你仔细看看
java.lang.Object
extended by java.util.AbstractCollection<E>
extended by java.util.AbstractList<E>
extended by java.util.ArrayList<E>
ArrrayList继承了AbstractCollection
而AbstractCollection则实现了Iterator接口
java.lang.Object
extended by java.util.AbstractCollection<E>
extended by java.util.AbstractSet<E>
extended by java.util.HashSet<E>
java.lang.Object
extended by java.util.AbstractCollection<E>
extended by java.util.AbstractList<E>
extended by java.util.AbstractSequentialList<E>
extended by java.util.LinkedList<E>