java - 为什么不能在Java中将Vector <Vector <Number >>转换为AbstractList <AbstractList <Number >>？

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Why we can't do List<Parent> mylist = ArrayList<child>(); [duplicate]
                                
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为什么

AbstractList<AbstractList<Number>> list = new Vector<Vector<Number>();

产生以下错误：

Type mismatch: cannot convert from Vector<Vector<Number>> to AbstractList<AbstractList<Number>>

而Vector扩展了AbstractList？

最佳答案

使用Java泛型时，必须使用? extends AbstractList：

AbstractList<? extends AbstractList<Number>> list = new Vector<Vector<Number>>();