java面试基础篇（一）
- 2019 年 10 月 5 日
- 筆記
最近想深入的理解一下java 的工作机制，也是便于后期的面试。
1、A：HashMap和Hashtable有什么区别？
Q：HashMap和Hashtable都实现了Map接口，因此很多特性非常相似。但是，他们有以下不同点：
HashMap允许键和值是null，而Hashtable不允许键或者值是null。
Hashtable是同步的，而HashMap不是。因此，HashMap更适合于单线程环境，而Hashtable适合于多线程环境。
我们再来剖析一下这个问题，如果你答些，面试官几乎不会满意的，下面我来深入的看一下，具体的细节和延伸
1,看一下什么是Map，Map是一个接口，在 api 中的定义为
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
翻译软件译文为：将键映射到值的对象。映射不能包含重复的键；每个键最多可以映射到一个值。也就是说Map是一个不能重复键的键值对接口。
我们再来看一下Map源码：
/* * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ package java.util; import java.util.function.BiConsumer; import java.util.function.BiFunction; import java.util.function.Function; import java.io.Serializable; /** * An object that maps keys to values. A map cannot contain duplicate keys; * each key can map to at most one value. * * <p>This interface takes the place of the <tt>Dictionary</tt> class, which * was a totally abstract class rather than an interface. * * <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which * allow a map's contents to be viewed as a set of keys, collection of values, * or set of key-value mappings. The <i>order</i> of a map is defined as * the order in which the iterators on the map's collection views return their * elements. Some map implementations, like the <tt>TreeMap</tt> class, make * specific guarantees as to their order; others, like the <tt>HashMap</tt> * class, do not. * * <p>Note: great care must be exercised if mutable objects are used as map * keys. The behavior of a map is not specified if the value of an object is * changed in a manner that affects <tt>equals</tt> comparisons while the * object is a key in the map. A special case of this prohibition is that it * is not permissible for a map to contain itself as a key. While it is * permissible for a map to contain itself as a value, extreme caution is * advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer * well defined on such a map. * * <p>All general-purpose map implementation classes should provide two * "standard" constructors: a void (no arguments) constructor which creates an * empty map, and a constructor with a single argument of type <tt>Map</tt>, * which creates a new map with the same key-value mappings as its argument. * In effect, the latter constructor allows the user to copy any map, * producing an equivalent map of the desired class. There is no way to * enforce this recommendation (as interfaces cannot contain constructors) but * all of the general-purpose map implementations in the JDK comply. * * <p>The "destructive" methods contained in this interface, that is, the * methods that modify the map on which they operate, are specified to throw * <tt>UnsupportedOperationException</tt> if this map does not support the * operation. If this is the case, these methods may, but are not required * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would * have no effect on the map. For example, invoking the {@link #putAll(Map)} * method on an unmodifiable map may, but is not required to, throw the * exception if the map whose mappings are to be "superimposed" is empty. * * <p>Some map implementations have restrictions on the keys and values they * may contain. For example, some implementations prohibit null keys and * values, and some have restrictions on the types of their keys. Attempting * to insert an ineligible key or value throws an unchecked exception, * typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. * Attempting to query the presence of an ineligible key or value may throw an * exception, or it may simply return false; some implementations will exhibit * the former behavior and some will exhibit the latter. More generally, * attempting an operation on an ineligible key or value whose completion * would not result in the insertion of an ineligible element into the map may * throw an exception or it may succeed, at the option of the implementation. * Such exceptions are marked as "optional" in the specification for this * interface. * * <p>Many methods in Collections Framework interfaces are defined * in terms of the {@link Object#equals(Object) equals} method. For * example, the specification for the {@link #containsKey(Object) * containsKey(Object key)} method says: "returns <tt>true</tt> if and * only if this map contains a mapping for a key <tt>k</tt> such that * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt> * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to * be invoked for any key <tt>k</tt>. Implementations are free to * implement optimizations whereby the <tt>equals</tt> invocation is avoided, * for example, by first comparing the hash codes of the two keys. (The * {@link Object#hashCode()} specification guarantees that two objects with * unequal hash codes cannot be equal.) More generally, implementations of * the various Collections Framework interfaces are free to take advantage of * the specified behavior of underlying {@link Object} methods wherever the * implementor deems it appropriate. * * <p>Some map operations which perform recursive traversal of the map may fail * with an exception for self-referential instances where the map directly or * indirectly contains itself. This includes the {@code clone()}, * {@code equals()}, {@code hashCode()} and {@code toString()} methods. * Implementations may optionally handle the self-referential scenario, however * most current implementations do not do so. * * <p>This interface is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @param <K> the type of keys maintained by this map * @param <V> the type of mapped values * * @author Josh Bloch * @see HashMap * @see TreeMap * @see Hashtable * @see SortedMap * @see Collection * @see Set * @since 1.2 */ public interface Map<K,V> { // Query Operations /** * Returns the number of key-value mappings in this map. If the * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns * <tt>Integer.MAX_VALUE</tt>. * * @return the number of key-value mappings in this map */ int size(); /** * Returns <tt>true</tt> if this map contains no key-value mappings. * * @return <tt>true</tt> if this map contains no key-value mappings */ boolean isEmpty(); /** * Returns <tt>true</tt> if this map contains a mapping for the specified * key. More formally, returns <tt>true</tt> if and only if * this map contains a mapping for a key <tt>k</tt> such that * <tt>(key==null ? k==null : key.equals(k))</tt>. (There can be * at most one such mapping.) * * @param key key whose presence in this map is to be tested * @return <tt>true</tt> if this map contains a mapping for the specified * key * @throws ClassCastException if the key is of an inappropriate type for * this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified key is null and this map * does not permit null keys * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) */ boolean containsKey(Object key); /** * Returns <tt>true</tt> if this map maps one or more keys to the * specified value. More formally, returns <tt>true</tt> if and only if * this map contains at least one mapping to a value <tt>v</tt> such that * <tt>(value==null ? v==null : value.equals(v))</tt>. This operation * will probably require time linear in the map size for most * implementations of the <tt>Map</tt> interface. * * @param value value whose presence in this map is to be tested * @return <tt>true</tt> if this map maps one or more keys to the * specified value * @throws ClassCastException if the value is of an inappropriate type for * this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified value is null and this * map does not permit null values * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) */ boolean containsValue(Object value); /** * Returns the value to which the specified key is mapped, * or {@code null} if this map contains no mapping for the key. * * <p>More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code (key==null ? k==null : * key.equals(k))}, then this method returns {@code v}; otherwise * it returns {@code null}. (There can be at most one such mapping.) * * <p>If this map permits null values, then a return value of * {@code null} does not <i>necessarily</i> indicate that the map * contains no mapping for the key; it's also possible that the map * explicitly maps the key to {@code null}. The {@link #containsKey * containsKey} operation may be used to distinguish these two cases. * * @param key the key whose associated value is to be returned * @return the value to which the specified key is mapped, or * {@code null} if this map contains no mapping for the key * @throws ClassCastException if the key is of an inappropriate type for * this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified key is null and this map * does not permit null keys * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) */ V get(Object key); // Modification Operations /** * Associates the specified value with the specified key in this map * (optional operation). If the map previously contained a mapping for * the key, the old value is replaced by the specified value. (A map * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only * if {@link #containsKey(Object) m.containsKey(k)} would return * <tt>true</tt>.) * * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt>. * (A <tt>null</tt> return can also indicate that the map * previously associated <tt>null</tt> with <tt>key</tt>, * if the implementation supports <tt>null</tt> values.) * @throws UnsupportedOperationException if the <tt>put</tt> operation * is not supported by this map * @throws ClassCastException if the class of the specified key or value * prevents it from being stored in this map * @throws NullPointerException if the specified key or value is null * and this map does not permit null keys or values * @throws IllegalArgumentException if some property of the specified key * or value prevents it from being stored in this map */ V put(K key, V value); /** * Removes the mapping for a key from this map if it is present * (optional operation). More formally, if this map contains a mapping * from key <tt>k</tt> to value <tt>v</tt> such that * <code>(key==null ? k==null : key.equals(k))</code>, that mapping * is removed. (The map can contain at most one such mapping.) * * <p>Returns the value to which this map previously associated the key, * or <tt>null</tt> if the map contained no mapping for the key. * * <p>If this map permits null values, then a return value of * <tt>null</tt> does not <i>necessarily</i> indicate that the map * contained no mapping for the key; it's also possible that the map * explicitly mapped the key to <tt>null</tt>. * * <p>The map will not contain a mapping for the specified key once the * call returns. * * @param key key whose mapping is to be removed from the map * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt>. * @throws UnsupportedOperationException if the <tt>remove</tt> operation * is not supported by this map * @throws ClassCastException if the key is of an inappropriate type for * this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified key is null and this * map does not permit null keys * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) */ V remove(Object key); // Bulk Operations /** * Copies all of the mappings from the specified map to this map * (optional operation). The effect of this call is equivalent to that * of calling {@link #put(Object,Object) put(k, v)} on this map once * for each mapping from key <tt>k</tt> to value <tt>v</tt> in the * specified map. The behavior of this operation is undefined if the * specified map is modified while the operation is in progress. * * @param m mappings to be stored in this map * @throws UnsupportedOperationException if the <tt>putAll</tt> operation * is not supported by this map * @throws ClassCastException if the class of a key or value in the * specified map prevents it from being stored in this map * @throws NullPointerException if the specified map is null, or if * this map does not permit null keys or values, and the * specified map contains null keys or values * @throws IllegalArgumentException if some property of a key or value in * the specified map prevents it from being stored in this map */ void putAll(Map<? extends K, ? extends V> m); /** * Removes all of the mappings from this map (optional operation). * The map will be empty after this call returns. * * @throws UnsupportedOperationException if the <tt>clear</tt> operation * is not supported by this map */ void clear(); // Views /** * Returns a {@link Set} view of the keys contained in this map. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. If the map is modified * while an iteration over the set is in progress (except through * the iterator's own <tt>remove</tt> operation), the results of * the iteration are undefined. The set supports element removal, * which removes the corresponding mapping from the map, via the * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> * operations. It does not support the <tt>add</tt> or <tt>addAll</tt> * operations. * * @return a set view of the keys contained in this map */ Set<K> keySet(); /** * Returns a {@link Collection} view of the values contained in this map. * The collection is backed by the map, so changes to the map are * reflected in the collection, and vice-versa. If the map is * modified while an iteration over the collection is in progress * (except through the iterator's own <tt>remove</tt> operation), * the results of the iteration are undefined. The collection * supports element removal, which removes the corresponding * mapping from the map, via the <tt>Iterator.remove</tt>, * <tt>Collection.remove</tt>, <tt>removeAll</tt>, * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not * support the <tt>add</tt> or <tt>addAll</tt> operations. * * @return a collection view of the values contained in this map */ Collection<V> values(); /** * Returns a {@link Set} view of the mappings contained in this map. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. If the map is modified * while an iteration over the set is in progress (except through * the iterator's own <tt>remove</tt> operation, or through the * <tt>setValue</tt> operation on a map entry returned by the * iterator) the results of the iteration are undefined. The set * supports element removal, which removes the corresponding * mapping from the map, via the <tt>Iterator.remove</tt>, * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and * <tt>clear</tt> operations. It does not support the * <tt>add</tt> or <tt>addAll</tt> operations. * * @return a set view of the mappings contained in this map */ Set<Map.Entry<K, V>> entrySet(); /** * A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns * a collection-view of the map, whose elements are of this class. The * <i>only</i> way to obtain a reference to a map entry is from the * iterator of this collection-view. These <tt>Map.Entry</tt> objects are * valid <i>only</i> for the duration of the iteration; more formally, * the behavior of a map entry is undefined if the backing map has been * modified after the entry was returned by the iterator, except through * the <tt>setValue</tt> operation on the map entry. * * @see Map#entrySet() * @since 1.2 */ interface Entry<K,V> { /** * Returns the key corresponding to this entry. * * @return the key corresponding to this entry * @throws IllegalStateException implementations may, but are not * required to, throw this exception if the entry has been * removed from the backing map. */ K getKey(); /** * Returns the value corresponding to this entry. If the mapping * has been removed from the backing map (by the iterator's * <tt>remove</tt> operation), the results of this call are undefined. * * @return the value corresponding to this entry * @throws IllegalStateException implementations may, but are not * required to, throw this exception if the entry has been * removed from the backing map. */ V getValue(); /** * Replaces the value corresponding to this entry with the specified * value (optional operation). (Writes through to the map.) The * behavior of this call is undefined if the mapping has already been * removed from the map (by the iterator's <tt>remove</tt> operation). * * @param value new value to be stored in this entry * @return old value corresponding to the entry * @throws UnsupportedOperationException if the <tt>put</tt> operation * is not supported by the backing map * @throws ClassCastException if the class of the specified value * prevents it from being stored in the backing map * @throws NullPointerException if the backing map does not permit * null values, and the specified value is null * @throws IllegalArgumentException if some property of this value * prevents it from being stored in the backing map * @throws IllegalStateException implementations may, but are not * required to, throw this exception if the entry has been * removed from the backing map. */ V setValue(V value); /** * Compares the specified object with this entry for equality. * Returns <tt>true</tt> if the given object is also a map entry and * the two entries represent the same mapping. More formally, two * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping * if<pre> * (e1.getKey()==null ? * e2.getKey()==null : e1.getKey().equals(e2.getKey())) && * (e1.getValue()==null ? * e2.getValue()==null : e1.getValue().equals(e2.getValue())) * </pre> * This ensures that the <tt>equals</tt> method works properly across * different implementations of the <tt>Map.Entry</tt> interface. * * @param o object to be compared for equality with this map entry * @return <tt>true</tt> if the specified object is equal to this map * entry */ boolean equals(Object o); /** * Returns the hash code value for this map entry. The hash code * of a map entry <tt>e</tt> is defined to be: <pre> * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^ * (e.getValue()==null ? 0 : e.getValue().hashCode()) * </pre> * This ensures that <tt>e1.equals(e2)</tt> implies that * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries * <tt>e1</tt> and <tt>e2</tt>, as required by the general * contract of <tt>Object.hashCode</tt>. * * @return the hash code value for this map entry * @see Object#hashCode() * @see Object#equals(Object) * @see #equals(Object) */ int hashCode(); /** * Returns a comparator that compares {@link Map.Entry} in natural order on key. * * <p>The returned comparator is serializable and throws {@link * NullPointerException} when comparing an entry with a null key. * * @param <K> the {@link Comparable} type of then map keys * @param <V> the type of the map values * @return a comparator that compares {@link Map.Entry} in natural order on key. * @see Comparable * @since 1.8 */ public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> comparingByKey() { return (Comparator<Map.Entry<K, V>> & Serializable) (c1, c2) -> c1.getKey().compareTo(c2.getKey()); } /** * Returns a comparator that compares {@link Map.Entry} in natural order on value. * * <p>The returned comparator is serializable and throws {@link * NullPointerException} when comparing an entry with null values. * * @param <K> the type of the map keys * @param <V> the {@link Comparable} type of the map values * @return a comparator that compares {@link Map.Entry} in natural order on value. * @see Comparable * @since 1.8 */ public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> comparingByValue() { return (Comparator<Map.Entry<K, V>> & Serializable) (c1, c2) -> c1.getValue().compareTo(c2.getValue()); } /** * Returns a comparator that compares {@link Map.Entry} by key using the given * {@link Comparator}. * * <p>The returned comparator is serializable if the specified comparator * is also serializable. * * @param <K> the type of the map keys * @param <V> the type of the map values * @param cmp the key {@link Comparator} * @return a comparator that compares {@link Map.Entry} by the key. * @since 1.8 */ public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) { Objects.requireNonNull(cmp); return (Comparator<Map.Entry<K, V>> & Serializable) (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey()); } /** * Returns a comparator that compares {@link Map.Entry} by value using the given * {@link Comparator}. * * <p>The returned comparator is serializable if the specified comparator * is also serializable. * * @param <K> the type of the map keys * @param <V> the type of the map values * @param cmp the value {@link Comparator} * @return a comparator that compares {@link Map.Entry} by the value. * @since 1.8 */ public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) { Objects.requireNonNull(cmp); return (Comparator<Map.Entry<K, V>> & Serializable) (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue()); } } // Comparison and hashing /** * Compares the specified object with this map for equality. Returns * <tt>true</tt> if the given object is also a map and the two maps * represent the same mappings. More formally, two maps <tt>m1</tt> and * <tt>m2</tt> represent the same mappings if * <tt>m1.entrySet().equals(m2.entrySet())</tt>. This ensures that the * <tt>equals</tt> method works properly across different implementations * of the <tt>Map</tt> interface. * * @param o object to be compared for equality with this map * @return <tt>true</tt> if the specified object is equal to this map */ boolean equals(Object o); /** * Returns the hash code value for this map. The hash code of a map is * defined to be the sum of the hash codes of each entry in the map's * <tt>entrySet()</tt> view. This ensures that <tt>m1.equals(m2)</tt> * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of * {@link Object#hashCode}. * * @return the hash code value for this map * @see Map.Entry#hashCode() * @see Object#equals(Object) * @see #equals(Object) */ int hashCode(); // Defaultable methods /** * Returns the value to which the specified key is mapped, or * {@code defaultValue} if this map contains no mapping for the key. * * @implSpec * The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. * * @param key the key whose associated value is to be returned * @param defaultValue the default mapping of the key * @return the value to which the specified key is mapped, or * {@code defaultValue} if this map contains no mapping for the key * @throws ClassCastException if the key is of an inappropriate type for * this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified key is null and this map * does not permit null keys * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @since 1.8 */ default V getOrDefault(Object key, V defaultValue) { V v; return (((v = get(key)) != null) || containsKey(key)) ? v : defaultValue; } /** * Performs the given action for each entry in this map until all entries * have been processed or the action throws an exception. Unless * otherwise specified by the implementing class, actions are performed in * the order of entry set iteration (if an iteration order is specified.) * Exceptions thrown by the action are relayed to the caller. * * @implSpec * The default implementation is equivalent to, for this {@code map}: * <pre> {@code * for (Map.Entry<K, V> entry : map.entrySet()) * action.accept(entry.getKey(), entry.getValue()); * }</pre> * * The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. * * @param action The action to be performed for each entry * @throws NullPointerException if the specified action is null * @throws ConcurrentModificationException if an entry is found to be * removed during iteration * @since 1.8 */ default void forEach(BiConsumer<? super K, ? super V> action) { Objects.requireNonNull(action); for (Map.Entry<K, V> entry : entrySet()) { K k; V v; try { k = entry.getKey(); v = entry.getValue(); } catch(IllegalStateException ise) { // this usually means the entry is no longer in the map. throw new ConcurrentModificationException(ise); } action.accept(k, v); } } /** * Replaces each entry's value with the result of invoking the given * function on that entry until all entries have been processed or the * function throws an exception. Exceptions thrown by the function are * relayed to the caller. * * @implSpec * <p>The default implementation is equivalent to, for this {@code map}: * <pre> {@code * for (Map.Entry<K, V> entry : map.entrySet()) * entry.setValue(function.apply(entry.getKey(), entry.getValue())); * }</pre> * * <p>The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. * * @param function the function to apply to each entry * @throws UnsupportedOperationException if the {@code set} operation * is not supported by this map's entry set iterator. * @throws ClassCastException if the class of a replacement value * prevents it from being stored in this map * @throws NullPointerException if the specified function is null, or the * specified replacement value is null, and this map does not permit null * values * @throws ClassCastException if a replacement value is of an inappropriate * type for this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if function or a replacement value is null, * and this map does not permit null keys or values * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws IllegalArgumentException if some property of a replacement value * prevents it from being stored in this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws ConcurrentModificationException if an entry is found to be * removed during iteration * @since 1.8 */ default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) { Objects.requireNonNull(function); for (Map.Entry<K, V> entry : entrySet()) { K k; V v; try { k = entry.getKey(); v = entry.getValue(); } catch(IllegalStateException ise) { // this usually means the entry is no longer in the map. throw new ConcurrentModificationException(ise); } // ise thrown from function is not a cme. v = function.apply(k, v); try { entry.setValue(v); } catch(IllegalStateException ise) { // this usually means the entry is no longer in the map. throw new ConcurrentModificationException(ise); } } } /** * If the specified key is not already associated with a value (or is mapped * to {@code null}) associates it with the given value and returns * {@code null}, else returns the current value. * * @implSpec * The default implementation is equivalent to, for this {@code * map}: * * <pre> {@code * V v = map.get(key); * if (v == null) * v = map.put(key, value); * * return v; * }</pre> * * <p>The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. * * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with the specified key, or * {@code null} if there was no mapping for the key. * (A {@code null} return can also indicate that the map * previously associated {@code null} with the key, * if the implementation supports null values.) * @throws UnsupportedOperationException if the {@code put} operation * is not supported by this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws ClassCastException if the key or value is of an inappropriate * type for this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified key or value is null, * and this map does not permit null keys or values * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws IllegalArgumentException if some property of the specified key * or value prevents it from being stored in this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @since 1.8 */ default V putIfAbsent(K key, V value) { V v = get(key); if (v == null) { v = put(key, value); } return v; } /** * Removes the entry for the specified key only if it is currently * mapped to the specified value. * * @implSpec * The default implementation is equivalent to, for this {@code map}: * * <pre> {@code * if (map.containsKey(key) && Objects.equals(map.get(key), value)) { * map.remove(key); * return true; * } else * return false; * }</pre> * * <p>The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. * * @param key key with which the specified value is associated * @param value value expected to be associated with the specified key * @return {@code true} if the value was removed * @throws UnsupportedOperationException if the {@code remove} operation * is not supported by this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws ClassCastException if the key or value is of an inappropriate * type for this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified key or value is null, * and this map does not permit null keys or values * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @since 1.8 */ default boolean remove(Object key, Object value) { Object curValue = get(key); if (!Objects.equals(curValue, value) || (curValue == null && !containsKey(key))) { return false; } remove(key); return true; } /** * Replaces the entry for the specified key only if currently * mapped to the specified value. * * @implSpec * The default implementation is equivalent to, for this {@code map}: * * <pre> {@code * if (map.containsKey(key) && Objects.equals(map.get(key), value)) { * map.put(key, newValue); * return true; * } else * return false; * }</pre> * * The default implementation does not throw NullPointerException * for maps that do not support null values if oldValue is null unless * newValue is also null. * * <p>The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. * * @param key key with which the specified value is associated * @param oldValue value expected to be associated with the specified key * @param newValue value to be associated with the specified key * @return {@code true} if the value was replaced * @throws UnsupportedOperationException if the {@code put} operation * is not supported by this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws ClassCastException if the class of a specified key or value * prevents it from being stored in this map * @throws NullPointerException if a specified key or newValue is null, * and this map does not permit null keys or values * @throws NullPointerException if oldValue is null and this map does not * permit null values * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws IllegalArgumentException if some property of a specified key * or value prevents it from being stored in this map * @since 1.8 */ default boolean replace(K key, V oldValue, V newValue) { Object curValue = get(key); if (!Objects.equals(curValue, oldValue) || (curValue == null && !containsKey(key))) { return false; } put(key, newValue); return true; } /** * Replaces the entry for the specified key only if it is * currently mapped to some value. * * @implSpec * The default implementation is equivalent to, for this {@code map}: * * <pre> {@code * if (map.containsKey(key)) { * return map.put(key, value); * } else * return null; * }</pre> * * <p>The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. * * @param key key with which the specified value is associated * @param value value to be associated with the specified key * @return the previous value associated with the specified key, or * {@code null} if there was no mapping for the key. * (A {@code null} return can also indicate that the map * previously associated {@code null} with the key, * if the implementation supports null values.) * @throws UnsupportedOperationException if the {@code put} operation * is not supported by this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws ClassCastException if the class of the specified key or value * prevents it from being stored in this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified key or value is null, * and this map does not permit null keys or values * @throws IllegalArgumentException if some property of the specified key * or value prevents it from being stored in this map * @since 1.8 */ default V replace(K key, V value) { V curValue; if (((curValue = get(key)) != null) || containsKey(key)) { curValue = put(key, value); } return curValue; } /** * If the specified key is not already associated with a value (or is mapped * to {@code null}), attempts to compute its value using the given mapping * function and enters it into this map unless {@code null}. * * <p>If the function returns {@code null} no mapping is recorded. If * the function itself throws an (unchecked) exception, the * exception is rethrown, and no mapping is recorded. The most * common usage is to construct a new object serving as an initial * mapped value or memoized result, as in: * * <pre> {@code * map.computeIfAbsent(key, k -> new Value(f(k))); * }</pre> * * <p>Or to implement a multi-value map, {@code Map<K,Collection<V>>}, * supporting multiple values per key: * * <pre> {@code * map.computeIfAbsent(key, k -> new HashSet<V>()).add(v); * }</pre> * * * @implSpec * The default implementation is equivalent to the following steps for this * {@code map}, then returning the current value or {@code null} if now * absent: * * <pre> {@code * if (map.get(key) == null) { * V newValue = mappingFunction.apply(key); * if (newValue != null) * map.put(key, newValue); * } * }</pre> * * <p>The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. In particular, all implementations of * subinterface {@link java.util.concurrent.ConcurrentMap} must document * whether the function is applied once atomically only if the value is not * present. * * @param key key with which the specified value is to be associated * @param mappingFunction the function to compute a value * @return the current (existing or computed) value associated with * the specified key, or null if the computed value is null * @throws NullPointerException if the specified key is null and * this map does not support null keys, or the mappingFunction * is null * @throws UnsupportedOperationException if the {@code put} operation * is not supported by this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws ClassCastException if the class of the specified key or value * prevents it from being stored in this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @since 1.8 */ default V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) { Objects.requireNonNull(mappingFunction); V v; if ((v = get(key)) == null) { V newValue; if ((newValue = mappingFunction.apply(key)) != null) { put(key, newValue); return newValue; } } return v; } /** * If the value for the specified key is present and non-null, attempts to * compute a new mapping given the key and its current mapped value. * * <p>If the function returns {@code null}, the mapping is removed. If the * function itself throws an (unchecked) exception, the exception is * rethrown, and the current mapping is left unchanged. * * @implSpec * The default implementation is equivalent to performing the following * steps for this {@code map}, then returning the current value or * {@code null} if now absent: * * <pre> {@code * if (map.get(key) != null) { * V oldValue = map.get(key); * V newValue = remappingFunction.apply(key, oldValue); * if (newValue != null) * map.put(key, newValue); * else * map.remove(key); * } * }</pre> * * <p>The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. In particular, all implementations of * subinterface {@link java.util.concurrent.ConcurrentMap} must document * whether the function is applied once atomically only if the value is not * present. * * @param key key with which the specified value is to be associated * @param remappingFunction the function to compute a value * @return the new value associated with the specified key, or null if none * @throws NullPointerException if the specified key is null and * this map does not support null keys, or the * remappingFunction is null * @throws UnsupportedOperationException if the {@code put} operation * is not supported by this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws ClassCastException if the class of the specified key or value * prevents it from being stored in this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @since 1.8 */ default V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) { Objects.requireNonNull(remappingFunction); V oldValue; if ((oldValue = get(key)) != null) { V newValue = remappingFunction.apply(key, oldValue); if (newValue != null) { put(key, newValue); return newValue; } else { remove(key); return null; } } else { return null; } } /** * Attempts to compute a mapping for the specified key and its current * mapped value (or {@code null} if there is no current mapping). For * example, to either create or append a {@code String} msg to a value * mapping: * * <pre> {@code * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre> * (Method {@link #merge merge()} is often simpler to use for such purposes.) * * <p>If the function returns {@code null}, the mapping is removed (or * remains absent if initially absent). If the function itself throws an * (unchecked) exception, the exception is rethrown, and the current mapping * is left unchanged. * * @implSpec * The default implementation is equivalent to performing the following * steps for this {@code map}, then returning the current value or * {@code null} if absent: * * <pre> {@code * V oldValue = map.get(key); * V newValue = remappingFunction.apply(key, oldValue); * if (oldValue != null ) { * if (newValue != null) * map.put(key, newValue); * else * map.remove(key); * } else { * if (newValue != null) * map.put(key, newValue); * else * return null; * } * }</pre> * * <p>The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. In particular, all implementations of * subinterface {@link java.util.concurrent.ConcurrentMap} must document * whether the function is applied once atomically only if the value is not * present. * * @param key key with which the specified value is to be associated * @param remappingFunction the function to compute a value * @return the new value associated with the specified key, or null if none * @throws NullPointerException if the specified key is null and * this map does not support null keys, or the * remappingFunction is null * @throws UnsupportedOperationException if the {@code put} operation * is not supported by this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws ClassCastException if the class of the specified key or value * prevents it from being stored in this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @since 1.8 */ default V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) { Objects.requireNonNull(remappingFunction); V oldValue = get(key); V newValue = remappingFunction.apply(key, oldValue); if (newValue == null) { // delete mapping if (oldValue != null || containsKey(key)) { // something to remove remove(key); return null; } else { // nothing to do. Leave things as they were. return null; } } else { // add or replace old mapping put(key, newValue); return newValue; } } /** * If the specified key is not already associated with a value or is * associated with null, associates it with the given non-null value. * Otherwise, replaces the associated value with the results of the given * remapping function, or removes if the result is {@code null}. This * method may be of use when combining multiple mapped values for a key. * For example, to either create or append a {@code String msg} to a * value mapping: * * <pre> {@code * map.merge(key, msg, String::concat) * }</pre> * * <p>If the function returns {@code null} the mapping is removed. If the * function itself throws an (unchecked) exception, the exception is * rethrown, and the current mapping is left unchanged. * * @implSpec * The default implementation is equivalent to performing the following * steps for this {@code map}, then returning the current value or * {@code null} if absent: * * <pre> {@code * V oldValue = map.get(key); * V newValue = (oldValue == null) ? value : * remappingFunction.apply(oldValue, value); * if (newValue == null) * map.remove(key); * else * map.put(key, newValue); * }</pre> * * <p>The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. In particular, all implementations of * subinterface {@link java.util.concurrent.ConcurrentMap} must document * whether the function is applied once atomically only if the value is not * present. * * @param key key with which the resulting value is to be associated * @param value the non-null value to be merged with the existing value * associated with the key or, if no existing value or a null value * is associated with the key, to be associated with the key * @param remappingFunction the function to recompute a value if present * @return the new value associated with the specified key, or null if no * value is associated with the key * @throws UnsupportedOperationException if the {@code put} operation * is not supported by this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws ClassCastException if the class of the specified key or value * prevents it from being stored in this map * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified key is null and this map * does not support null keys or the value or remappingFunction is * null * @since 1.8 */ default V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) { Objects.requireNonNull(remappingFunction); Objects.requireNonNull(value); V oldValue = get(key); V newValue = (oldValue == null) ? value : remappingFunction.apply(oldValue, value); if(newValue == null) { remove(key); } else { put(key, newValue); } return newValue; } }
我们再点击开HashMap的源码，我们可以看到HashMap继承了AbstractMap<K,V>，同时实现了Map<K,V>, Cloneable, Serializable。
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable
我们再点击开AbstractMap的源码，我会惊奇的发现了，AbstractMap已经实现了Map接口，那么为什么HashMap还有再次实现Map接口呢？如果有人问你这个问题，你就打死他，百度了很多资料，据说是当时开发人员写多了，比较悲催。
<!– p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Menlo; color: #7f0055} span.s1 {color: #000000} –>
public abstract class AbstractMap<K,V> implements Map<K,V>
我们接着回到HashMap的源码中，我们在236行看到 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 也就是说一个map在无参数的情况下被创建出来，默认的大小就是 1<<4 （16）
在248行我们可以看到DEFAULT_LOAD_FACTOR 默认负载因子 0.75， 这个非常重要，在后面的扩容会用到。
HashMap 提供了4个构造方法，可以接收修改初始化大小和负载因子，但是一般情况下就不要去修改了，避免设置得不好性能上出现问题。
MAXIMUM_CAPACITY 最大容量 1 << 30。1左移30位二进制的形势下就是 0100 0000 0000 0000 0000 0000 0000 0000，这个的意思是2的30次方，十进制下是 1073741824。注释说了 MUST be a power of two（一定要是2的n次方）， 再多移动一位 1<<31 就变成负数了。
TREEIFY_THRESHOLD，UNTREEIFY_THRESHOLD， MIN_TREEIFY_CAPACITY 这几个参数是后面当红黑树的参数。分别为最大阈值，取消阈值验证，和最小阈值。
我们再来看一下279行代码 static class Node<K,V> implements Map.Entry<K,V> 和396行的transient Node<K,V>[] table 这2个东西就是 HashMap 的本质了。读到这里，我们知道了，其实 HashMap 就是一个由 Node 类组成的一个二维数组，Node 是 Map.Entry 的具体实现类。当put一个对象的时候会根据对象的hash值计算出它在数组中存放的位置（通过扰动函数计算，后面会讲到），然后判断这个位置上有没有已经存在的对象，如果没有就直接放到这个位置，如果有将已存在对象的next指向当前对象形成一个链表，当链表长度超过一定数量之后，链表会转换成红黑树（这是java8之后的修改，为了提升查询效率）。所以hashmap本质上是一个二维数组加链表加红黑树的组合。
我们知道HashMap的内部结构了，那么我们来看一下他为什么是线程不安全的。我们在上述文章中，可以知道HashMap是链表结构的，也就是说总有一个指针指向下一项或上一项。
我们在声明HashMap时，使用的都是默认的构造方法：HashMap<K,V>，看了代码你会发现，它还有其它的构造方法：HashMap(int initialCapacity, float loadFactor)，
其中参数initialCapacity为初始容量，loadFactor为加载因子，扩容就是在put加入元素的个数超过initialCapacity * loadFactor的时候就会将内部Entry数组大小扩大至原来的2倍，然后将数组元素按照新的数组大小重新计算索引，放在新
的数组中，同时修改每个节点的链表关系（主要是next和节点在链表中的位置）。
概括的来说：HashMap在put的时候，插入的元素超过了容量（由负载因子决定）的范围就会触发扩容操作，就是rehash，这个会重新将原数组的内容重新hash到新的扩容数组中，在多线程的环境下，存在同时其他的元素也在进行put操作，如果hash值相同，可能出现同时在同一数组下用链表表示，造成闭环，导致在get时会出现死循环，所以HashMap是线程不安全的。
HashTable就是因为加了synchronized线程锁，而不是因为他们的结构不一致才保证的线程安全，HashMap和HashTable都是单向链表结构的。
如果我们真的理解了上面所说的一切，我觉得面试官不会不满意你对于HashMap和Hashtable的面试答案吧，可能会问到一些扩展问题，不如，怎么才能做到线程安全，什么是扰动函数，红黑树又是什么？下面我来依依作答这些问题。
2、Q：怎么才能做到线程的安全。
A：首先我们应该知道什么是线程，线程thread是操作系统能够进行运算调度的最小单位。它被包含在进程之中，是进程中的实际运作单位。一条线程指的是进程中一个单一顺序的控制流，一个进程中可以并发多个线程，每条线程并行执行不同的任务。在Unix System V及SunOS中也被称为轻量进程，但轻量进程更多指内核线程（kernel thread），而我们一般把用户线程（user thread）称为线程。
然后我们应该知道什么是多线程，提到多线程这里要说两个概念，就是串行和并行，搞清楚这个我们才能更好的理解多线程，串行是指，A、B、C三个任务，执行完任务A，才能执行B，B执行完，才能执行C，而并行是指三个任务一起执行。
什么是线程安全，既然是线程安全问题，那么毫无疑问所有的隐患都是出现在多个线程访问的情况下产生的，也就是我们要确保在多条线程访问的时候，我们的程序还能按照我们预期的行为去执行。当多个线程访问某个方法时，不管你通过怎样的调用方式或者说这些线程如何交替的执行，我们在主程序中不需要去做任何的同步，这个类的结果行为都是我们设想的正确行为，那么我们就可以说这个类时线程安全的。那么我们来不如正题来说下java如何做到线程的安全，也可以理解为线程的同步。
我们先来看一段代码：
package com; public class Demo { static int tickets = 10; class SellTickets implements Runnable { @Override public void run() { // 未加同步时产生脏数据 while (tickets > 0) { System.out.println(Thread.currentThread().getName() + "--->售出第： " + tickets + " 票"); tickets--; try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } if (tickets <= 0) { System.out.println(Thread.currentThread().getName() + "--->售票结束！"); } } } public static void main(String[] args) { SellTickets sell = new Demo().new SellTickets(); Thread thread1 = new Thread(sell, "1号窗口"); Thread thread2 = new Thread(sell, "2号窗口"); Thread thread3 = new Thread(sell, "3号窗口"); Thread thread4 = new Thread(sell, "4号窗口"); thread1.start(); thread2.start(); thread3.start(); thread4.start(); } }
我运行可以发现，有重复售票的情况，那么说明现在的方式是不安全的，我们可以采用以下方法来控制线程的安全。
第一种实现线程安全的方式，同步代码块
这样我们就可以保证线程的一致性了。
第二种方式Lock锁机制， 通过创建Lock对象，采用lock()加锁，unlock()解锁，来保护指定的代码块
package com; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; public class Demo { static int tickets = 10; class SellTickets implements Runnable { Lock lock = new ReentrantLock(); @Override public void run() { // Lock锁机制 while (tickets > 0) { try { lock.lock(); if (tickets <= 0) { return; } System.out.println(Thread.currentThread().getName() + "--->售出第： " + tickets + " 票"); tickets--; } catch (Exception e1) { // TODO Auto-generated catch block e1.printStackTrace(); } finally { lock.unlock(); try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); } } } if (tickets <= 0) { System.out.println(Thread.currentThread().getName() + "--->售票结束！"); } } } public static void main(String[] args) { SellTickets sell = new Demo().new SellTickets(); Thread thread1 = new Thread(sell, "1号窗口"); Thread thread2 = new Thread(sell, "2号窗口"); Thread thread3 = new Thread(sell, "3号窗口"); Thread thread4 = new Thread(sell, "4号窗口"); thread1.start(); thread2.start(); thread3.start(); thread4.start(); } }
总结：由于synchronized是在JVM层面实现的，因此系统可以监控锁的释放与否；而ReentrantLock是使用代码实现的，系统无法自动释放锁，需要在代码中的finally子句中显式释放锁lock.unlock()。另外，在并发量比较小的情况下，使用synchronized是个不错的选择；但是在并发量比较高的情况下，其性能下降会很严重，此时ReentrantLock是个不错的方案。
补充：在使用synchronized 代码块时,可以与wait()、notify()、nitifyAll()一起使用，从而进一步实现线程的通信。其中，wait()方法会释放占有的对象锁，当前线程进入等待池，释放cpu,而其他正在等待的线程即可抢占此锁，获得锁的线程即可运行程序；线程的sleep()方法则表示，当前线程会休眠一段时间，休眠期间，会暂时释放cpu，但并不释放对象锁，也就是说，在休眠期间，其他线程依然无法进入被同步保护的代码内部，当前线程休眠结束时，会重新获得cpu执行权,从而执行被同步保护的代码。wait()和sleep()最大的不同在于wait()会释放对象锁，而sleep()不会释放对象锁。notify()方法会唤醒因为调用对象的wait()而处于等待状态的线程，从而使得该线程有机会获取对象锁。调用notify()后，当前线程并不会立即释放锁，而是继续执行当前代码，直到synchronized中的代码全部执行完毕，才会释放对象锁。JVM会在等待的线程中调度一个线程去获得对象锁，执行代码。需要注意的是，wait()和notify()必须在synchronized代码块中调用。notifyAll()是唤醒所有等待的线程。
说了这么多，其实就是一个由Map引发的“血案”，其实还有很多很多的扩展问题。明天我们再来继续讨论这些问题。
