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JDK5.0的11个主要新特征(4)

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7 元数据(Meta data) 8 Building Strings(StringBuilder类) 9 控制台输入(Console Input)

作者：中国IT实验室来源：中国IT实验室 2007年8月21日

关键字：

7 元数据(Meta data)

请参考

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类)

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。

9 控制台输入(Console Input)

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();

10 Covariant Return Types(不晓得怎么翻译，大概是改变返回类型)

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();

11 格式化I/O(Formatted I/O)

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

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

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.

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