java面試基礎篇（一）
- 2019 年 10 月 5 日
- 筆記
最近想深入的理解一下java 的工作機制，也是便於後期的面試。
1、A：HashMap和Hashtable有什麼區別？
Q：HashMap和Hashtable都實現了Map介面，因此很多特性非常相似。但是，他們有以下不同點：
HashMap允許鍵和值是null，而Hashtable不允許鍵或者值是null。
Hashtable是同步的，而HashMap不是。因此，HashMap更適合於單執行緒環境，而Hashtable適合於多執行緒環境。
我們再來剖析一下這個問題，如果你答些，面試官幾乎不會滿意的，下面我來深入的看一下，具體的細節和延伸
1,看一下什麼是Map，Map是一個介面，在 api 中的定義為
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
翻譯軟體譯文為：將鍵映射到值的對象。映射不能包含重複的鍵；每個鍵最多可以映射到一個值。也就是說Map是一個不能重複鍵的鍵值對介面。
我們再來看一下Map源碼：
/* * 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; } }
我們再點擊開HashMap的源碼，我們可以看到HashMap繼承了AbstractMap<K,V>，同時實現了Map<K,V>, Cloneable, Serializable。
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable
我們再點擊開AbstractMap的源碼，我會驚奇的發現了，AbstractMap已經實現了Map介面，那麼為什麼HashMap還有再次實現Map介面呢？如果有人問你這個問題，你就打死他，百度了很多資料，據說是當時開發人員寫多了，比較悲催。
<!– 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>
我們接著回到HashMap的源碼中，我們在236行看到 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 也就是說一個map在無參數的情況下被創建出來，默認的大小就是 1<<4 （16）
在248行我們可以看到DEFAULT_LOAD_FACTOR 默認負載因子 0.75， 這個非常重要，在後面的擴容會用到。
HashMap 提供了4個構造方法，可以接收修改初始化大小和負載因子，但是一般情況下就不要去修改了，避免設置得不好性能上出現問題。
MAXIMUM_CAPACITY 最大容量 1 << 30。1左移30位二進位的形勢下就是 0100 0000 0000 0000 0000 0000 0000 0000，這個的意思是2的30次方，十進位下是 1073741824。注釋說了 MUST be a power of two（一定要是2的n次方）， 再多移動一位 1<<31 就變成負數了。
TREEIFY_THRESHOLD，UNTREEIFY_THRESHOLD， MIN_TREEIFY_CAPACITY 這幾個參數是後面當紅黑樹的參數。分別為最大閾值，取消閾值驗證，和最小閾值。
我們再來看一下279行程式碼 static class Node<K,V> implements Map.Entry<K,V> 和396行的transient Node<K,V>[] table 這2個東西就是 HashMap 的本質了。讀到這裡，我們知道了，其實 HashMap 就是一個由 Node 類組成的一個二維數組，Node 是 Map.Entry 的具體實現類。當put一個對象的時候會根據對象的hash值計算出它在數組中存放的位置（通過擾動函數計算，後面會講到），然後判斷這個位置上有沒有已經存在的對象，如果沒有就直接放到這個位置，如果有將已存在對象的next指向當前對象形成一個鏈表，當鏈表長度超過一定數量之後，鏈表會轉換成紅黑樹（這是java8之後的修改，為了提升查詢效率）。所以hashmap本質上是一個二維數組加鏈表加紅黑樹的組合。
我們知道HashMap的內部結構了，那麼我們來看一下他為什麼是執行緒不安全的。我們在上述文章中，可以知道HashMap是鏈表結構的，也就是說總有一個指針指向下一項或上一項。
我們在聲明HashMap時，使用的都是默認的構造方法：HashMap<K,V>，看了程式碼你會發現，它還有其它的構造方法：HashMap(int initialCapacity, float loadFactor)，
其中參數initialCapacity為初始容量，loadFactor為載入因子，擴容就是在put加入元素的個數超過initialCapacity * loadFactor的時候就會將內部Entry數組大小擴大至原來的2倍，然後將數組元素按照新的數組大小重新計算索引，放在新
的數組中，同時修改每個節點的鏈表關係（主要是next和節點在鏈表中的位置）。
概括的來說：HashMap在put的時候，插入的元素超過了容量（由負載因子決定）的範圍就會觸發擴容操作，就是rehash，這個會重新將原數組的內容重新hash到新的擴容數組中，在多執行緒的環境下，存在同時其他的元素也在進行put操作，如果hash值相同，可能出現同時在同一數組下用鏈表表示，造成閉環，導致在get時會出現死循環，所以HashMap是執行緒不安全的。
HashTable就是因為加了synchronized執行緒鎖，而不是因為他們的結構不一致才保證的執行緒安全，HashMap和HashTable都是單向鏈表結構的。
如果我們真的理解了上面所說的一切，我覺得面試官不會不滿意你對於HashMap和Hashtable的面試答案吧，可能會問到一些擴展問題，不如，怎麼才能做到執行緒安全，什麼是擾動函數，紅黑樹又是什麼？下面我來依依作答這些問題。
2、Q：怎麼才能做到執行緒的安全。
A：首先我們應該知道什麼是執行緒，執行緒thread是作業系統能夠進行運算調度的最小單位。它被包含在進程之中，是進程中的實際運作單位。一條執行緒指的是進程中一個單一順序的控制流，一個進程中可以並發多個執行緒，每條執行緒並行執行不同的任務。在Unix System V及SunOS中也被稱為輕量進程，但輕量進程更多指內核執行緒（kernel thread），而我們一般把用戶執行緒（user thread）稱為執行緒。
然後我們應該知道什麼是多執行緒，提到多執行緒這裡要說兩個概念，就是串列和並行，搞清楚這個我們才能更好的理解多執行緒，串列是指，A、B、C三個任務，執行完任務A，才能執行B，B執行完，才能執行C，而並行是指三個任務一起執行。
什麼是執行緒安全，既然是執行緒安全問題，那麼毫無疑問所有的隱患都是出現在多個執行緒訪問的情況下產生的，也就是我們要確保在多條執行緒訪問的時候，我們的程式還能按照我們預期的行為去執行。當多個執行緒訪問某個方法時，不管你通過怎樣的調用方式或者說這些執行緒如何交替的執行，我們在主程式中不需要去做任何的同步，這個類的結果行為都是我們設想的正確行為，那麼我們就可以說這個類時執行緒安全的。那麼我們來不如正題來說下java如何做到執行緒的安全，也可以理解為執行緒的同步。
我們先來看一段程式碼：
package com; public class Demo { static int tickets = 10; class SellTickets implements Runnable { @Override public void run() { // 未加同步時產生臟數據 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() + "--->售票結束！"); } } } public static void main(String[] args) { SellTickets sell = new Demo().new SellTickets(); Thread thread1 = new Thread(sell, "1號窗口"); Thread thread2 = new Thread(sell, "2號窗口"); Thread thread3 = new Thread(sell, "3號窗口"); Thread thread4 = new Thread(sell, "4號窗口"); thread1.start(); thread2.start(); thread3.start(); thread4.start(); } }
我運行可以發現，有重複售票的情況，那麼說明現在的方式是不安全的，我們可以採用以下方法來控制執行緒的安全。
第一種實現執行緒安全的方式，同步程式碼塊
這樣我們就可以保證執行緒的一致性了。
第二種方式Lock鎖機制， 通過創建Lock對象，採用lock()加鎖，unlock()解鎖，來保護指定的程式碼塊
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鎖機制 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() + "--->售票結束！"); } } } public static void main(String[] args) { SellTickets sell = new Demo().new SellTickets(); Thread thread1 = new Thread(sell, "1號窗口"); Thread thread2 = new Thread(sell, "2號窗口"); Thread thread3 = new Thread(sell, "3號窗口"); Thread thread4 = new Thread(sell, "4號窗口"); thread1.start(); thread2.start(); thread3.start(); thread4.start(); } }
總結：由於synchronized是在JVM層面實現的，因此系統可以監控鎖的釋放與否；而ReentrantLock是使用程式碼實現的，系統無法自動釋放鎖，需要在程式碼中的finally子句中顯式釋放鎖lock.unlock()。另外，在並發量比較小的情況下，使用synchronized是個不錯的選擇；但是在並發量比較高的情況下，其性能下降會很嚴重，此時ReentrantLock是個不錯的方案。
補充：在使用synchronized 程式碼塊時,可以與wait()、notify()、nitifyAll()一起使用，從而進一步實現執行緒的通訊。其中，wait()方法會釋放佔有的對象鎖，當前執行緒進入等待池，釋放cpu,而其他正在等待的執行緒即可搶佔此鎖，獲得鎖的執行緒即可運行程式；執行緒的sleep()方法則表示，當前執行緒會休眠一段時間，休眠期間，會暫時釋放cpu，但並不釋放對象鎖，也就是說，在休眠期間，其他執行緒依然無法進入被同步保護的程式碼內部，當前執行緒休眠結束時，會重新獲得cpu執行權,從而執行被同步保護的程式碼。wait()和sleep()最大的不同在於wait()會釋放對象鎖，而sleep()不會釋放對象鎖。notify()方法會喚醒因為調用對象的wait()而處於等待狀態的執行緒，從而使得該執行緒有機會獲取對象鎖。調用notify()後，當前執行緒並不會立即釋放鎖，而是繼續執行當前程式碼，直到synchronized中的程式碼全部執行完畢，才會釋放對象鎖。JVM會在等待的執行緒中調度一個執行緒去獲得對象鎖，執行程式碼。需要注意的是，wait()和notify()必須在synchronized程式碼塊中調用。notifyAll()是喚醒所有等待的執行緒。
說了這麼多，其實就是一個由Map引發的「血案」，其實還有很多很多的擴展問題。明天我們再來繼續討論這些問題。
