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.
* <p>
* 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.
* <p>
* If <code>dest</code> is <code>null</code>, then a
* <code>NullPointerException</code> is thrown.
* <p>
* If <code>src</code> is <code>null</code>, then a
* <code>NullPointerException</code> is thrown and the destination
* array is not modified.
* <p>
* 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>
* <p>
* 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>
* <p>
* 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
* <b><i>k</i></b> be the smallest nonnegative integer less than
* length such that <code>src[srcPos+</code><i>k</i><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><i>k</i><code>-1</code>
* will already have been copied to destination array positions
* <code>destPos</code> through
* <code>destPos+</code><i>k</I><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);