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。。。