java.lang.UnsupportedOperationException

第一次遇到這個異常，有點懵，後來看源碼才知道。

Arrays.asList(Object aobj[])傳回的ArrayList是這樣的。

public class Arrays

{

private static class ArrayList extends AbstractList

implements RandomAccess, Serializable

{

public int size()
    {
        return a.length;
    }

    public Object[] toArray()
    {
        return (Object[])((Object []) (a)).clone();
    }

    public Object[] toArray(Object aobj[])
    {
        int i = size();
        if(aobj.length < i)
            return Arrays.copyOf(a, i, ((Object) (aobj)).getClass());
        System.arraycopy(((Object) (a)), 0, ((Object) (aobj)), 0, i);
        if(aobj.length > i)
            aobj[i] = null;
        return aobj;
    }

    public Object get(int i)
    {
        return a[i];
    }

    public Object set(int i, Object obj)
    {
        Object obj1 = a[i];
        a[i] = obj;
        return obj1;
    }

    public int indexOf(Object obj)
    {
        if(obj == null)
        {
            for(int i = 0; i < a.length; i++)
                if(a[i] == null)
                    return i;

        } else
        {
            for(int j = 0; j < a.length; j++)
                if(obj.equals(a[j]))
                    return j;

        }
        return -1;
    }

    public boolean contains(Object obj)
    {
        return indexOf(obj) != -1;
    }

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

    ArrayList(Object aobj[])
    {
        if(aobj == null)
        {
            throw new NullPointerException();
        } else
        {
            a = aobj;
            return;
        }
    }
}
           

在AbstractList中 add、remove、set方法是這樣的

那麼報錯原因清晰了。

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