java数组复制的方式和效率比较

java中，数组的复制有以下三种方式：

1. 调用System.arraycopy（Arrays.copyOfRange可以当作第四种，但是底层调用的是System.arraycopy，所以，认为是同一种，下面是Arrays.copyOfRange的方法实现）

 

 public static <T,U> T[] copyOfRange(U[] original, int from, int to, Class<? extends T[]> newType) {

        int newLength = to - from;

        if (newLength < 0)

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

        T[] copy = ((Object)newType == (Object)Object[].class)

            ? (T[]) new Object[newLength]

            : (T[]) Array.newInstance(newType.getComponentType(), newLength);

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

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

        return copy;

    }

 

2. 调用clone方法

3.利用for循环自己实现

效率比较上，通过下面程序可以看出

 

/**

 * 

 */

package collection;



import java.util.Arrays;

import java.util.Date;



/**

 * @author yfei

 * Date:2013-8-26

 */

public class Array {

	public static final int size = 1000000;



	public static void copyByArrayCopy(String[] strArray){

		Long startTime = System.currentTimeMillis();

		String[] destArray = new String[size];

		System.arraycopy(strArray,0,destArray,0,strArray.length);

		//printArr(destArray);

		Long endTime = System.currentTimeMillis();

		System.out.println("copyByArrayCopy cost time is "+(endTime-startTime));

	}

	

	public static void copyByLoop(String[] strArray){

		Long startTime = System.currentTimeMillis();

		String[] destArray = new String[size];

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

			destArray[i] = strArray[i];

		}

		//printArr(destArray);

		Long endTime = System.currentTimeMillis();

		System.out.println("copyByLoop cost time is "+(endTime-startTime));

	}

	

	public static void copyByClone(String[] strArray){

		Long startTime = System.currentTimeMillis();

		String[] destArray = strArray.clone();

		Long endTime = System.currentTimeMillis();

		System.out.println("copyByClone cost time is "+(endTime-startTime));

	}



	public static void main(String args[]){

		String arr1[] = new String[size];

		for(int i=0;i<size;i++){

			arr1[i] = "this is a test"+i;

			//arr1[i] = "shishangzhiyoumamahaoyouamdehaizixiangkuaibaotoujinmamadehuaibaoxingfuxiangbuliaoshishangzhiyoumamahaomeimadehaizixianggencaolikaimamadehuaibaoxingfunalizhaoshishangzhiyoumamahaomeimadehaizibuzhidaoyaoshitazhidaomengliyehuixiao"+i;



		}

		String arr2[] = new String[size];

		for(int i=0;i<size;i++){

			arr2[i] = "this is a test"+i;

			//arr2[i] = "shishangzhiyoumamahaoyouamdehaizixiangkuaibaotoujinmamadehuaibaoxingfuxiangbuliaoshishangzhiyoumamahaomeimadehaizixianggencaolikaimamadehuaibaoxingfunalizhaoshishangzhiyoumamahaomeimadehaizibuzhidaoyaoshitazhidaomengliyehuixiao"+i;



		}

		String arr3[] = new String[size];

		for(int i=0;i<size;i++){

			arr3[i] = "this is a test"+i;

			//arr3[i] = "shishangzhiyoumamahaoyouamdehaizixiangkuaibaotoujinmamadehuaibaoxingfuxiangbuliaoshishangzhiyoumamahaomeimadehaizixianggencaolikaimamadehuaibaoxingfunalizhaoshishangzhiyoumamahaomeimadehaizibuzhidaoyaoshitazhidaomengliyehuixiao"+i;



		}

		copyByClone(arr1);

		copyByLoop(arr2);

		copyByArrayCopy(arr3);

		

	}

	public static void printArr(String[] strArray){

		for(String str:strArray){

			System.out.println(str);

		}

	}

}

 

结果(ms单位)：

copyByClone cost time is 2
copyByLoop cost time is 352
copyByArrayCopy cost time is 3

由此可以看出，clone和arraycopy的效率基本是一致的，而通过循环copy的效率最慢。

看了一下源代码：

clone：

 

 protected native Object clone() throws CloneNotSupportedException;

arraycopy

 

 

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

                                        Object dest, int destPos,

                                        int length);

两种方式都是用native  method。

 

简单地讲，一个Native Method就是一个java调用非java代码的接口。一个Native Method是这样一个java的方法：该方法的实现由非java语言实现，比如C。这个特征并非java所特有，很多其它的编程语言都有这一机制，比如在C＋＋中，你可以用extern "C"告知C＋＋编译器去调用一个C的函数。

java的性能是不能言说的伤啊。。。