/** * Constructs an empty <tt>HashMap</tt> with the default initial capacity * (16) and the default load factor (0.75). */ public HashMap() { this.loadFactor = DEFAulT_LOAD_FACTOR; // all other fIElds defaulted } /** * The maximum capacity, used if a higher value is implicitly specifIEd * by either of the constructors with arguments. * MUST be a power of two <= 1<<30. */ static final int MAXIMUM_CAPACITY = 1 << 30; /** * Constructs an empty <tt>HashMap</tt> with the specifIEd initial * capacity and load factor. * * @param initialCapacity the initial capacity * @param loadFactor the load factor * @throws IllegalArgumentException if the initial capacity is negative * or the load factor is nonpositive */ public HashMap(int initialCapacity, float loadFactor) { //小于0走异常处理。 if (initialCapacity < 0) throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity); //最大不能超过1左移30位，也就是2的30次方，非常大的数。 if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; //如果扩容因子小于0，或者不是浮点数，报异常处理。 if (loadFactor <= 0 || float.isNaN(loadFactor)) throw new IllegalArgumentException("Illegal load factor: " + loadFactor); this.loadFactor = loadFactor; this.threshold = tableSizefor(initialCapacity); } /** * Constructs a new <tt>HashMap</tt> with the same mapPings as the * specifIEd <tt>Map</tt>. The <tt>HashMap</tt> is created with * default load factor (0.75) and an initial capacity sufficIEnt to * hold the mapPings in the specifIEd <tt>Map</tt>. * * @param m the map whose mapPings are to be placed in this map * @throws NullPointerException if the specifIEd map is null */ public HashMap(Map<? extends K, ? extends V> m) { this.loadFactor = DEFAulT_LOAD_FACTOR; putMapEntrIEs(m, false); } /** * Associates the specifIEd value with the specifIEd key in this map. * If the map prevIoUsly contained a mapPing for the key, the old * value is replaced. * * @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>.) */ public V put(K key, V value) { return putVal(hash(key), key, value, false, true); } /** * Computes key.hashCode() and spreads (XORs) higher bits of hash * to lower. Because the table uses power-of-two masking, sets of * hashes that vary only in bits above the current mask will * always collIDe. (Among kNown examples are sets of float keys * holding consecutive whole numbers in small tables.) So we * apply a transform that spreads the impact of higher bits * downward. There is a tradeoff between speed, utility, and * quality of bit-spreading. Because many common sets of hashes * are already reasonably distributed (so don't benefit from * spreading), and because we use trees to handle large sets of * collisions in bins, we just XOR some shifted bits in the * cheapest possible way to reduce systematic lossage, as well as * to incorporate impact of the highest bits that would otherwise * never be used in index calculations because of table bounds. */ static final int hash(Object key) { int h; //如果key为空，就返回0 //否者key的哈希码 异或 key无符号右移16位的结果 return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); } /** * Basic hash bin node, used for most entrIEs. (See below for * TreeNode subclass, and in linkedHashMap for its Entry subclass.) */ static class Node<K,V> implements Map.Entry<K,V> { final int hash; final K key; V value; //单链表结构 Node<K,V> next; Node(int hash, K key, V value, Node<K,V> next) { this.hash = hash; this.key = key; this.value = value; this.next = next; } public final K getKey() { return key; } public final V getValue() { return value; } public final String toString() { return key + "=" + value; } public final int hashCode() { return Objects.hashCode(key) ^ Objects.hashCode(value); } public final V setValue(V newValue) { V oldValue = value; value = newValue; return oldValue; } //重写equals public final boolean equals(Object o) { if (o == this) return true; if (o instanceof Map.Entry) { Map.Entry<?,?> e = (Map.Entry<?,?>)o; //对比key和value if (Objects.equals(key, e.getKey()) && Objects.equals(value, e.getValue())) return true; } return false; } }这个方法看到Node<K,V>[]，HashMap的结构就能理解了吧。
HashMap为了解决散列冲突，就用了链表法。
存放数据的链表，在长度在 8以下 的时候，是 链表 存储，8以上 或者数组长度大于64时就红黑树存储。
由于方法细节太多，直接写注释一步步好理解。
/** * The bin count threshold for using a tree rather than List for a * bin. Bins are converted to trees when adding an element to a * bin with at least this many nodes. The value must be greater * than 2 and should be at least 8 to mesh with assumptions in * tree removal about conversion back to plain bins upon * shrinkage. */ static final int TREEIFY_THRESHolD = 8; /** * Implements Map.put and related methods * * @param hash hash for key * @param key the key * @param value the value to put * @param onlyIfAbsent if true, don't change existing value * @param evict if false, the table is in creation mode. * @return prevIoUs value, or null if none */ final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { //分别是散列表，节点，散列表长度，索引位置 Node<K,V>[] tab; Node<K,V> p; int n, i; //如果散列表为空或者散列表长度为0 if ((tab = table) == null || (n = tab.length) == 0) //resize()是创建哈希表，长度为16，并且将长度赋值给n n = (tab = resize()).length; //找到hash值在当前哈希表中的位置，该位置的节点赋值给p，且判断该位置是否为空 if ((p = tab[i = (n - 1) & hash]) == null) //如果为空就把这个元素放在这个位置 tab[i] = newNode(hash, key, value, null); else { Node<K,V> e; K k; //如果hash值相同，key也相同 if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) //将节点p赋值给e e = p; //如果p是树节点 else if (p instanceof TreeNode) //创建一个树节点赋值给e e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); else { //链表节点，就遍历链表 for (int binCount = 0; ; ++binCount) { //将p的下一节点赋值给e，且为空 if ((e = p.next) == null) { //找到链表尾部，插入新的节点 p.next = newNode(hash, key, value, null); //如果链表的长度大于8的时候 if (binCount >= TREEIFY_THRESHolD - 1) // -1 for 1st //链表转树结构 treeifyBin(tab, hash); break; } //遍历到的位置已经有元素了，这里就是遍历链表一直循环next，直到为空停止 if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) break; p = e; } } //如果当前节点不为空，前面的 *** 作除了最后一个else,其他就是找到已存在的节点 if (e != null) { // existing mapPing for key //当前节点的值赋值给oldValue V oldValue = e.value; //如果不修改值，或者oldValue 为空（存储value为空） if (!onlyIfAbsent || oldValue == null) //就修改当前节点的值 e.value = value; afterNodeAccess(e); //修改值，在这return,不再增加数据的长度 return oldValue; } } ++modCount; //添加好元素，长度+1 if (++size > threshold) //扩容 *** 作 resize(); afterNodeInsertion(evict); return null; } /** * Replaces all linked nodes in bin at index for given hash unless * table is too small, in which case resizes instead. */ final voID treeifyBin(Node<K,V>[] tab, int hash) { int n, index; Node<K,V> e; //如果哈希表为空，或者哈希表长度小于64，优先扩容，而不是转为红黑树 if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY) //扩容 resize(); //否则判断不为空就转为树节点 else if ((e = tab[index = (n - 1) & hash]) != null) { TreeNode<K,V> hd = null, tl = null; do { TreeNode<K,V> p = replacementTreeNode(e, null); if (tl == null) hd = p; else { p.prev = tl; tl.next = p; } tl = p; } while ((e = e.next) != null); if ((tab[index] = hd) != null) hd.treeify(tab); } } /** * Initializes or doubles table size. If null, allocates in * accord with initial capacity target held in fIEld threshold. * Otherwise, because we are using power-of-two expansion, the * elements from each bin must either stay at same index, or move * with a power of two offset in the new table. * * @return the table */ final Node<K,V>[] resize() { Node<K,V>[] oldTab = table; int oldCap = (oldTab == null) ? 0 : oldTab.length; int oldThr = threshold; int newCap, newThr = 0; if (oldCap > 0) { //最大不能超过Integer.MAX_VALUE if (oldCap >= MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return oldTab; } //扩容为原来的2倍 else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAulT_INITIAL_CAPACITY) newThr = oldThr << 1; // double threshold } else if (oldThr > 0) // initial capacity was placed in threshold newCap = oldThr; else { // zero initial threshold signifIEs using defaults //创建默认大小，长度16 newCap = DEFAulT_INITIAL_CAPACITY; //阈值0.75 * 16 newThr = (int)(DEFAulT_LOAD_FACTOR * DEFAulT_INITIAL_CAPACITY); } if (newThr == 0) { float ft = (float)newCap * loadFactor; newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ? (int)ft : Integer.MAX_VALUE); } threshold = newThr; @SuppressWarnings({"rawtypes","unchecked"}) Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; table = newTab; if (oldTab != null) { for (int j = 0; j < oldCap; ++j) { Node<K,V> e; if ((e = oldTab[j]) != null) { oldTab[j] = null; if (e.next == null) newTab[e.hash & (newCap - 1)] = e; else if (e instanceof TreeNode) ((TreeNode<K,V>)e).split(this, newTab, j, oldCap); else { // preserve order Node<K,V> lohead = null, loTail = null; Node<K,V> hihead = null, hiTail = null; Node<K,V> next; do { next = e.next; if ((e.hash & oldCap) == 0) { if (loTail == null) lohead = e; else loTail.next = e; loTail = e; } else { if (hiTail == null) hihead = e; else hiTail.next = e; hiTail = e; } } while ((e = next) != null); if (loTail != null) { loTail.next = null; newTab[j] = lohead; } if (hiTail != null) { hiTail.next = null; newTab[j + oldCap] = hihead; } } } } } return newTab; } /** * 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>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. * * @see #put(Object, Object) */ public V get(Object key) { Node<K,V> e; return (e = getNode(hash(key), key)) == null ? null : e.value; } /** * Implements Map.get and related methods * * @param hash hash for key * @param key the key * @return the node, or null if none */ final Node<K,V> getNode(int hash, Object key) { Node<K,V>[] tab; Node<K,V> first, e; int n; K k; if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) { if (first.hash == hash && // always check first node ((k = first.key) == key || (key != null && key.equals(k)))) return first; if ((e = first.next) != null) { if (first instanceof TreeNode) return ((TreeNode<K,V>)first).getTreeNode(hash, key); do { if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } while ((e = e.next) != null); } } return null; } /** * Removes the mapPing for the specifIEd key from this map if present. * * @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>. * (A <tt>null</tt> return can also indicate that the map * prevIoUsly associated <tt>null</tt> with <tt>key</tt>.) */ public V remove(Object key) { Node<K,V> e; return (e = removeNode(hash(key), key, null, false, true)) == null ? null : e.value; } /** * Implements Map.remove and related methods * * @param hash hash for key * @param key the key * @param value the value to match if matchValue, else ignored * @param matchValue if true only remove if value is equal * @param movable if false do not move other nodes while removing * @return the node, or null if none */ final Node<K,V> removeNode(int hash, Object key, Object value, boolean matchValue, boolean movable) { Node<K,V>[] tab; Node<K,V> p; int n, index; //如果散列表不为空且hash对应位置有元素，找到赋值给p if ((tab = table) != null && (n = tab.length) > 0 && (p = tab[index = (n - 1) & hash]) != null) { Node<K,V> node = null, e; K k; V v; //如果p就是要找的元素，赋值给node if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) node = p; //如果p不是要找的元素 else if ((e = p.next) != null) { //如果是树节点，遍历树，找到节点赋值给node if (p instanceof TreeNode) node = ((TreeNode<K,V>)p).getTreeNode(hash, key); else { //如果是链表，就不停的next，找节点赋值给node do { if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) { node = e; break; } p = e; } while ((e = e.next) != null); } } //如果找的节点不为空，且value值符合 if (node != null && (!matchValue || (v = node.value) == value || (value != null && value.equals(v)))) { //树节点就用树的删除 if (node instanceof TreeNode) ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable); //链表只有一个的情况next为空，相当于置空 else if (node == p) tab[index] = node.next; //链表后面有元素，直接指向next else p.next = node.next; ++modCount; --size; afterNodeRemoval(node); return node; } } return null; } /** * Removes all of the mapPings from this map. * The map will be empty after this call returns. */ public voID clear() { Node<K,V>[] tab; modCount++; if ((tab = table) != null && size > 0) { size = 0; for (int i = 0; i < tab.length; ++i) tab[i] = null; } } /** * Returns the number of key-value mapPings in this map. * * @return the number of key-value mapPings in this map */ public int size() { return size; } /** * 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 */ public Set<Map.Entry<K,V>> entrySet() { Set<Map.Entry<K,V>> es; return (es = entrySet) == null ? (entrySet = new EntrySet()) : es; }HashMap<String, String> hashMap = new HashMap<>();Iterator iterator = hashMap.entrySet().iterator();while (iterator.hasNext()) { Map.Entry<String, String> entry = (Map.Entry<String, String>) iterator.next(); String key = entry.getKey(); String value = entry.getValue();} /** * 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 */ public Set<K> keySet() { Set<K> ks = keySet; if (ks == null) { ks = new KeySet(); keySet = ks; } return ks; }HashMap<String, String> hashMap = new HashMap<>();Iterator iterator = hashMap.keySet().iterator();while (iterator.hasNext()){ String key = (String) iterator.next(); String value = hashMap.get(key);}最后，如果有错误，欢迎大家指出，我会继续学习修改，谢谢~~
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