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
|
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.
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