System.arraycopy in java

背景

System.arraycopy 是System包下面的函数，主要是从一个数组复制元素到另外一个数组

为什么要介绍一下这个函数呢？
因为lucene经常会用到这个函数复制内容

注释

public static void arraycopy(Object src,
                             int srcPos,
                             Object dest,
                             int destPos,
                             int length)

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 src to the destination array referenced by dest. The number of components copied is equal to the length argument. The components at positions srcPos through srcPos+length-1 in the source array are copied into positions destPos through destPos+length-1, respectively, of the destination array.

If the src and dest arguments refer to the same array object, then the copying is performed as if the components at positions srcPos through srcPos+length-1 were first copied to a temporary array with length components and then the contents of the temporary array were copied into positions destPos through destPos+length-1 of the destination array.

If dest is null, then a NullPointerException is thrown.

If src is null, then a NullPointerException is thrown and the destination array is not modified.

Otherwise, if any of the following is true, an ArrayStoreException is thrown and the destination is not modified:

    The src argument refers to an object that is not an array.
    The dest argument refers to an object that is not an array.
    The src argument and dest argument refer to arrays whose component types are different primitive types.
    The src argument refers to an array with a primitive component type and the dest argument refers to an array with a reference component type.
    The src argument refers to an array with a reference component type and the dest argument refers to an array with a primitive component type. 

Otherwise, if any of the following is true, an IndexOutOfBoundsException is thrown and the destination is not modified:

    The srcPos argument is negative.
    The destPos argument is negative.
    The length argument is negative.
    srcPos+length is greater than src.length, the length of the source array.
    destPos+length is greater than dest.length, the length of the destination array. 

Otherwise, if any actual component of the source array from position srcPos through srcPos+length-1 cannot be converted to the component type of the destination array by assignment conversion, an ArrayStoreException is thrown. In this case, let k be the smallest nonnegative integer less than length such that src[srcPos+k] cannot be converted to the component type of the destination array; when the exception is thrown, source array components from positions srcPos through srcPos+k-1 will already have been copied to destination array positions destPos through destPos+k-1 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.)

Parameters:
    src - the source array.
    srcPos - starting position in the source array.
    dest - the destination array.
    destPos - starting position in the destination data.
    length - the number of array elements to be copied.
Throws:
    IndexOutOfBoundsException - if copying would cause access of data outside array bounds.
    ArrayStoreException - if an element in the src array could not be stored into the dest array because of a type mismatch.
    NullPointerException - if either src or dest is null.