Java 实现向数组指定位置写入元素

来源：互联网发布：多系统数据共享编辑：程序博客网时间：2024/05/22 04:24

思路

1. 因为数组长度在初始化的时候是指定的并且不可变的，所以不能在原有的数组上直接进行添加操作，需要新建一个长度为当前长度加1的数组

2. 向新数组写数据

/**     * insert the input element into the specified position by copy the input     * array     *      * @param array     * @param position     * @param value     * @return     */    public static String[] insertElementByCopy(String[] array, int position, String value) {        int length = array.length;        if (position < 0 || position > length + 1) {            throw new IndexOutOfBoundsException("the position is out of the array indices");        }        long startTime = System.currentTimeMillis();        String[] newArray = new String[array.length + 1];        int index = position - 1;        if (index == 0) {            // copy 0~end elements to the new array 1~end            System.arraycopy(array, 0, newArray, 1, length);        } else {            // copy 0~index elements to the new array 0~index            System.arraycopy(array, 0, newArray, 0, index);            if (index < length)                // copy index~end elements to the new array index+1~end                System.arraycopy(array, index, newArray, index + 1, length - index);        }        // set the input value to the new array at specified index        newArray[index] = value;        System.out.println("took:" + (System.currentTimeMillis() - startTime) + " ms by copy solution");        return newArray;    }    /**     * insert the input element into the specified position by loop the input     * array     *      * @param array     * @param position     * @param value     * @return     */    public static String[] insertElementByLoop(String[] array, int position, String value) {        if (position < 0 || position > array.length + 1) {            throw new IndexOutOfBoundsException("the position is out of the array indices");        }        long startTime = System.currentTimeMillis();        String[] newArray = new String[array.length + 1];        int index = position - 1;        for (int i = 0; i < newArray.length; i++) {            // it should insert input value when the input position -1 equals i            String newValue = value;            if (i < index) {                // it should insert the original value when the input position                // -1 is greater than i                newValue = array[i];            } else if (i > index) {                // it should insert the previous value when the input position                // -1 is less than i                newValue = array[i - 1];            }            newArray[i] = newValue;        }        System.out.println("took:" + (System.currentTimeMillis() - startTime) + " ms by loop solution");        return newArray;    }

对比:
从时间复杂度来说insertElementByCopy的性能能优于insertElementByLoop，因为insertElementByLoop是0(n)而insertElementByCopy是0(1)。

从空间复杂度来说insertElementByCopy的性能能优于insertElementByLoop，因为insertElementByCopy需要更多次的swap。

下面是测试结果
1. 当原数组长度较少的时候.

List<String> elements = new ArrayList<String>();for (int i = 0; i <90000; i++) {    elements.add(i + "");}String[] array = elements.toArray(new String[elements.size()]);int position = 5000;String value = "T";insertElementByCopy(array, position, value);insertElementByLoop(array, position, value);

---->
took:0 ms by copy solution
took:2 ms by loop solution

2. 当原数组长度较大的时候

List<String> elements = new ArrayList<String>();for (int i = 0; i <90000; i++) {    elements.add(i + "");}String[] array = elements.toArray(new String[elements.size()]);int position = 5000;String value = "T";insertElementByCopy(array, position, value);insertElementByLoop(array, position, value);

---->
took:8 ms by copy solution
took:6 ms by loop solution

从测试结果可以看出来，在执行时间上两种处理方式都差不多。因为他们都需要维护新插入数据到指定的position。

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