? ? ? ? List集合轉(zhuǎn)換成集合，List類本身提供了兩個(gè)api:

Object[] toArray();
<T> T[] toArray(T[] a);

????????一個(gè)是把集合轉(zhuǎn)換成元素?cái)?shù)據(jù)類型為Object的數(shù)組；另外一個(gè)則是一個(gè)泛型函數(shù)。其中泛型函數(shù)這個(gè)api是比較常用的，因?yàn)樗D(zhuǎn)換后得到的數(shù)組的元素類型仍然是列表中的數(shù)據(jù)元素類型，而不是Object類型。

? ? ? ? 在實(shí)際使用中，它的用法如下:

List<String> list = new ArrayList<>();
list.add("xxx1");
list.add("xxx2");
list.add("xxx3");
String[] strings = list.toArray(new String[0]);
for (String str : strings) {
    out.println(str);
}

????????注意到toArray()函數(shù)傳入的實(shí)參為new String[0]。代表一個(gè)數(shù)組長(zhǎng)度為0的字符串?dāng)?shù)組。在java中允許數(shù)組長(zhǎng)度為0。這一點(diǎn)在編寫一個(gè)結(jié)果為數(shù)組的方法時(shí)，如果碰巧結(jié)果為空時(shí)，new String[0]這種寫法就非常有用了。 在這里我們主要是利用了字符串?dāng)?shù)組的數(shù)據(jù)類型信息，并沒(méi)有實(shí)際地開(kāi)辟一塊內(nèi)存，這樣就不會(huì)浪費(fèi)內(nèi)存了。 那么list.toArray(new String[0])是如何做到傳入一個(gè)長(zhǎng)度為0 的數(shù)組，返回的卻是list集合的數(shù)組形式呢？我們來(lái)看這個(gè)函數(shù)的實(shí)現(xiàn)：

/**
 * Returns an array containing all of the elements in this list in
 * proper sequence (from first to last element); the runtime type of
 * the returned array is that of the specified array.  If the list fits
 * in the specified array, it is returned therein.  Otherwise, a new
 * array is allocated with the runtime type of the specified array and
 * the size of this list.
 *
 * <p>If the list fits in the specified array with room to spare (i.e.,
 * the array has more elements than the list), the element in the array
 * immediately following the end of the list is set to <tt>null</tt>.
 * (This is useful in determining the length of the list <i>only</i> if
 * the caller knows that the list does not contain any null elements.)
 *
 * <p>Like the {@link #toArray()} method, this method acts as bridge between
 * array-based and collection-based APIs.  Further, this method allows
 * precise control over the runtime type of the output array, and may,
 * under certain circumstances, be used to save allocation costs.
 *
 * <p>Suppose <tt>x</tt> is a list known to contain only strings.
 * The following code can be used to dump the list into a newly
 * allocated array of <tt>String</tt>:
 *
 * <pre>{@code
 *     String[] y = x.toArray(new String[0]);
 * }</pre>
 *
 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
 * <tt>toArray()</tt>.
 *
 * @param a the array into which the elements of this list are to
 *          be stored, if it is big enough; otherwise, a new array of the
 *          same runtime type is allocated for this purpose.
 * @return an array containing the elements of this list
 * @throws ArrayStoreException if the runtime type of the specified array
 *         is not a supertype of the runtime type of every element in
 *         this list
 * @throws NullPointerException if the specified array is null
 */
<T> T[] toArray(T[] a);

我們來(lái)看其在ArrayList上的具體實(shí)現(xiàn)：

public <T> T[] toArray(T[] a) {
    if (a.length < size)
        // Make a new array of a's runtime type, but my contents:
        return (T[]) Arrays.copyOf(elementData, size, a.getClass());
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)
        a[size] = null;
    return a;
}

? ? ? ? 首先比較形參數(shù)組a的數(shù)組長(zhǎng)度和列表的長(zhǎng)度，如果a.length<size，則調(diào)用Arrays.copyOf(elementData, size, a.getClass())，其內(nèi)部實(shí)現(xiàn)為：

public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
    @SuppressWarnings("unchecked")
    T[] copy = ((Object)newType == (Object)Object[].class)
        ? (T[]) new Object[newLength]
        : (T[]) Array.newInstance(newType.getComponentType(), newLength);
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

????????可以看到其內(nèi)部會(huì)開(kāi)辟出一塊和列表長(zhǎng)度一樣的數(shù)組空間大小，并且數(shù)組元素類型和傳進(jìn)來(lái)的元素類型一致。調(diào)用System.arraycopy()函數(shù)進(jìn)行數(shù)組拷貝。

????????如果a.length>size,則不需要單獨(dú)開(kāi)辟空間了，因?yàn)樾螀⒌目臻g足夠大了。然后也是調(diào)用System.arraycopy()函數(shù)進(jìn)行數(shù)組拷貝。最后把形參數(shù)組a中剩余空間的值全部賦值為null。

? ? ? ? 所以終于解開(kāi)了List的toArray()函數(shù)的面紗了。文章來(lái)源地址http://www.zghlxwxcb.cn/news/detail-692532.html

到了這里，關(guān)于List集合轉(zhuǎn)換成數(shù)組list.toArray的文章就介紹完了。如果您還想了解更多內(nèi)容，請(qǐng)?jiān)谟疑辖撬阉鱐OY模板網(wǎng)以前的文章或繼續(xù)瀏覽下面的相關(guān)文章，希望大家以后多多支持TOY模板網(wǎng)！