Java源碼分析之HashMap

成員變量

//默認的初始容量，空間必須爲2的冪
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;
//默認的加載因子,這裏解釋一下加載因子，在map被創建後，就有了一個容量，在put鍵值對時，會首先算key的hashcode，然後根據hash值來裝進“桶”，加載因子的取值範圍是(0,1]，加載因子乘初始容量，就是能裝的最大值，舉個例子，加入初始容量是16，初始加載因子是0.75，0.75*16=12，現在如果16個桶中已經裝了12個，當再來一個，並且該對應的值還沒有被裝進通，那麼容量就會擴大，變成原來的2倍，加載因子有什麼用呢，就是平衡時間和空間，假如值太小了，那麼在沒有裝幾個的時候，就會擴容，對空間要求比較高，當值太大的時候，衝突會增加的比較多，所以裝的也就更多，對時間要求比較高，所以折中是比較好的選擇
static final float DEFAULT_LOAD_FACTOR = 0.75f;
int threshold;
final float loadFactor;

構造方法

public HashMap(int initialCapacity, float loadFactor) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                           initialCapacity);
    if (initialCapacity > MAXIMUM_CAPACITY)
        initialCapacity = MAXIMUM_CAPACITY;
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal load factor: " +
                                           loadFactor);
    this.loadFactor = loadFactor;
    this.threshold = tableSizeFor(initialCapacity);
}
//返回2的冪
static final int tableSizeFor(int cap) {
    int n = cap - 1;
    n |= n >>> 1;
    n |= n >>> 2;
    n |= n >>> 4;
    n |= n >>> 8;
    n |= n >>> 16;
    return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}

public HashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
public HashMap(Map<? extends K, ? extends V> m) {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    putMapEntries(m, false);
}

Node< K, V>

//通過這個類，來保存鍵值對
static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    V value;
    //鏈表，下個結點
    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; }

    //鍵值對的hash值是將key，value的hash值異或起來
    public final int hashCode() {
        return Objects.hashCode(key) ^ Objects.hashCode(value);
    }

    public final V setValue(V newValue) {
        V oldValue = value;
        value = newValue;
        return oldValue;
    }

    public final boolean equals(Object o) {
        if (o == this)
            return true;
        if (o instanceof Map.Entry) {
            Map.Entry<?,?> e = (Map.Entry<?,?>)o;
            if (Objects.equals(key, e.getKey()) &&
                Objects.equals(value, e.getValue()))
                return true;
        }
        return false;
    }
}

基本的方法

put方法

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

//onlyIfAbsent如果爲真時，如果原map中已經存在了key值的對，那麼不改變原來的對
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    //如果爲空，則創建
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    //如果該桶內爲空，直接裝入
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        //如果存在了和要裝入key值相同的對
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        //如果是TreeNode，插入
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            //遍歷鏈表，準備插入
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    //如果長度大於8，鏈表轉爲紅黑樹
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        //當已經存在key時
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            //如果oldValue爲空，或者onlyIfAbsent 爲false，則覆蓋
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    //是否需要擴容
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
    int s = m.size();
    if (s > 0) {
        //如果不存在，初始化
        if (table == null) { // pre-size
            float ft = ((float)s / loadFactor) + 1.0F;
            int t = ((ft < (float)MAXIMUM_CAPACITY) ?
                     (int)ft : MAXIMUM_CAPACITY);
            if (t > threshold)
                threshold = tableSizeFor(t);
        }
        else if (s > threshold)
            resize();
        //全部put進去，這也給我們了一個遍歷map的模板
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
            K key = e.getKey();
            V value = e.getValue();
            putVal(hash(key), key, value, false, evict);
        }
    }
}

public void putAll(Map<? extends K, ? extends V> m){
    putMapEntries(m, true);
}

get方法

public V get(Object key) {
    Node<K,V> e;
    return (e = getNode(hash(key), key)) == null ? null : e.value;
}

final Node<K,V> getNode(int hash, Object key) {
    Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
    //看對應的桶是否爲空
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (first = tab[(n - 1) & hash]) != null) {
        //先看頭結點
        if (first.hash == hash && // always check first node
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        //若不是頭結點，並且頭結點下面還有其他結點，分紅黑樹，還是鏈表，然後繼續找
        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;
}

//是否包含key
public boolean containsKey(Object key) {
    return getNode(hash(key), key) != null;
}

//查找是否存在value，從全部中找 O(n)
public boolean containsValue(Object value) {
    Node<K,V>[] tab; V v;
    if ((tab = table) != null && size > 0) {
        for (int i = 0; i < tab.length; ++i) {
            for (Node<K,V> e = tab[i]; e != null; e = e.next) {
                if ((v = e.value) == value ||
                    (value != null && value.equals(v)))
                    return true;
            }
        }
    }
    return false;
}

remove方法

public V remove(Object key) {
    Node<K,V> e;
    return (e = removeNode(hash(key), key, null, false, true)) == null ?
        null : e.value;
}

//移除key和value都相等的數，注意，倒數第二個參數是true
public boolean remove(Object key, Object value) {
    return removeNode(hash(key), key, value, true, true) != null;
}

//matchValue表示刪除時候，是否需要value也相等，movable表示是否可以移動
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;
    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;
        //如果在頭部
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            node = p;
        else if ((e = p.next) != null) {
            //如果是紅黑樹
            if (p instanceof TreeNode)
                node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
            else {
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key ||
                         (key != null && key.equals(k)))) {
                        node = e;
                        break;
                    }
                    p = e;
                } while ((e = e.next) != null);
            }
        }
        //找到了，並且看是否需要value也相等，並判斷value是否相等
        if (node != null && (!matchValue || (v = node.value) == value ||
                             (value != null && value.equals(v)))) {
            if (node instanceof TreeNode)
                ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
            //是否在頭部
            else if (node == p)
                tab[index] = node.next;
            else
                p.next = node.next;
            ++modCount;
            --size;
            afterNodeRemoval(node);
            return node;
        }
    }
    return null;
}

size() 與 isEmpty()

public int size() {
   return size;
}

public boolean isEmpty() {
   return size == 0;
}

clear()

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;
    }
}

keySet()

//keySet是繼承自AbstractSet
public Set<K> keySet() {
    Set<K> ks = keySet;
    if (ks == null) {
        ks = new KeySet();
        keySet = ks;
    }
    return ks;
}

//居然叫values，不叫valuesSet，神奇
public Collection<V> values() {
    Collection<V> vs = values;
    if (vs == null) {
        vs = new Values();
        values = vs;
    }
    return vs;
}

//entrySet，可以用來遍歷
public Set<Map.Entry<K,V>> entrySet() {
    Set<Map.Entry<K,V>> es;
    return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
}

replace方法

//先取到Node，再改變，返回舊的值
public V replace(K key, V value) {
    Node<K,V> e;
    if ((e = getNode(hash(key), key)) != null) {
        V oldValue = e.value;
        e.value = value;
        afterNodeAccess(e);
        return oldValue;
    }
    return null;
}

//有舊值的替換
public boolean replace(K key, V oldValue, V newValue) {
    Node<K,V> e; V v;
    if ((e = getNode(hash(key), key)) != null &&
        ((v = e.value) == oldValue || (v != null && v.equals(oldValue)))) {
        e.value = newValue;
        afterNodeAccess(e);
        return true;
    }
    return false;
}

Java源碼分析之HashMap

成員變量

構造方法

Node< K, V>

基本的方法

put方法

remove方法

size() 與 isEmpty()

clear()

keySet()

replace方法

modCount

是爲了檢測多線程中，是否被其他線程修改，雖然如此，但依舊不是線程安全的

除了這些，還有迭代器這些，先不說了，另外，裏面有紅黑樹的實現，可以改日好好研究一下

hdu5875 Function

Codeforces 678E Another Sith Tournament(狀壓dp，概率dp)

hdu5726 && hdu5869

codeforces716c Plus and Square Root

hdu5894 hannnnah_j’s Biological Test

Mac下配置sublime實現LaTeX

https://yachay.unat.edu.pe/blog/index.php?comment_area=format_blog&comment_component=blog&comment_co

linux以太網驅動總結