System.arraycopy_牛客博客

源码

 /** * 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);

方法理解

arraycopy 方法是static修饰的我们可以直接通过类名去调用此方法
其次还发现方法中有native 关键字，说明这个方法的实现不是由java实现的，而是有本地方法栈中C或C++实现。

参数

Object src 数据源数组，
int srcPos 从元数据的起始位置开始
Object dest 目标数组
int destPos 目标数组的开始起始位置
int length 要copy的数组的长度

案例

    public static void main(String[] args){
   
        // 数据源数组
        int[] a1 = {
   1, 2, 3, 4};

        // 目标数组
        int[] a2 = new int[3];

        /** * 从目标数组的a1的0开始拷贝 * 到目标数组a2的0位置 * 拷贝2个 * */
        System.arraycopy(a1, 0, a2, 0, 2);

        // 结果1 2 0 (会发现最后一个为0 那是因为a2初始化长度为3 默认值都是0 我们只从a1中拷贝了2个数
        for (int i : a2) {
   
            System.out.println(i); 
        }
    }