無為

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。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);
}
　　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

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 %<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