一句话：把src从第srcPos开始取length个，插入到dest的destPos位置后

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

public void test(){
        int[] arr1 = {1,2,3};
        int[] arr2 = new int[5+arr1.length];

        int index = 0;
        arr2[index++] = 4;
        arr2[index++] = 5;
        arr2[index++] = 6;
        arr2[index++] = 7;
        arr2[index++] = 8;
        // arr1从0开始取arr1.length个，插入到arr2的index位置后
        System.arraycopy(arr1,0, arr2, index, arr1.length);

        for (int i = 0; i < arr2.length; i++) {
            System.out.println(arr2[i]);
        }
    }