Iterator And Unmodifiable Wrappers,Null references
迭代器是一个对象,它遍历一组元素并逐个返回元素。IteratorsareusedunderthecoversinJavawhenyou’reusingafor(…:…)looptostepthroughaListorarray.遍历时默认调用迭代器Aniteratorhastwomethods:–next()returnsthenextelementinthecollection—thisisam