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Why do many Collection classes in Java extend the abstract class and implement the interface as well?10个
HashMap类实现了Map接口：
HashMap类实现了Map接口：
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>,Cloneable,Serializable
LinkedHashMap类扩展了HashMap,这意味着它应该默认实现Map接口.为什么明确实现Map接口？
public class LinkedHashMap<K,V> extends HashMap<K,V>
