JAVA开发中有人使用集合吗 java集合实现原理

Java常见集合实现原理

• ArrayList

• add()方法
• remove()
• get()
• size()

• LinkedList

• add()
• remove
• get()

• HashMap

• put(K key, V value)
• get(Object key)
• containsKey

• ConcurrentHashMap
• Hashtable

• put()
• get()

• HashSet

• add()
• remove()
• size()

• StringBuilder

• append(String str)
• delete(int start,int end)

• StringBuffer

ArrayList

private static final long serialVersionUID = 8683452581122892189L;

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    transient Object[] elementData; // non-private to simplify nested class access


    private int size;	//ArrayList元素个数

    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

上面定义了一些常量.
可以看到ArrayList使用Object[] elementData对象数组来存放数据.size属性记录ArrayList的大小.
上面定义了两个构造函数,无参构造函数将数组长度设置为0;ArrayList(int initialCapacity)创建一个长度为initialCapacity的数组并赋给elementData.

add()方法

public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

调用了ensureCapacityInternal()方法来确保容量足够,size++将ArrayList大小加一,再将要添加的e放进数组.

ensureCapacityInternal():

private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

minCapacity表示至少需要的数组容量,如果elementData 为{},表示还没有添加过数据,那么设置数组容量为10.
接着调用ensureExplicitCapacity()方法.

ensureExplicitCapacity():

private void ensureExplicitCapacity(int minCapacity) {
        modCount++;//修改次数

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

modCount是ArrayList父类的属性,记录链表修改次数,这里先不管.

如果minCapacity >elementData.length表示最小需要长度大于现在的长度,需要进行扩容.调用grow()方法.

grow():

private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);//扩容为原容量的1.5倍
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

将数组扩容为1.5倍.
elementData = Arrays.copyOf(elementData, newCapacity)将复制并将elementData 长度设置为newCapacity.

总结:当ArrayList没有元素时,此时add()添加第一个元素,elementData长度将扩容为10,当继续添加元素直到elementData长度不够时,将扩容为原来的1.5倍.

remove()

public E remove(int index) {
        rangeCheck(index);

        modCount++;	//修改次数.
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

首先进行范围判断.
将被删除元素位置后面的元素前移,设置最后一个为null使其被GC回收,再将size减一.

get()

public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

调用rangeCheck()判断是否越界,调用elementData()获取值.

@SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

直接通过数组访问.

size()

直接返回size属性
判断是否为空直接size是否为0.

public int size() {
        return size;
    }

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

LinkedList

双向链表实现.

Node节点:
作为静态内部类

private static class Node<E> {
        E item;
        Node<E> next;
        Node<E> prev;

        Node(Node<E> prev, E element, Node<E> next) {
            this.item = element;
            this.next = next;
            this.prev = prev;
        }
    }

有指向前一个元素和后一个元素的引用.item保存数据.

属性:

transient int size = 0;

/**
 * Pointer to first node.
 * Invariant: (first == null && last == null) ||
 *            (first.prev == null && first.item != null)
 */
transient Node<E> first;

/**
 * Pointer to last node.
 * Invariant: (first == null && last == null) ||
 *            (last.next == null && last.item != null)
 */
transient Node<E> last;

size保存LinkedList大小,first保存头节点,last保存最后节点.

add()

public boolean add(E e) {
        linkLast(e);
        return true;
    }

调用linkLast()方法在尾部插入:

void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)	//如果链表为空则插入到第一个元素
            first = newNode;
        else
            l.next = newNode;	//在最后插入
        size++;			//大小增一
        modCount++;
    }

remove

public boolean remove(Object o) {
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

使用for循环找到要删除的节点,再调用unlink()移除:

E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

        if (prev == null) {//如果前一个为null,表示要删除第一个节点,直接将first指向next
            first = next;
        } else {//将前一个节点的next指向下一个节点,x的prev置空
            prev.next = next;
            x.prev = null;
        }

        if (next == null) {//如果被删是尾节点,更新last为前一个节点
            last = prev;
        } else {//将后一个节点的prev指向前一个节点,x的next置空
            next.prev = prev;
            x.next = null;
        }

        x.item = null;	//item值空,让GC回收.
        size--;
        modCount++;
        return element;
    }

get()

public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }

先调用checkElementIndex()检测是否越界:

private void checkElementIndex(int index) {
        if (!isElementIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

再调用node()获取值:

Node<E> node(int index) {
        // assert isElementIndex(index);

        if (index < (size >> 1)) {//如果在左半边,从左往右找
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {//如果在右半边,从右往左找
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }

HashMap

采用数组+链表+红黑树实现.当链表长度超过8时使用红黑树储存.

常量:

//默认容量16
	static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * 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.
     */
     //最大容量2^30
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     */
     
     //默认装载因子75%
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * 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.
     */
	//转为红黑树的阈值
    static final int TREEIFY_THRESHOLD = 8;

    /**
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     */
     
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     * The smallest table capacity for which bins may be treeified.
     * (Otherwise the table is resized if too many nodes in a bin.)
     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
     * between resizing and treeification thresholds.
     */
	/*当需要将解决 hash 冲突的链表转变为红黑树时，需要判断下此时数组容量，若是由于数组
	容量太小（小于　MIN_TREEIFY_CAPACITY　）导致的 hash 冲突太多，则不进行链表转变为
	红黑树操作，转为利用　resize() 函数对　hashMap 扩容　*/
    static final int MIN_TREEIFY_CAPACITY = 64;

链表节点Node:
作为内部类.

是一个单向链表.

/**
     * 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;	//保存hash码
        final K key;	//保存key
        V value;		//保存value
        Node<K,V> next;		//下一个节点

        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;
        }

        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;
        }
    }

属性:

/* ---------------- Fields -------------- */

    /**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     */
    transient Node<K,V>[] table;

    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     */
    transient Set<Map.Entry<K,V>> entrySet;

    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient int modCount;

    /**
     * The next size value at which to resize (capacity * load factor).
     *
     * @serial
     */
    // (The javadoc description is true upon serialization.
    // Additionally, if the table array has not been allocated, this
    // field holds the initial array capacity, or zero signifying
    // DEFAULT_INITIAL_CAPACITY.)
    int threshold;	//粗糙理解为数组容量*装载因子

    /**
     * The load factor for the hash table.
     *
     * @serial
     */
    final float loadFactor;

有一个Node数组table。loadFactor为装填因子.threshold是在数组容量和装载因子确定的情况下,如果存放的数据个数(size)超过此值数组就进行扩容,扩容为原来的2倍,避免频繁的哈希冲突.

红黑树节点:

500多行代码,还是有点复杂.

static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
        TreeNode<K,V> parent;  // 父节点
        TreeNode<K,V> left;		//左子树
        TreeNode<K,V> right;	//右子树
        TreeNode<K,V> prev;    // 上一节点
        boolean red;
        TreeNode(int hash, K key, V val, Node<K,V> next) {
            super(hash, key, val, next);
        }
		//......
}

属性:

/* ---------------- Fields -------------- */

    /**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     */
    transient Node<K,V>[] table;	//链表节点数组

    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     */
    transient Set<Map.Entry<K,V>> entrySet;

    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;		

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient int modCount;		//修改次数

    /**
     * The next size value at which to resize (capacity * load factor).
     *
     * @serial
     */
    // (The javadoc description is true upon serialization.
    // Additionally, if the table array has not been allocated, this
    // field holds the initial array capacity, or zero signifying
    // DEFAULT_INITIAL_CAPACITY.)
    int threshold;

    /**
     * The load factor for the hash table.
     *
     * @serial
     */
    final float loadFactor;	//装填因子

数组类型为Node.

构造函数:

/**
     * 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) {
        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);
    }

    /**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and the default load factor (0.75).
     *
     * @param  initialCapacity the initial capacity.
     * @throws IllegalArgumentException if the initial capacity is negative.
     */
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    /**
     * 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
    }

这些构造函数都是做一些初始化工作，设定装载因子（默认75%）、初始容量大小（默认16）等，但没有创建数组。

put(K key, V value)

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

对key求hash值，再调用putVal（）方法。
hash（）：

static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

获取key的hashCode值、高位运算.

可以看到这里使用了Object的hashCode()方法,所以我们用自定义类做HashMap的键时要重写hashCode()方法,如果使用默认的不同的对象会得到相同的hashCode.

putVal（）：

/**
     * 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) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        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;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

如果table为空，则先调用resize()调整数组大小.
先来看一下resize():

/**
     * 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) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            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
            newCap = DEFAULT_INITIAL_CAPACITY;
            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;
    }

初始化或加倍table数组.

初始化table数组:
当原threhold为0,即原数组为空时,执行:

else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }

之后执行:

threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;

创建初始容量大小(默认16)的数组并更新threhold值.
这样就完成了数组的初始化.

加倍:
如果oldCap > 0表示原数组中有元素,已经初始化过了,执行代码:

if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }

将数组大小增加为原来的2倍,同样threhold也变为2倍.

每次扩容为原来的2倍,保证了数组长度为2的n次方,这种设计主要是为了在取模和扩容时做优化，同时为了减少冲突.

再回来看putVal()方法,

if ((p = tab[i = (n - 1) & hash]) == null)
     tab[i] = newNode(hash, key, value, null);

(n - 1) & hash其实是对hash取模操作,找到key对应数组索引位置,如果数组该位置为null,表示还没有在该位置插入过元素,直接在该位置创建节点来存放key和value值.

接下来看看数组该位置已存在节点的情况:

Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;

p.hash == hash表示hash冲突了,此时看一下第一个节点key是否相同,如果相同即(k = p.key) == key或者key.equals(k)则直接用新的value替换旧的value.

如果第一个key不相同先判断一下是否是红黑树节点,如果是则调用红黑树的插入方法.

else if (p instanceof TreeNode)
      e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);

如果又不是红黑树,那就是链表

else {
	for (int binCount = 0; ; ++binCount) {
	    if ((e = p.next) == null) {	//链表到了尾节点
	        p.next = newNode(hash, key, value, null);//在尾节点后面插入
	        if (binCount >= TREEIFY_THRESHOLD - 1) // 链表长度大于8则转为红黑树
	            treeifyBin(tab, hash);
	        break;
	    }
	    if (e.hash == hash &&
	        ((k = e.key) == key || (key != null && key.equals(k))))
	        break;		//链表中的key与要插入的key相同,则插入成功或已存在key,退出循环
	    p = e;	//指向下一个节点
	}
}
if (e != null) { // existing mapping for key 如果已存在key且不是新插入的
    V oldValue = e.value;
    if (!onlyIfAbsent || oldValue == null)
        e.value = value;		//更新value值
    afterNodeAccess(e);
    return oldValue;
}

get(Object key)

可以看出来在底层泛型还是被转换成了Object.

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

先计算key的hash值,再调用了getNode()方法:

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) {//数组对应位置的首节点不为空
         if (first.hash == hash && // always check first node
             ((k = first.key) == key || (key != null && key.equals(k))))
             return first;//首节点就是要找的节点,直接返回
         if ((e = first.next) != null) {
             if (first instanceof TreeNode)	//如果是红黑树,调用红黑树的get
                 return ((TreeNode<K,V>)first).getTreeNode(hash, key);
             do {	//如果是链表则遍历
                 if (e.hash == hash &&
                     ((k = e.key) == key || (key != null && key.equals(k))))
                     return e;
             } while ((e = e.next) != null);
         }
     }
     return null;
 }

containsKey

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

直接调用getNode()方法,如果获取vlaue非空则包括,否则不包含.

ConcurrentHashMap

线程安全的Map,也是用数组+链表+红黑树实现.

采用CAS算法保证线程安全,采用乐观锁而没有使用synchronized(悲观锁),所以效率比Hashtable高.

乐观锁适用于读多写少的情况.

Hashtable

Hashtable的所有方法都是synchronized,所以是线程安全的.

属性:

/**
     * The hash table data.
     */
    private transient Entry<?,?>[] table;

    /**
     * The total number of entries in the hash table.
     */
    private transient int count;

    /**
     * The table is rehashed when its size exceeds this threshold.  (The
     * value of this field is (int)(capacity * loadFactor).)
     *
     * @serial
     */
    private int threshold;

    /**
     * The load factor for the hashtable.
     *
     * @serial
     */
    private float loadFactor;

    /**
     * The number of times this Hashtable has been structurally modified
     * Structural modifications are those that change the number of entries in
     * the Hashtable or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the Hashtable fail-fast.  (See ConcurrentModificationException).
     */
    private transient int modCount = 0;

这里的数组类型是Entry<?,?>.

构造方法:

public Hashtable(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal Load: "+loadFactor);

        if (initialCapacity==0)
            initialCapacity = 1;
        this.loadFactor = loadFactor;
        table = new Entry<?,?>[initialCapacity];
        threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
    }

    /**
     * Constructs a new, empty hashtable with the specified initial capacity
     * and default load factor (0.75).
     *
     * @param     initialCapacity   the initial capacity of the hashtable.
     * @exception IllegalArgumentException if the initial capacity is less
     *              than zero.
     */
    public Hashtable(int initialCapacity) {
        this(initialCapacity, 0.75f);
    }

    /**
     * Constructs a new, empty hashtable with a default initial capacity (11)
     * and load factor (0.75).
     */
    public Hashtable() {
        this(11, 0.75f);
    }

可以看到默认的初始容量为11,装载因子为75%.并且在创建Hashtable时就创建设置,这里和HashMap有点不一样.

put()

貌似只用了数组+链表,没有使用红黑树.

public synchronized V put(K key, V value) {
        // Make sure the value is not null
        if (value == null) {
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry<?,?> tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
        @SuppressWarnings("unchecked")
        Entry<K,V> entry = (Entry<K,V>)tab[index];
        for(; entry != null ; entry = entry.next) {
            if ((entry.hash == hash) && entry.key.equals(key)) {//如果已存在key则更新
                V old = entry.value;
                entry.value = value;
                return old;
            }
        }

        addEntry(hash, key, value, index);//如果不存在key则调用addEntry()插入
        return null;
    }

addEntry():

private void addEntry(int hash, K key, V value, int index) {
        modCount++;

        Entry<?,?> tab[] = table;
        if (count >= threshold) {//如果需要扩容则调用rehash进行扩容
            // Rehash the table if the threshold is exceeded
            rehash();

            tab = table;
            hash = key.hashCode();
            index = (hash & 0x7FFFFFFF) % tab.length;
        }

        // Creates the new entry.
        @SuppressWarnings("unchecked")
        Entry<K,V> e = (Entry<K,V>) tab[index];
        tab[index] = new Entry<>(hash, key, value, e);//插入,链表的头插法
        count++;
    }

rehash():

@SuppressWarnings("unchecked")
    protected void rehash() {
        int oldCapacity = table.length;
        Entry<?,?>[] oldMap = table;

        // overflow-conscious code
        int newCapacity = (oldCapacity << 1) + 1;//新容量为原来的2倍加1
        if (newCapacity - MAX_ARRAY_SIZE > 0) {
            if (oldCapacity == MAX_ARRAY_SIZE)
                // Keep running with MAX_ARRAY_SIZE buckets
                return;
            newCapacity = MAX_ARRAY_SIZE;
        }
        Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];//创建新数组

        modCount++;
        threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
        table = newMap;

        for (int i = oldCapacity ; i-- > 0 ;) {//转移原值到新数组
            for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
                Entry<K,V> e = old;
                old = old.next;

                int index = (e.hash & 0x7FFFFFFF) % newCapacity;
                e.next = (Entry<K,V>)newMap[index];
                newMap[index] = e;
            }
        }
    }

这里扩容为原来的2倍加一.

get()

@SuppressWarnings("unchecked")
    public synchronized V get(Object key) {
        Entry<?,?> tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {//遍历链表
            if ((e.hash == hash) && e.key.equals(key)) {
                return (V)e.value;//找到key则返回值
            }
        }
        return null;
    }

HashSet

使用HashMap来实现,利用HashMap没有重复key的特性.

属性:

private transient HashMap<E,Object> map;

    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();//用于插入map的value.

构造函数:
调用HashMap的构造函数.

public HashSet() {
        map = new HashMap<>();
    }

    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * the specified initial capacity and the specified load factor.
     *
     * @param      initialCapacity   the initial capacity of the hash map
     * @param      loadFactor        the load factor of the hash map
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero, or if the load factor is nonpositive
     */
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * the specified initial capacity and default load factor (0.75).
     *
     * @param      initialCapacity   the initial capacity of the hash table
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero
     */
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

    /**
     * Constructs a new, empty linked hash set.  (This package private
     * constructor is only used by LinkedHashSet.) The backing
     * HashMap instance is a LinkedHashMap with the specified initial
     * capacity and the specified load factor.
     *
     * @param      initialCapacity   the initial capacity of the hash map
     * @param      loadFactor        the load factor of the hash map
     * @param      dummy             ignored (distinguishes this
     *             constructor from other int, float constructor.)
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero, or if the load factor is nonpositive
     */
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

add()

public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

map.put()插入后返回的是原来的旧值,如果map没有key,则插入后返回的是null,表示插入成功;如果原来有key则put返回的旧值不是null,表示key已存在,Set的add失败,返回false.

remove()

public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

调用map移除key.

size()

public int size() {
        return map.size();
    }

StringBuilder

字符串要想加上字符串,String的话把字符串当成常量放在常量池,要加上字符串就要新构造一个新的字符串,所以效率较低.StringBuiler添加操作是在同一个对象上操作.

线程不安全,要想线程安全应使用StringBuffer

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{
	//......
}

继承自AbstractStringBuilder,先来看一下父类:

abstract class AbstractStringBuilder implements Appendable, CharSequence {
    /**
     * The value is used for character storage.
     */
    char[] value;

    /**
     * The count is the number of characters used.
     */
    int count;

    /**
     * This no-arg constructor is necessary for serialization of subclasses.
     */
    AbstractStringBuilder() {
    }

    /**
     * Creates an AbstractStringBuilder of the specified capacity.
     */
    AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }


    @Override
    public int length() {
        return count;
    }

使用char[] value来保存字符串.count记录字符串的长度.

有参构造函数创建长度为指定大小的数组.

StringBuilder构造函数:

public StringBuilder() {
        super(16);
    }

    /**
     * Constructs a string builder with no characters in it and an
     * initial capacity specified by the {@code capacity} argument.
     *
     * @param      capacity  the initial capacity.
     * @throws     NegativeArraySizeException  if the {@code capacity}
     *               argument is less than {@code 0}.
     */
    public StringBuilder(int capacity) {
        super(capacity);
    }

    /**
     * Constructs a string builder initialized to the contents of the
     * specified string. The initial capacity of the string builder is
     * {@code 16} plus the length of the string argument.
     *
     * @param   str   the initial contents of the buffer.
     */
    public StringBuilder(String str) {
        super(str.length() + 16);
        append(str);
    }

无参构造函数将设置数组大小为16,StringBuilder(String str)也会设置长度多16.

append(String str)

@Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }

调用了父类的append.

public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

先调用ensureCapacityInternal()来确保value数组的长度足够来添加str,再使用str.getChars()方法将str复制到value数组后面,就实现了添加.

ensureCapacityInternal():

private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        if (minimumCapacity - value.length > 0) {
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }

如果minimumCapacity > value.length表示容量不够了,需要扩容,调用newCapacity(minimumCapacity)计算扩容后的容量Arrays.copyOf()复制并扩容.

newCapacity()方法:

private int newCapacity(int minCapacity) {
        // overflow-conscious code
        int newCapacity = (value.length << 1) + 2;
        if (newCapacity - minCapacity < 0) {
            newCapacity = minCapacity;
        }
        return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
            ? hugeCapacity(minCapacity)
            : newCapacity;
    }

(value.length << 1) + 2将扩容为原来的两倍再加2.

delete(int start,int end)

public AbstractStringBuilder delete(int start, int end) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (end > count)
            end = count;
        if (start > end)
            throw new StringIndexOutOfBoundsException();
        int len = end - start;
        if (len > 0) {
            System.arraycopy(value, start+len, value, start, count-end);
            count -= len;
        }
        return this;
    }

将要删除的段后面的前移.

StringBuffer

StringBuffer和StringBuilder差不多,只不过所有方法加上了synchronized,所以是线程安全的,但效率就变慢了.