JDK5.0的11個主要新特征
轉自 kmlzkma 的 Blog?
?
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(tài)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(shù)據(jù)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(shù)
(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(xiàn)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(shù)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(shù)
(type parameter)
的受限關系
.
例如
,
我們只需要一個存放數(shù)字的列表
,
包括整數(shù)
(Long, Integer, Short),
實數(shù)
(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(shù)
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(shù)
(type parameter)
限制為數(shù)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(shù)在
catch
塊中不允許出現(xiàn)，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(xiàn)
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(shù)組
T[] ts= new T[10];?? //
編譯錯誤
不能實例化泛型參數(shù)數(shù)
Pair<String>[] table = new Pair<String>(10); // ERROR
類的靜態(tài)變量不能聲明為類型參數(shù)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(huán)
(Enhanced for Loop)
舊的循環(huán)
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(huán)
//for(Iterator iter = list.iterator(); iter.hasNext(); ) {
//Integer stringObject = (String)iter.next();
// ... more statements to use stringObject...
//}
新的循環(huán)
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(shù)
(Variable Arguments)
實現(xiàn)了更靈活的方法參數(shù)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(xiàn)裝箱和解箱操作
(Boxing/Unboxing Conversions)
說明：實現(xiàn)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(xiàn)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(xiàn)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(tài)導入
(Static Imports)
很簡單的東西，看一個例子：
沒有靜態(tài)導入
Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));
有了靜態(tài)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(tài)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(tài)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(shù)據(jù)(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(shù)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.