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摘要: JDK5.0的11个主要新特征
1.1 说明
增强了java的类型安全,可以在编译期间对容器内的对象进行类型检查,在运行期不必进行类型的转换。而在j2se5之前必须在运行期动态进行容器内对象的检查及转换
减少含糊的容器,可以定义什么类型的数据放入容器
ArrayList
Integer integerObject;
listOfIntegers = new ArrayList
listOfIntegers.add(new Integer(10)); // 只能是Integer类型
integerObject = listOfIntegers.get(0); // 取出对象不需要转换
1.2 用法
声明及实例化泛型类:
HashMap
file://不能使用原始类型
GenList
J2SE 5.0目前不支持原始类型作为类型参数(type parameter)
定义泛型接口:
public interface GenInterface
void func(T t);
}
定义泛型类:
public class ArrayList
public class GenMap
例1:
public class MyList
{
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.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
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
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
受限泛型是指类型参数的取值范围是受到限制的. extends关键字不仅仅可以用来声明类的继承关系, 也可以用来声明类型参数(type parameter)的受限关系.例如, 我们只需要一个存放数字的列表, 包括整数(Long, Integer, Short), 实数(Double, Float), 不能用来存放其他类型, 例如字符串(String), 也就是说, 要把类型参数T的取值泛型限制在Number极其子类中.在这种情况下, 我们就可以使用extends关键字把类型参数(type parameter)限制为数字
示例
public class Limited
public static void main(String[] args) {
Limited
Limited
}
}
1.4 泛型与异常
类型参数在catch块中不允许出现,但是能用在方法的throws之后。例:
import java.io.*;
interface Executor
void execute() throws E;
}
public class GenericExceptionTest {
public static void main(String args[]) {
try {
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();
}
}
}
1.5 泛型的通配符"?"
"?"可以用来代替任何类型, 例如使用通配符来实现print方法。
public static void print(GenList<?> list) {})
1.6 泛型的一些局限型
不能实例化泛型
T t = new T(); file://error
不能实例化泛型类型的数组
T[] ts= new T[10]; file://编译错误
不能实例化泛型参数数
Pair
类的静态变量不能声明为类型参数类型
public class GenClass
private static T t; file://编译错误
}
泛型类不能继承自Throwable以及其子类
public GenExpection
不能用于基础类型int等
Pair
Pair
旧的循环
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));
file://或者用以下循环
file://for(Iterator iter = list.iterator(); iter.hasNext(); ) {
file://Integer stringObject = (String)iter.next();
// ... more statements to use stringObject...
file://}
新的循环
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
实现了更灵活的方法参数传入方式,System.out.printf是个很好的例子
用法:void test(Object … args)
一个很容易理解的例子
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);
}
说明:实现了基本类型与外覆类之间的隐式转换。基本类型至外覆类的转换称为装箱,外覆类至基本类型的转换为解箱。这些类包括
Primitive Type Reference Type
boolean Boolean
byte Byte
char Character
short Short
int Integer
long Long
float Float
double Double
例如,旧的实现方式
Integer intObject;
int intPrimitive;
ArrayList arrayList = new ArrayList();
intPrimitive = 11;
intObject = new Integer(intPrimitive);
arrayList.put(intObject); // 不能放入int类型,只能使Integer
新的实现方式
int intPrimitive;
ArrayList arrayList = new ArrayList();
intPrimitive = 11;
file://在这里intPrimitive被自动的转换为Integer类型
arrayList.put(intPrimitive);
很简单的东西,看一个例子:
没有静态导入
Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));
有了静态导入
import static java.lang.Math.*;
sqrt(pow(x, 2) + pow(y, 2));
其中import static java.lang.Math.*;就是静态导入的语法,它的意思是导入Math类中的所有static方法和属性。这样我们在使用这些方法和属性时就不必写类名。
需要注意的是默认包无法用静态导入,另外如果导入的类中有重复的方法和属性则需要写出类名,否则编译时无法通过。
用法:public enum Name {types, ….}
简单的例子:
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);
}
又一个简单例子:
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;
}
}
}
应运enum简写的例子:
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类中拥有方法的一个例子:
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());
}
}
请参考
http://www-900.ibm.com/developerWorks/cn/java/j-annotate1/
http://www-900.ibm.com/developerworks/cn/java/j-annotate2.shtml
在JDK5.0中引入了StringBuilder类,该类的方法不是同步(synchronized)的,这使得它比StringBuffer更加轻量级和有效。
在JDK5.0之前我们只能通过JOptionPane.showInputDialog进行输入,但在5.0中我们可以通过类Scanner在控制台进行输入操作
例如在1.4中的输入
String input = JOptionPane.showInputDialog(prompt);
int n = Integer.parseInt(input);
double x = Double.parseDouble(input);
s = input;
在5.0中我们可以
Scanner in = new Scanner(System.in);
System.out.print(prompt);
int n = in.nextInt();
double x = in.nextDouble();
String s = in.nextLine();
JDK5之前我们覆盖一个方法时我们无法改变被方法的返回类型,但在JDK5中我们可以改变它
例如1.4中我们只能
public Object clone() { ... }
...
Employee cloned = (Employee) e.clone();
但是在5.0中我们可以改变返回类型为Employee
public Employee clone() { ... }
...
Employee cloned = e.clone();
增加了类似C的格式化输入输出,简单的例子:
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);
}
}
输出结果为:
150000 10
249f0 a
5.01 3.1
5.010e+00 3.140e+02
下面是一些格式化参数说明(摘自Core Java 2 Volume I - Fundamentals, Seventh Edition)
Table 3-5. Conversions for printf
<table bgColor=#efefef border=0 cellPadding=0 cellSpacing=1 width=545>
Conversion Character
</td> <td bgColor=#ffffff width=324>Type
</td> <td bgColor=#ffffff width=175>Example
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>d
</td> <td bgColor=#ffffff width=324>Decimal integer
</td> <td bgColor=#ffffff width=175>159
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>x
</td> <td bgColor=#ffffff width=324>Hexadecimal integer
</td> <td bgColor=#ffffff width=175>9f
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>o
</td> <td bgColor=#ffffff width=324>Octal integer
</td> <td bgColor=#ffffff width=175>237
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>f
</td> <td bgColor=#ffffff width=324>Fixed-point floating-point
</td> <td bgColor=#ffffff width=175>15.9
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>e
</td> <td bgColor=#ffffff width=324>Exponential floating-point
</td> <td bgColor=#ffffff width=175>1.59E+01
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>g
</td> <td bgColor=#ffffff width=324>General floating-point (the shorter of e and f)
</td> <td bgColor=#ffffff width=175> </td></tr> <tr> <td bgColor=#ffffff height=22 width=85>a
</td> <td bgColor=#ffffff width=324>Hexadecimal floating point
</td> <td bgColor=#ffffff width=175>0x1.fccdp3
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>s
</td> <td bgColor=#ffffff width=324>String
</td> <td bgColor=#ffffff width=175>Hello
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>c
</td> <td bgColor=#ffffff width=324>Character
</td> <td bgColor=#ffffff width=175>H
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>b
</td> <td bgColor=#ffffff width=324>Boolean
</td> <td bgColor=#ffffff width=175>TRUE
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>h
</td> <td bgColor=#ffffff width=324>Hash code
</td> <td bgColor=#ffffff width=175>42628b2
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>tx
</td> <td bgColor=#ffffff width=324>Date and time
</td> <td bgColor=#ffffff width=175>See Table 3-7
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>%
</td> <td bgColor=#ffffff width=324>The percent symbol
</td> <td bgColor=#ffffff width=175>%
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>n
</td> <td bgColor=#ffffff width=324>The platform-dependent line separator
</td> <td bgColor=#ffffff width=175> </td></tr></table>Table 3-7. Date and Time Conversion Characters
<table bgColor=#efefef border=0 cellPadding=0 cellSpacing=1 width=545>
<tr> <td bgColor=#ffffff height=22 width=85>Conversion Character
</td> <td bgColor=#ffffff width=304>Type
</td> <td bgColor=#ffffff width=195>Example
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>C
</td> <td bgColor=#ffffff width=304>Complete date and time
</td> <td bgColor=#ffffff width=195>Mon Feb 09 18:05:19 PST 2004
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>F
</td> <td bgColor=#ffffff width=304>ISO 8601 date
</td> <td bgColor=#ffffff width=195>2004-02-09
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>D
</td> <td bgColor=#ffffff width=304>U.S. formatted date (month/day/year)
</td> <td bgColor=#ffffff width=195>02/09/2004
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>T
</td> <td bgColor=#ffffff width=304>24-hour time
</td> <td bgColor=#ffffff width=195>18:05:19
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>r
</td> <td bgColor=#ffffff width=304>12-hour time
</td> <td bgColor=#ffffff width=195>06:05:19 pm
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>R
</td> <td bgColor=#ffffff width=304>24-hour time, no seconds
</td> <td bgColor=#ffffff width=195>18:05
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>Y
</td> <td bgColor=#ffffff width=304>Four-digit year (with leading zeroes)
</td> <td bgColor=#ffffff width=195>2004
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>y
</td> <td bgColor=#ffffff width=304>Last two digits of the year (with leading zeroes)
</td> <td bgColor=#ffffff width=195>04
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>C
</td> <td bgColor=#ffffff width=304>First two digits of the year (with leading zeroes)
</td> <td bgColor=#ffffff width=195>20
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>B
</td> <td bgColor=#ffffff width=304>Full month name
</td> <td bgColor=#ffffff width=195>February
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>b or h
</td> <td bgColor=#ffffff width=304>Abbreviated month name
</td> <td bgColor=#ffffff width=195>Feb
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>m
</td> <td bgColor=#ffffff width=304>Two-digit month (with leading zeroes)
</td> <td bgColor=#ffffff width=195>02
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>d
</td> <td bgColor=#ffffff width=304>Two-digit day (with leading zeroes)
</td> <td bgColor=#ffffff width=195>09
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>e
</td> <td bgColor=#ffffff width=304>Two-digit day (without leading zeroes)
</td> <td bgColor=#ffffff width=195>9
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>A
</td> <td bgColor=#ffffff width=304>Full weekday name
</td> <td bgColor=#ffffff width=195>Monday
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>a
</td> <td bgColor=#ffffff width=304>Abbreviated weekday name
</td> <td bgColor=#ffffff width=195>Mon
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>j
</td> <td bgColor=#ffffff width=304>Three-digit day of year (with leading zeroes), between 001 and 366
</td> <td bgColor=#ffffff width=195>069
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>H
</td> <td bgColor=#ffffff width=304>Two-digit hour (with leading zeroes), between 00 and 23
</td> <td bgColor=#ffffff width=195>18
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>k
</td> <td bgColor=#ffffff width=304>Two-digit hour (without leading zeroes), between 0 and 23
</td> <td bgColor=#ffffff width=195>18
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>I
</td> <td bgColor=#ffffff width=304>Two-digit hour (with leading zeroes), between 01 and 12
</td> <td bgColor=#ffffff width=195>06
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>l
</td> <td bgColor=#ffffff width=304>Two-digit hour (without leading zeroes), between 1 and 12
</td> <td bgColor=#ffffff width=195>6
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>M
</td> <td bgColor=#ffffff width=304>Two-digit minutes (with leading zeroes)
</td> <td bgColor=#ffffff width=195>05
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>S
</td> <td bgColor=#ffffff width=304>Two-digit seconds (with leading zeroes)
</td> <td bgColor=#ffffff width=195>19
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>L
</td> <td bgColor=#ffffff width=304>Three-digit milliseconds (with leading zeroes)
</td> <td bgColor=#ffffff width=195>047
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>N
</td> <td bgColor=#ffffff width=304>Nine-digit nanoseconds (with leading zeroes)
</td> <td bgColor=#ffffff width=195>047000000
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>P
</td> <td bgColor=#ffffff width=304>Uppercase morning or afternoon marker
</td> <td bgColor=#ffffff width=195>PM
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>p
</td> <td bgColor=#ffffff width=304>Lowercase morning or afternoon marker
</td> <td bgColor=#ffffff width=195>pm
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>z
</td> <td bgColor=#ffffff width=304>RFC 822 numeric offset from GMT
</td> <td bgColor=#ffffff width=195>-0800
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>Z
</td> <td bgColor=#ffffff width=304>Time zone
</td> <td bgColor=#ffffff width=195>PST
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>s
</td> <td bgColor=#ffffff width=304>Seconds since 1970-01-01 00:00:00 GMT
</td> <td bgColor=#ffffff width=195>1078884319
</td></tr> <tr> <td bgColor=#ffffff height=22 width=85>E
</td> <td bgColor=#ffffff width=304>Milliseconds since 1970-01-01 00:00:00 GMT
</td> <td bgColor=#ffffff width=195>1078884319047
</td></tr></table>Table 3-6. Flags for printf
<table bgColor=#efefef border=0 cellPadding=0 cellSpacing=1 width=545>
<tr> <td bgColor=#ffffff height=22 width=157>Flag
</td> <td bgColor=#ffffff width=232>Purpose
</td> <td bgColor=#ffffff width=195>Example
</td></tr> <tr> <td bgColor=#ffffff height=22 width=157>+
</td> <td bgColor=#ffffff width=232>Prints sign for positive and negative numbers
</td> <td bgColor=#ffffff width=195>+3333.33
</td></tr> <tr> <td bgColor=#ffffff height=22 width=157>space
</td> <td bgColor=#ffffff width=232>Adds a space before positive numbers
</td> <td bgColor=#ffffff width=195>| 3333.33|
</td></tr> <tr> <td bgColor=#ffffff height=22 width=157>0
</td> <td bgColor=#ffffff width=232>Adds leading zeroes
</td> <td bgColor=#ffffff width=195>003333.33
</td></tr> <tr> <td bgColor=#ffffff height=22 width=157>-
</td> <td bgColor=#ffffff width=232>Left-justifies field
</td> <td bgColor=#ffffff width=195>|3333.33 |
</td></tr> <tr> <td bgColor=#ffffff height=22 width=157>(
</td> <td bgColor=#ffffff width=232>Encloses negative number in parentheses
</td> <td bgColor=#ffffff width=195>(3333.33)
</td></tr> <tr> <td bgColor=#ffffff height=22 width=157>,
</td> <td bgColor=#ffffff width=232>Adds group separators
</td> <td bgColor=#ffffff width=195>3,333.33
</td></tr> <tr> <td bgColor=#ffffff height=22 width=157># (for f format)
</td> <td bgColor=#ffffff width=232>Always includes a decimal point
</td> <td bgColor=#ffffff width=195>3,333.
</td></tr> <tr> <td bgColor=#ffffff height=22 width=157># (for x or o format)
</td> <td bgColor=#ffffff width=232>Adds 0x or 0 prefix
</td> <td bgColor=#ffffff width=195>0xcafe
</td></tr> <tr> <td bgColor=#ffffff height=22 width=157>^
</td> <td bgColor=#ffffff width=232>Converts to upper case
</td> <td bgColor=#ffffff width=195>0XCAFE
</td></tr> <tr> <td bgColor=#ffffff height=22 width=157>$
</td> <td bgColor=#ffffff width=232>Specifies the index of the argument to be formatted; for example, %1$d %1$x prints the first argument in decimal and hexadecimal
</td> <td bgColor=#ffffff width=195>159 9F
</td></tr> <tr> <td bgColor=#ffffff height=22 width=157><
</td> <td bgColor=#ffffff width=232>Formats the same value as the previous specification; for example, %d %
这里是一些简单的介绍,更详细的说明请参考:
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,中文名称就是《Java核心技术》。只有第七版才有J2SE5.0的介绍,但是第七版好像还没有中文版。本文还参考了Professional Java JDK - 5th Edition.
</td> </tr> </table>