Arrays.asList()返回一個List，但是這種情況下，其底層的實現是一個final數組，因此不能調整其尺寸
如下代碼片段：
package chapter11.t1;
import java.util.*;
public class AddingGroups {
public static void main(String[] args) {
List list = Arrays.asList(1,2,3,4,5);
list.add(6);
}
}
通過Arrays.asList()生成list對象，然后再向list中添加元素6，程序報錯如下：Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.AbstractList.add(AbstractList.java:148)
at java.util.AbstractList.add(AbstractList.java:108)
at chapter11.t1.AddingGroups.main(AddingGroups.java:8)
看下Arrays類的源碼如下：
asList()方法返回一個內部類ArrayList的對象
public static List asList(T... a) {
return new ArrayList<>(a);
}
下面看下Arrays類中的ArrayList內部類的源碼：
private static class ArrayList extends AbstractList
implements RandomAccess, java.io.Serializable
{
private static final long serialVersionUID = -2764017481108945198L;
private final E[] a;
ArrayList(E[] array) {
a = Objects.requireNonNull(array);
}
@Override
public int size() {
return a.length;
}
@Override
public Object[] toArray() {
return a.clone();
}
@Override
@SuppressWarnings("unchecked")
public T[] toArray(T[] a) {
int size = size();
if (a.length < size)
return Arrays.copyOf(this.a, size,
(Class extends T[]>) a.getClass());
System.arraycopy(this.a, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
@Override
public E get(int index) {
return a[index];
}
@Override
public E set(int index, E element) {
E oldValue = a[index];
a[index] = element;
return oldValue;
}
@Override
public int indexOf(Object o) {
E[] a = this.a;
if (o == null) {
for (int i = 0; i < a.length; i++)
if (a[i] == null)
return i;
} else {
for (int i = 0; i < a.length; i++)
if (o.equals(a[i]))
return i;
}
return -1;
}
@Override
public boolean contains(Object o) {
return indexOf(o) != -1;
}
@Override
public Spliterator spliterator() {
return Spliterators.spliterator(a, Spliterator.ORDERED);
}
@Override
public void forEach(Consumer super E> action) {
Objects.requireNonNull(action);
for (E e : a) {
action.accept(e);
}
}
@Override
public void replaceAll(UnaryOperator operator) {
Objects.requireNonNull(operator);
E[] a = this.a;
for (int i = 0; i < a.length; i++) {
a[i] = operator.apply(a[i]);
}
}
@Override
public void sort(Comparator super E> c) {
Arrays.sort(a, c);
}
}
該內部類的構造方法如下：
ArrayList(E[] array) {
a = Objects.requireNonNull(array);
}
也就是說Arrays.asList()返回一個final數組，因此不可以改變次數組的大小。當我們試圖用add()或delete()方法在這種列表中添加或刪除元素的時候，會引發數組尺寸的改變。因此在運行時會拋java.lang.UnsupportedOperationException不支持的操作這樣的異常。
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