執行下面代碼時報錯:

List centerList =WebConstants.SUPPORT_BIG_CENTERS_LIST; // WebConstants.SUPPORT_BIG_CENTERS_LIST是Arrays.asList轉化來的

centerList.removeAll(WebConstants.SUPPORT_CENTERS_LIST);

centerList.addAll(WebConstants.SUPPORT_CENTERS_LIST);

最后一行報錯：

java.lang.UnsupportedOperationException: nullat java.util.AbstractList.add(AbstractList.java:148) ~[na:1.8.0_101]

at java.util.AbstractList.add(AbstractList.java:108) ~[na:1.8.0_101]

at java.util.AbstractCollection.addAll(AbstractCollection.java:344) ~[na:1.8.0_101]

查看Arrays$ArrayList的源代碼，如下(1.8版本)：

private static class ArrayList extends AbstractList

implementsRandomAccess, java.io.Serializable

{private static final long serialVersionUID = -2764017481108945198L;private finalE[] a;

ArrayList(E[] array) {

a=Objects.requireNonNull(array);

}

@Overridepublic intsize() {returna.length;

}

@OverridepublicObject[] toArray() {returna.clone();

}

@Override

@SuppressWarnings("unchecked")public T[] toArray(T[] a) {int size =size();if (a.length

(Class extends T[]>) a.getClass());

System.arraycopy(this.a, 0, a, 0, size);if (a.length >size)

a[size]= null;returna;

}

@Overridepublic E get(intindex) {returna[index];

}

@Overridepublic E set(intindex, E element) {

E oldValue=a[index];

a[index]=element;returnoldValue;

}

@Overridepublic intindexOf(Object o) {

E[] a= this.a;if (o == null) {for (int i = 0; i < a.length; i++)if (a[i] == null)returni;

}else{for (int i = 0; i < a.length; i++)if(o.equals(a[i]))returni;

}return -1;

}

@Overridepublic booleancontains(Object o) {return indexOf(o) != -1;

}

@Overridepublic Spliteratorspliterator() {returnSpliterators.spliterator(a, Spliterator.ORDERED);

}

@Overridepublic void forEach(Consumer super E>action) {

Objects.requireNonNull(action);for(E e : a) {

action.accept(e);

}

}

@Overridepublic void replaceAll(UnaryOperatoroperator) {

Objects.requireNonNull(operator);

E[] a= this.a;for (int i = 0; i < a.length; i++) {

a[i]=operator.apply(a[i]);

}

}

@Overridepublic void sort(Comparator super E>c) {

Arrays.sort(a, c);

}

}

進入父類找下addAll方法:

public boolean addAll(int index, Collection extends E>c) {

rangeCheckForAdd(index);boolean modified = false;for(E e : c) {

add(index++, e);

modified= true;

}returnmodified;

}

實現只存在于父類中，如下所示：

public void add(intindex, E element) {throw newUnsupportedOperationException();

}

父類AbstractList add方法直接拋出異常。

所以問題就在這里，我們改下代碼，如下就不報錯了：

List centerList = new ArrayList<>();

if (null != WebConstants.SUPPORT_BIG_CENTERS_LIST) {? //addAll的目標是null會報錯

centerList.addAll(WebConstants.SUPPORT_BIG_CENTERS_LIST);

}

if (null != WebConstants.SUPPORT_CENTERS_LIST) {

centerList.removeAll(WebConstants.SUPPORT_CENTERS_LIST);

centerList.addAll(WebConstants.SUPPORT_CENTERS_LIST);

}

它調用的就是arrayList的addAll方法了

總結

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