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1?????????? 泛型 (Generic)

1.1????????? 說明

增強了 java 的類型安全，可以在編譯期間對容器內的對象進行類型檢查，在運行期不必進行類型的轉換。而在 j2se5 之前必須在運行期動態進行容器內對象的檢查及轉換

減少含糊的容器，可以定義什么類型的數據放入容器

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 類型

integerObject = listOfIntegers.get(0); // 取出對象不需要轉換

1.2????????? 用法

聲明及實例化泛型類：

HashMap<String,Float> hm = new HashMap<String,Float>();

// 不能使用原始類型

GenList<int> nList = new GenList<int>();? // 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<T> {

??? void func(T t);

}

定義泛型類：

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) { // 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));

????????????? }

}

1.3????????? 受限泛型

　　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<T extends Number> {

?????? public static void main(String[] args) {

????????????? Limited<Integer> number;?? // 正確

????????????? Limited<String> str;?????? // 編譯錯誤

?????? }

}

1.4????????? 泛型與異常

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

類的靜態變量不能聲明為類型參數類型

public class GenClass<T> {

???? private static T t;??? // 編譯錯誤

}

泛型類不能繼承自 Throwable 以及其子類

public GenExpection<T> extends Exception{}??? // 編譯錯誤

不能用于基礎類型int等

Pair<double> //error

Pair<Double> //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<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;

}

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?????????? 自動實現裝箱和解箱操作 (Boxing/Unboxing Conversions)

說明：實現了基本類型與外覆類之間的隱式轉換?；绢愋椭镣飧差惖霓D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);

}

又一個簡單例子：

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;

??????? }

??? }

}

應運enum簡寫的例子：

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 類中擁有方法的一個例子：

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????????? 元數據(Meta data)

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);

??? }

}

輸出結果為：

150000?? 10

249f 0 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 %<x prints the same number in decimal and hexadecimal

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.