java System.arraycopy 复制数组

骑猪看日落 2021-09-26 16:52 297阅读 0赞

### 简介 ###

System.arraycopy，对数组进行复制的方法。使用数组的封装类的底层实现基本都会出现他的身影。比如Arrays，ArrayList。

### 源码 ###

* @param src the source array.
 * @param srcPos starting position in the source array.
 * @param dest the destination array.
 * @param destPos starting position in the destination data.
 * @param length the number of array elements to be copied.
 * @exception IndexOutOfBoundsException if copying would cause
 * access of data outside array bounds.
 * @exception ArrayStoreException if an element in the <code>src</code>
 * array could not be stored into the <code>dest</code> array
 * because of a type mismatch.
 * @exception NullPointerException if either <code>src</code> or
 * <code>dest</code> is <code>null</code>.
 */
 public static native void arraycopy(Object src, int srcPos,
 Object dest, int destPos,
 int length);

这是一个native方法。

src：source；源数组。  
srcPos：source，position；从源数组的哪个下标处开始复制。  
dest：destination；目标数组。  
destPos：destination，position；复制到目标数组的下标处。  
length：length；复制的长度。

### 使用示例 ###

int[] src = {
       1, 2, 3, 4, 5, 6}; //源数组
            System.out.println(Arrays.toString(src));
            int[] dest = new int[6];    //目标数组
            System.arraycopy(src, 1, dest, 0, 2);   //源数组下标1处开始，向后复制2个数至目标数组下标0处
            System.out.println(Arrays.toString(dest));
            System.arraycopy(src, 1, src, 0, 2);    //下标1处开始，向后复制2个数至本数组下标0处
            System.out.println(Arrays.toString(src));

输出

[1, 2, 3, 4, 5, 6]
    [2, 3, 0, 0, 0, 0]
    [2, 3, 3, 4, 5, 6]

### 是否为深复制 ###

import java.util.Arrays;
    
    /** * * @author nayi224 * https://blog.csdn.net/nayi_224 * 2018-5-12下午09:12:19 */
    public class Test { 
    
        /** * @param args */
        public static void main(String[] args) {
            // TODO Auto-generated method stub
            Bean[] src = {
       new Bean("1")};
            Bean[] dest = new Bean[1];
            System.arraycopy(src, 0, dest, 0, 1);
            System.out.println(src[0].a);
            System.out.println(dest[0].a);
            dest[0].a = "2";
            System.out.println(src[0].a);
            System.out.println(dest[0].a);
        }
    }
    
    class Bean{
        public String a;
        public Bean(String a){
            this.a = a;
        }
    }

输出

1
    1
    2
    2

并不是。用之前想好自己在干什么。

### 效率 ###

非常高，主流JVM中有专门的优化。

### 完整注释（1.6） ###

/** * Copies an array from the specified source array, beginning at the * specified position, to the specified position of the destination array. * A subsequence of array components are copied from the source * array referenced by <code>src</code> to the destination array * referenced by <code>dest</code>. The number of components copied is * equal to the <code>length</code> argument. The components at * positions <code>srcPos</code> through * <code>srcPos+length-1</code> in the source array are copied into * positions <code>destPos</code> through * <code>destPos+length-1</code>, respectively, of the destination * array. * * If the <code>src</code> and <code>dest</code> arguments refer to the * same array object, then the copying is performed as if the * components at positions <code>srcPos</code> through * <code>srcPos+length-1</code> were first copied to a temporary * array with <code>length</code> components and then the contents of * the temporary array were copied into positions * <code>destPos</code> through <code>destPos+length-1</code> of the * destination array. * * If <code>dest</code> is <code>null</code>, then a * <code>NullPointerException</code> is thrown. * * If <code>src</code> is <code>null</code>, then a * <code>NullPointerException</code> is thrown and the destination * array is not modified. * * Otherwise, if any of the following is true, an * <code>ArrayStoreException</code> is thrown and the destination is * not modified: * <ul> * <li>The <code>src</code> argument refers to an object that is not an * array. * <li>The <code>dest</code> argument refers to an object that is not an * array. * <li>The <code>src</code> argument and <code>dest</code> argument refer * to arrays whose component types are different primitive types. * <li>The <code>src</code> argument refers to an array with a primitive * component type and the <code>dest</code> argument refers to an array * with a reference component type. * <li>The <code>src</code> argument refers to an array with a reference * component type and the <code>dest</code> argument refers to an array * with a primitive component type. * </ul> * * Otherwise, if any of the following is true, an * <code>IndexOutOfBoundsException</code> is * thrown and the destination is not modified: * <ul> * <li>The <code>srcPos</code> argument is negative. * <li>The <code>destPos</code> argument is negative. * <li>The <code>length</code> argument is negative. * <li><code>srcPos+length</code> is greater than * <code>src.length</code>, the length of the source array. * <li><code>destPos+length</code> is greater than * <code>dest.length</code>, the length of the destination array. * </ul> * * Otherwise, if any actual component of the source array from * position <code>srcPos</code> through * <code>srcPos+length-1</code> cannot be converted to the component * type of the destination array by assignment conversion, an * <code>ArrayStoreException</code> is thrown. In this case, let * k be the smallest nonnegative integer less than * length such that <code>src[srcPos+</code>k<code>]</code> * cannot be converted to the component type of the destination * array; when the exception is thrown, source array components from * positions <code>srcPos</code> through * <code>srcPos+</code>k<code>-1</code> * will already have been copied to destination array positions * <code>destPos</code> through * <code>destPos+</code>k<code>-1</code> and no other * positions of the destination array will have been modified. * (Because of the restrictions already itemized, this * paragraph effectively applies only to the situation where both * arrays have component types that are reference types.) * * @param src the source array. * @param srcPos starting position in the source array. * @param dest the destination array. * @param destPos starting position in the destination data. * @param length the number of array elements to be copied. * @exception IndexOutOfBoundsException if copying would cause * access of data outside array bounds. * @exception ArrayStoreException if an element in the <code>src</code> * array could not be stored into the <code>dest</code> array * because of a type mismatch. * @exception NullPointerException if either <code>src</code> or * <code>dest</code> is <code>null</code>. */
 public static native void arraycopy(Object src, int srcPos,
 Object dest, int destPos,
 int length);