站内搜索: 请输入搜索关键词

当前页面: 开发资料首页J2SE 专题J2SE5.0新特性之Foreach

J2SE5.0新特性之Foreach

摘要: J2SE5.0新特性之Foreach

J2SE5.0新特性之Foreach<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

晁岳攀 smallnest@163.com

C#中提供了Foreach的用法:

<table class=MsoTableGrid style="BACKGROUND: #cccccc; BORDER-COLLAPSE: collapse; mso-yfti-tbllook: 480; mso-padding-alt: 0cm 5.4pt 0cm 5.4pt" cellSpacing=0 cellPadding=0 border=0> <tr style="mso-yfti-irow: 0; mso-yfti-firstrow: yes; mso-yfti-lastrow: yes"> <td style="BORDER-RIGHT: #d4d0c8; PADDING-RIGHT: 5.4pt; BORDER-TOP: #d4d0c8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: #d4d0c8; WIDTH: 426.1pt; PADDING-TOP: 0cm; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: transparent" vAlign=top width=568>

foreach (string item in f)

{

Console.WriteLine(item);

}

</td></tr></table>

Java也增加了这样的功能:

<table class=MsoTableGrid style="BACKGROUND: #cccccc; BORDER-COLLAPSE: collapse; mso-yfti-tbllook: 480; mso-padding-alt: 0cm 5.4pt 0cm 5.4pt" cellSpacing=0 cellPadding=0 border=0> <tr style="mso-yfti-irow: 0; mso-yfti-firstrow: yes; mso-yfti-lastrow: yes"> <td style="BORDER-RIGHT: #d4d0c8; PADDING-RIGHT: 5.4pt; BORDER-TOP: #d4d0c8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: #d4d0c8; WIDTH: 426.1pt; PADDING-TOP: 0cm; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: transparent" vAlign=top width=568>

package com.kuaff.jdk5;

import java.util.*;

import java.util.Collection;

public class Foreach

{

private Collection c = null;

private String[] belle = new String[4];

public Foreach()

{

belle[0] = "西施";

belle[1] = "王昭君";

belle[2] = "貂禅";

belle[3] = "杨贵妃";

c = Arrays.asList(belle);

}

public void testCollection()

{

for (String b : c)

{

System.out.println("曾经的风化绝代:" + b);

}

}

public void testArray()

{

for (String b : belle)

{

System.out.println("曾经的青史留名:" + b);

}

}

public static void main(String[] args)

{

Foreach each = new Foreach();

each.testCollection();

each.testArray();

}

}

</td></tr></table>

对于集合类型和数组类型的,我们都可以通过foreach语法来访问它。上面的例子中,以前我们要依次访问数组,挺麻烦:

<table class=MsoTableGrid style="BACKGROUND: #cccccc; BORDER-COLLAPSE: collapse; mso-yfti-tbllook: 480; mso-padding-alt: 0cm 5.4pt 0cm 5.4pt" cellSpacing=0 cellPadding=0 border=0> <tr style="mso-yfti-irow: 0; mso-yfti-firstrow: yes; mso-yfti-lastrow: yes"> <td style="BORDER-RIGHT: #d4d0c8; PADDING-RIGHT: 5.4pt; BORDER-TOP: #d4d0c8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: #d4d0c8; WIDTH: 426.1pt; PADDING-TOP: 0cm; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: transparent" vAlign=top width=568>

for (int i = 0; i < belle.length; i++)

{

String b = belle[i];

System.out.println("曾经的风化绝代:" + b);

}

</td></tr></table>

现在只需下面简单的语句即可:

<table class=MsoTableGrid style="BACKGROUND: #cccccc; BORDER-COLLAPSE: collapse; mso-yfti-tbllook: 480; mso-padding-alt: 0cm 5.4pt 0cm 5.4pt" cellSpacing=0 cellPadding=0 border=0> <tr style="mso-yfti-irow: 0; mso-yfti-firstrow: yes; mso-yfti-lastrow: yes"> <td style="BORDER-RIGHT: #d4d0c8; PADDING-RIGHT: 5.4pt; BORDER-TOP: #d4d0c8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: #d4d0c8; WIDTH: 426.1pt; PADDING-TOP: 0cm; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: transparent" vAlign=top width=568>

for (String b : belle)

{

System.out.println("曾经的青史留名:" + b);

}

</td></tr></table>

对集合的访问效果更明显。以前我们访问集合的代码:

<table class=MsoTableGrid style="BACKGROUND: #cccccc; BORDER-COLLAPSE: collapse; mso-yfti-tbllook: 480; mso-padding-alt: 0cm 5.4pt 0cm 5.4pt" cellSpacing=0 cellPadding=0 border=0> <tr style="mso-yfti-irow: 0; mso-yfti-firstrow: yes; mso-yfti-lastrow: yes"> <td style="BORDER-RIGHT: #d4d0c8; PADDING-RIGHT: 5.4pt; BORDER-TOP: #d4d0c8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: #d4d0c8; WIDTH: 426.1pt; PADDING-TOP: 0cm; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: transparent" vAlign=top width=568>

for (Iterator it = c.iterator(); it.hasNext();)

{

String name = (String) it.next();

System.out.println("曾经的风化绝代:" + name);

}

</td></tr></table>

现在我们只需下面的语句:

<table class=MsoTableGrid style="BACKGROUND: #cccccc; BORDER-COLLAPSE: collapse; mso-yfti-tbllook: 480; mso-padding-alt: 0cm 5.4pt 0cm 5.4pt" cellSpacing=0 cellPadding=0 border=0> <tr style="mso-yfti-irow: 0; mso-yfti-firstrow: yes; mso-yfti-lastrow: yes"> <td style="BORDER-RIGHT: #d4d0c8; PADDING-RIGHT: 5.4pt; BORDER-TOP: #d4d0c8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: #d4d0c8; WIDTH: 426.1pt; PADDING-TOP: 0cm; BORDER-BOTTOM: #d4d0c8; BACKGROUND-COLOR: transparent" vAlign=top width=568>

for (String b : c)

{

System.out.println("曾经的风化绝代:" + b);

}

</td></tr></table>

Foreach也不是万能的,它也有以下的缺点:

在以前的代码中,我们可以通过Iterator执行remove操作。

for (Iterator it = c.iterator(); it.hasNext();)

{

itremove()

}

但是,在现在的foreach版中,我们无法删除集合包含的对象。你也不能替换对象。

同时,你也不能并行的foreach多个集合。所以,在我们编写代码时,还得看情况而使用它。



↑返回目录
前一篇: J2SE5.0新特性之范型编程
后一篇: J2SE5.0新特性之自动装箱解箱