存放數字的列表, 包括整數(Long, Integer,
Short), 實數(Double, Float), 不能用來存放其他類型,
例如字符串(String), 也就是說,
要把類型參數T的取值泛型限制在Number極其子類中.在這種情況下,
我們就可以使用extends關鍵字把類型參數(type
parameter)限制為數字

示例

public class Limited {

public static void main(String[] args) {

Limited number; //正確

Limited str; //編譯錯誤

}

類型參數在catch塊中不允許出現，但是能用在方法的throws之后。例：

import java.io.*;

interface Executor {

void execute() throws E;

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();

}

}

"?"可以用來代替任何類型,
例如使用通配符來實現print方法。

public static void print(GenList list) {})

1.6 泛型的一些局限型

不能實例化泛型

T t = new T(); //error

不能實例化泛型類型的數組

T[] ts= new T[10]; //編譯錯誤

不能實例化泛型參數數

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

類的靜態變量不能聲明為類型參數類型

public class GenClass {

private static T t; //編譯錯誤

public GenExpection extends Exception{} //編譯錯誤

不能用于基礎類型int等

Pair //error

Pair //right

2 增強循環(Enhanced for Loop)

舊的循環

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));

//或者用以下循環

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

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

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

//}

新的循環

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 可變參數(Variable Arguments)

實現了更靈活的方法參數傳入方式，System.out.printf是個很好的例子

用法：void test(Object ... args)

一個很容易理解的例子

public static int add(int ... args){

int total = 0;

for (int i = 0; i < args.length; i++)

total += args[i];

return total;

int a;

a = Varargs.add(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

System.out.println(a);

說明：實現了基本類型與外覆類之間的隱式轉換。基本類型至外覆類的轉換稱為裝箱，外覆類至基本類型的轉換為解箱。這些類包括

Primitive Type Reference Type

boolean Boolean

byte Byte

char Character

short Short

int Integer

long Long

float Float

double Double

例如，舊的實現方式

Integer intObject;

int intPrimitive;

ArrayList arrayList = new ArrayList();

intPrimitive = 11;

intObject = new Integer(intPrimitive);

arrayList.put(intObject); // 不能放入int類型，只能使Integer

新的實現方式

int intPrimitive;

ArrayList arrayList = new ArrayList();

intPrimitive = 11;

//在這里intPrimitive被自動的轉換為Integer類型

arrayList.put(intPrimitive);

5 靜態導入(Static Imports)

很簡單的東西，看一個例子：

沒有靜態導入

Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));

有了靜態導入

import static java.lang.Math.*;

sqrt(pow(x, 2) + pow(y, 2));

其中import static
java.lang.Math.*;就是靜態導入的語法，它的意思是導入Math類中的所有static方法和屬性。這樣我們在使用這些方法和屬性時就不必寫類­名。

需要注意的是默認包無法用靜態導入，另外如果導入的類中有重復的方法和屬性則需要寫出類名，否則編譯時無法通過。

6 枚舉類(Enumeration Classes)

用法：public enum Name {types, ....}

簡單的例子：

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);

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;

}

}

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 _.");

}

{

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 ProgramFlags {

showErrors(0x01),

includeFileOutput(0x02),

useAlternateProcessor(0x04);

private int bit;

ProgramFlags(int bitNumber) {

bit = bitNumber;

}

public int getBitNumber() {

return(bit);

}

public static void main(String args[]) {

ProgramFlags flag = ProgramFlags.showErrors;

System.out.println("Flag selected is: " +

flag.ordinal() +

" which is " +

flag.name());

}

請參考

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類)

在JDK5.0中引入了StringBuilder類，該類的方法不是同步(synchronized)的，這使得它比StringBuffer更加輕量級和有­效。

9 控制臺輸入(Console Input)

在JDK5.0之前我們只能通過JOptionPane.showInputDialog進行輸入，但在5.0中我們可以通過類Scanner在控制臺進行輸入­操作

例如在1.4中的輸入

String input = JOptionPane.showInputDialog(prompt);

int n = Integer.parseInt(input);

double x = Double.parseDouble(input);

s = input;

在5.0中我們可以

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(不曉得怎么翻譯，大概是
改變返回類型)

JDK5之前我們覆蓋一個方法時我們無法改變被方法的返回類型，但在JDK5中我們可以改變它

例如1.4中我們只能

public Object clone() { ... }

...

Employee cloned = (Employee) e.clone();

但是在5.0中我們可以改變返回類型為Employee

public Employee clone() { ... }

...

Employee cloned = e.clone();

11 格式化I/O(Formatted I/O)

增加了類似C的格式化輸入輸出，簡單的例子：

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);

}

150000 10

249f0 a

5.01 3.1

5.010e+00 3.140e+02

下面是一些格式化參數說明(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
%

這里是一些簡單的介紹，更詳細的說明請參考：

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，中文名稱就是《Java核心技術》。只有第七版才有J2SE5.0的介紹，但是第七版好像還沒有中文版。本文還參考了Profess­ional
Java JDK - 5th Edition.

摘自Blog：http://blog.csdn.net/kmlzkma/