Collections.emptyList()引发的java.lang.UnsupportedOperationException

最新推荐文章于 2022-09-24 19:15:56 发布

原创最新推荐文章于 2022-09-24 19:15:56 发布 · 4.6k 阅读

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#java #exception #异常 #arraylist

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探讨了使用Collections.emptyList()创建空列表时，addAll方法的行为及其实现原理，并提供了一个避免异常的示例。

缘由:

private static List<Integer> getEmptyList() {
return Collections.emptyList();
}

public static void main(String[] args) {
try {
List<Integer> list1 = new ArrayList<>(Arrays.asList(1,2,3,4,5));
List<Integer> list2 = getEmptyList();
list2.addAll(list1);
System.out.println(list1.toString());
} catch (Exception e) {
e.printStackTrace();
}
}

此方法会抛出以下异常:

java.lang.UnsupportedOperationException
at java.util.AbstractList.add(Unknown Source)
at java.util.AbstractList.add(Unknown Source)
at java.util.AbstractCollection.addAll(Unknown Source)

...

原因在于

ArrayList里面有addAll的实现, 详见源码

public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}

而emptyList里面没有, 详见源码

/**
* @serial include
*/
private static class EmptyList<E>
extends AbstractList<E>
implements RandomAccess, Serializable {
private static final long serialVersionUID = 8842843931221139166L;

public Iterator<E> iterator() {
return emptyIterator();
}
public ListIterator<E> listIterator() {
return emptyListIterator();
}

public int size() {return 0;}
public boolean isEmpty() {return true;}

public boolean contains(Object obj) {return false;}
public boolean containsAll(Collection<?> c) { return c.isEmpty(); }

public Object[] toArray() { return new Object[0]; }

public <T> T[] toArray(T[] a) {
if (a.length > 0)
a[0] = null;
return a;
}

public E get(int index) {
throw new IndexOutOfBoundsException("Index: "+index);
}

public boolean equals(Object o) {
return (o instanceof List) && ((List<?>)o).isEmpty();
}

public int hashCode() { return 1; }

// Preserves singleton property
private Object readResolve() {
return EMPTY_LIST;
}
}

但是, 如果我们这么写就可以避免这个异常

private static List<Integer> getEmptyList() {
return Collections.emptyList();
}

public static void main(String[] args) {
try {
List<Integer> list1 = new ArrayList<>(Arrays.asList(1,2,3,4,5));
List<Integer> list2 = getEmptyList();
list1.addAll(list2);
System.out.println(list1.toString());
} catch (Exception e) {
e.printStackTrace();
}
}