ArrayList可以通過toArray轉(zhuǎn)換為Array類型砂碉，觀察ArrayList類眯牧，發(fā)現(xiàn)重載的兩個toArray方法
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public Object[] toArray() { return Arrays.copyOf(elementData, size); }
其中transient Object[] elementData;中存儲ArrayList元素
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toArray():不包含參數(shù)，直接調(diào)用Arrays.copyOf,我們進一步探究
public static <T> T[] copyOf(T[] original, int newLength) {
return (T[]) copyOf(original, newLength, original.getClass());
}
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;
}
發(fā)現(xiàn)Arrays.copyOf新建一個指定大小的數(shù)組外恕，并調(diào)用System.arraycopy()對ArrayList中的元素進行復(fù)制
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toArray(T[] a)：傳入一個數(shù)組杆逗，當(dāng)數(shù)組大小小于ArrayList元素個數(shù)中，將會調(diào)用Arrays.copyOf(elementData, size, a.getClass());新建一個數(shù)組來復(fù)制元素鳞疲； 否則將直接調(diào)用System.arraycopy()復(fù)制元素
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;
}
