java 提供的System.arrayCopy()方法比自己寫的數組copy要快.

查看其源代碼:

public?static?native?void?arraycopy(Object?src,??int??srcPos,

????????????????????????????????????????Object?dest,?int?destPos,

????????????????????????????????????????int?length);

可以看到被定義為native方法...速度比自己寫的普通方法要快.

?

在jdk1.6中加入了新的數組拷貝方法.Arrays.copyOfRange().

其源代碼:

?public?static?byte[]?copyOfRange(byte[]?original,?int?from,?int?to)?{

????????int?newLength?=?to?-?from;

????????if?(newLength?<?0)

????????????throw?new?IllegalArgumentException(from?+?"?>?"?+?to);

????????byte[]?copy?=?new?byte[newLength];

????????System.arraycopy(original,?from,?copy,?0,

?????????????????????????Math.min(original.length?-?from,?newLength));

????????return?copy;

????}

?

其實就是System.arraycopy..暈死.

?

別為做的測試:

/*******************************************************************************

?*

?*?比較賦值與System.arraycopy誰快

?*

?*?復制的內容越多，System.arraycopy優勢更明顯

?*

?*?Author:?Java

?*

?*?Modified:?2007.09.16

?*

?******************************************************************************/

public?final?class?WhoFaster

{

??public?static?void?main(?String[]?args?)

??{

????/*/

????int?begin=100;?

????int?length=12;?

????String?temp="12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"黑客帝國忍者神龜變形金剛"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890";?

????int?times=10000000;??//千萬

????/*/

????int?begin=100;?

????int?length=120;?

????String?temp="12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"黑客帝國忍者神龜變形金剛"

???????????????+"黑客帝國忍者神龜變形金剛"

???????????????+"黑客帝國忍者神龜變形金剛"

???????????????+"黑客帝國忍者神龜變形金剛"

???????????????+"黑客帝國忍者神龜變形金剛"

???????????????+"黑客帝國忍者神龜變形金剛"

???????????????+"黑客帝國忍者神龜變形金剛"

???????????????+"黑客帝國忍者神龜變形金剛"

???????????????+"黑客帝國忍者神龜變形金剛"

???????????????+"黑客帝國忍者神龜變形金剛"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890"

???????????????+"12345678901234567890";?

????int?times=1000000;??//百萬

????//*/?

????char[]?oldArray=temp.toCharArray();?

????char[]?newArray=null;?

????long?start=0L;?

????

????//

????//?單純賦值

????//

????

????newArray=new?char[length];?

????start=System.currentTimeMillis();?

????for(?int?i=0;?i<times;?i++?)

???????{

?????????for(?int?j=0;?j<length;?j++?)

????????????{

??????????????newArray[j]=oldArray[begin+j];

????????????}

???????}?

????System.out.println(?new?String(?newArray?)+"　"+(?System.currentTimeMillis()-start?)?);?

????

????//

????//?System.arraycopy

????//

????

????newArray=new?char[length];?

????start=System.currentTimeMillis();?

????for(?int?i=0;?i<times;?i++?)

???????{

?????????System.arraycopy(?oldArray,?begin,?newArray,?0,?length?);

???????}?

????System.out.println(?new?String(?newArray?)+"　"+(?System.currentTimeMillis()-start?)?);

??}

}?

?

其結論:

在第一種情況，循環千萬，一個900，一個1000，兩者相差100毫秒

第二種情況就拉大了，循環千萬，一個6700，一個2200，相差4500毫秒

為防止JVM將字符數組作為常量保存在內存中，我分別屏蔽運行，效果一樣。

也就是說，對于很短的字符串復制，單純賦值略快，可以忽略

對于很長的字符串復制，用單純賦值就是臟代碼

總結

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