JDK源码之Vector与HashSet解析

Vector简介

ArrayList 和 Vector 其实大同小异，基本结构都差不多，但是一些细节上有区别：比如线程安全与否，扩容的大小等，Vector的线程安全通过在方法上直接加synchronized实现。扩容默认扩大为原来的2倍。

继承体系

从图中我们可以看出：Vector继承了AbstractList，实现了List，RandomAccess，Cloneable，Serializable接口，因此Vector支持快速随机访问，可以被克隆，支持序列化。

Vector的成员变量(属性)

// Object类型的数组
// 注意：访问修饰符有所不同，Vector用protected修饰，而ArrayList用private修饰。
// JavaSe中：private变量只能被当前类的方法访问，而protected可以被同一包中的所有类和其他包的子类访问
protected Object[] elementData;
// 动态数组的实际有效大小，即数组中存储的元素个数
protected int elementCount;
// 动态数组的增长系数：若开始事先没有指定，则默认是增加一倍的大小
protected int capacityIncrement;
// 序列版本号
private static final long serialVersionUID = -2767605614048989439L;

Vector的构造函数

Vector的构造函数有四个：

// 默认空参构造函数
public Vector() {
    // 调用指定初始容量的构造函数，初始容量为10
    this(10);
}
// 可以指定初始容量的构造函数
public Vector(int initialCapacity) {
    // 调用指定初始容量和增长系数的构造函数，增长系数设置为0
    this(initialCapacity, 0);
}
// 可以指定初始容量和增长系数的构造函数
public Vector(int initialCapacity, int capacityIncrement) {
    super();
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    // 根据初始容量创建一个Object类型的数组
    this.elementData = new Object[initialCapacity];
    // 给增长系数赋值
    this.capacityIncrement = capacityIncrement;
}
// 参数为集合类型的构造函数
public Vector(Collection<? extends E> c) {
    elementData = c.toArray();
    elementCount = elementData.length;
    // c.toArray might (incorrectly) not return Object[] (see 6260652)
    if (elementData.getClass() != Object[].class)
        // 将参数集合c 中的数据拷贝到elementData
        elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
}

Vector成员方法

get方法

// 获得指定下标的元素数据
public synchronized E get(int index) {
    if (index >= elementCount)
        throw new ArrayIndexOutOfBoundsException(index);
    return elementData(index);
}
@SuppressWarnings("unchecked")
E elementData(int index) {
    return (E) elementData[index];
}

set方法

// 修改指定下标的元素数据
public synchronized E set(int index, E element) {
    if (index >= elementCount)
        throw new ArrayIndexOutOfBoundsException(index);
    E oldValue = elementData(index);
    elementData[index] = element;
    return oldValue;
}

remove方法

// 删除某个元素数据
public boolean remove(Object o) {
    return removeElement(o);
}
//
public synchronized boolean removeElement(Object obj) {
    modCount++;
    // 找到指定元素的下标
    int i = indexOf(obj);
    if (i >= 0) {
        // 根据下标删除元素
        removeElementAt(i);
        return true;
    }
    return false;
}
// 根据下标删除元素
public synchronized void removeElementAt(int index) {
    modCount++;
    if (index >= elementCount) {
        throw new ArrayIndexOutOfBoundsException(index + " >= " +
                                                 elementCount);
    }
    else if (index < 0) {
        throw new ArrayIndexOutOfBoundsException(index);
    }
    // index之后的有效元素数量
    int j = elementCount - index - 1;
    if (j > 0) {
        // 旧数组替换新数组
        System.arraycopy(elementData, index + 1, elementData, index, j);
    }
    // 有效元素数量--
    elementCount--;
    elementData[elementCount] = null; /* to let gc do its work */
}

add方法

// 在数组末尾添加指定元素
public synchronized boolean add(E e) {
    modCount++;
    // 判断是否需要扩容
    ensureCapacityHelper(elementCount + 1);
    elementData[elementCount++] = e;
    return true;
}

其他方法

// 将数组Vector中的全部元素都拷贝到数组anArray中去，调用本地方法arraycopy
public synchronized void copyInto(Object[] anArray) {
    System.arraycopy(elementData, 0, anArray, 0, elementCount);
}
public synchronized void trimToSize() {
    modCount++;
    int oldCapacity = elementData.length;
    if (elementCount < oldCapacity) {
        elementData = Arrays.copyOf(elementData, elementCount);
    }
}
// 设置Vector数组的大小
public synchronized void setSize(int newSize) {
    // 修改次数++
    modCount++;
    // 判断设置的数组大小是否大于Vector中有存储的效元素的个数
    // 若 newSize > Vector中有存储的效元素的个数，则调整Vector的大小
    if (newSize > elementCount) {
        // 调用判断是否扩容的方法，如果需要扩容则该方法内部调用扩容方法grow()
        ensureCapacityHelper(newSize);
    } else {
        // 如果上述判断不成立，则将newSize位置之后开始的元素都设置为null
        for (int i = newSize ; i < elementCount ; i++) {
            elementData[i] = null;
        }
    }
    // 更新有效元素个数
    elementCount = newSize;
}
// 获取Vector的当前容量
public synchronized int capacity() {
    return elementData.length;
}
// 获取Vector里面的有效元素个数
public synchronized int size() {
    return elementCount;
}
// 判断Vecotor中是否包含元素 o
public boolean contains(Object o) {
    return indexOf(o, 0) >= 0;
}
// 获取Vector数组中第一次出现对象o的下标，如果不存在，那么返回-1
public int indexOf(Object o) {
    return indexOf(o, 0);
}
// 返回从index出开始第一次出现对象o的下标，如果不存在，那么返回-1
public synchronized int indexOf(Object o, int index) {
    if (o == null) {
        for (int i = index ; i < elementCount ; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = index ; i < elementCount ; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}
......

Vector的扩容方法

// 确定数组当前的容量大小
public synchronized void ensureCapacity(int minCapacity) {
    if (minCapacity > 0) {
        modCount++;
        ensureCapacityHelper(minCapacity);
    }
}
// 如果：当前容量 > 当前数组长度，就调用grow(minCapacity)方法进行扩容
// 由于该方法是在ensureCapacity()中被调用的，而ensureCapacity()方法中已经加上了synchronized锁，所以
// 该方法不需要再加锁
private void ensureCapacityHelper(int minCapacity) {
    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}
// 最大上限的数组容量大小
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE
// Vector集合中的核心扩容方法
private void grow(int minCapacity) {
    // overflow-conscious code
    // 获取旧数组的容量
    int oldCapacity = elementData.length;
    // 得到扩容后(如果需要扩容的话)的新数组容量
    int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                      capacityIncrement : oldCapacity);
    // 如果新容量 < 数组实际所需容量，则令newCapacity = minCapacity
    if (newCapacity - minCapacity < 0)
         newCapacity = minCapacity;
    // 如果当前所需容量 > MAX_ARRAY_SIZE，则新容量设为 Integer.MAX_VALUE，否则设为 MAX_ARRAY_SIZE
    if (newCapacity - MAX_ARRAY_SIZE > 0)
         newCapacity = hugeCapacity(minCapacity);
    elementData = Arrays.copyOf(elementData, newCapacity);
}
// 最大容量
private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
}

完整源码

public class Vector<E>
    extends AbstractList<E>
    implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    protected Object[] elementData;
    protected int elementCount;
    protected int capacityIncrement;
    private static final long serialVersionUID = -2767605614048989439L;
    public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
        this.capacityIncrement = capacityIncrement;
    }
    public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }
    public Vector() {
        this(10);
    }
    public Vector(Collection<? extends E> c) {
        elementData = c.toArray();
        elementCount = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
    }
    public synchronized void copyInto(Object[] anArray) {
        System.arraycopy(elementData, 0, anArray, 0, elementCount);
    }
    public synchronized void trimToSize() {
        modCount++;
        int oldCapacity = elementData.length;
        if (elementCount < oldCapacity) {
            elementData = Arrays.copyOf(elementData, elementCount);
        }
    }
    public synchronized void ensureCapacity(int minCapacity) {
        if (minCapacity > 0) {
            modCount++;
            ensureCapacityHelper(minCapacity);
        }
    }
    private void ensureCapacityHelper(int minCapacity) {
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                         capacityIncrement : oldCapacity);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }
    public synchronized void setSize(int newSize) {
        modCount++;
        if (newSize > elementCount) {
            ensureCapacityHelper(newSize);
        } else {
            for (int i = newSize ; i < elementCount ; i++) {
                elementData[i] = null;
            }
        }
        elementCount = newSize;
    }
    public synchronized int capacity() {
        return elementData.length;
    }
    public synchronized int size() {
        return elementCount;
    }
    public synchronized boolean isEmpty() {
        return elementCount == 0;
    }
    public Enumeration<E> elements() {
        return new Enumeration<E>() {
            int count = 0;
            public boolean hasMoreElements() {
                return count < elementCount;
            }
            public E nextElement() {
                synchronized (Vector.this) {
                    if (count < elementCount) {
                        return elementData(count++);
                    }
                }
                throw new NoSuchElementException("Vector Enumeration");
            }
        };
    }
    public boolean contains(Object o) {
        return indexOf(o, 0) >= 0;
    }
    public int indexOf(Object o) {
        return indexOf(o, 0);
    }
    public synchronized int indexOf(Object o, int index) {
        if (o == null) {
            for (int i = index ; i < elementCount ; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = index ; i < elementCount ; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }
    public synchronized int lastIndexOf(Object o) {
        return lastIndexOf(o, elementCount-1);
    }
    public synchronized int lastIndexOf(Object o, int index) {
        if (index >= elementCount)
            throw new IndexOutOfBoundsException(index + " >= "+ elementCount);
        if (o == null) {
            for (int i = index; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = index; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }
    public synchronized E elementAt(int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        }
        return elementData(index);
    }
    public synchronized E firstElement() {
        if (elementCount == 0) {
            throw new NoSuchElementException();
        }
        return elementData(0);
    }
    public synchronized E lastElement() {
        if (elementCount == 0) {
            throw new NoSuchElementException();
        }
        return elementData(elementCount - 1);
    }
    public synchronized void setElementAt(E obj, int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " +
                                                     elementCount);
        }
        elementData[index] = obj;
    }
    public synchronized void removeElementAt(int index) {
        modCount++;
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " +
                                                     elementCount);
        }
        else if (index < 0) {
            throw new ArrayIndexOutOfBoundsException(index);
        }
        int j = elementCount - index - 1;
        if (j > 0) {
            System.arraycopy(elementData, index + 1, elementData, index, j);
        }
        elementCount--;
        elementData[elementCount] = null; /* to let gc do its work */
    }
    public synchronized void insertElementAt(E obj, int index) {
        modCount++;
        if (index > elementCount) {
            throw new ArrayIndexOutOfBoundsException(index
                                                     + " > " + elementCount);
        }
        ensureCapacityHelper(elementCount + 1);
        System.arraycopy(elementData, index, elementData, index + 1, elementCount - index);
        elementData[index] = obj;
        elementCount++;
    }
    public synchronized void addElement(E obj) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
    }
    public synchronized boolean removeElement(Object obj) {
        modCount++;
        int i = indexOf(obj);
        if (i >= 0) {
            removeElementAt(i);
            return true;
        }
        return false;
    }
    public synchronized void removeAllElements() {
        modCount++;
        // Let gc do its work
        for (int i = 0; i < elementCount; i++)
            elementData[i] = null;
        elementCount = 0;
    }
    public synchronized Object clone() {
        try {
            @SuppressWarnings("unchecked")
                Vector<E> v = (Vector<E>) super.clone();
            v.elementData = Arrays.copyOf(elementData, elementCount);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }
    public synchronized Object[] toArray() {
        return Arrays.copyOf(elementData, elementCount);
    }
    @SuppressWarnings("unchecked")
    public synchronized <T> T[] toArray(T[] a) {
        if (a.length < elementCount)
            return (T[]) Arrays.copyOf(elementData, elementCount, a.getClass());
        System.arraycopy(elementData, 0, a, 0, elementCount);
        if (a.length > elementCount)
            a[elementCount] = null;
        return a;
    }
    // Positional Access Operations
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }
    public synchronized E get(int index) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        return elementData(index);
    }
    public synchronized E set(int index, E element) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
    public synchronized boolean add(E e) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
    }
    public boolean remove(Object o) {
        return removeElement(o);
    }
    public void add(int index, E element) {
        insertElementAt(element, index);
    }
    public synchronized E remove(int index) {
        modCount++;
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        E oldValue = elementData(index);
        int numMoved = elementCount - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--elementCount] = null; // Let gc do its work
        return oldValue;
    }
    public void clear() {
        removeAllElements();
    }
    // Bulk Operations
    public synchronized boolean containsAll(Collection<?> c) {
        return super.containsAll(c);
    }
    public synchronized boolean addAll(Collection<? extends E> c) {
        modCount++;
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityHelper(elementCount + numNew);
        System.arraycopy(a, 0, elementData, elementCount, numNew);
        elementCount += numNew;
        return numNew != 0;
    }
    public synchronized boolean removeAll(Collection<?> c) {
        return super.removeAll(c);
    }
    public synchronized boolean retainAll(Collection<?> c) {
        return super.retainAll(c);
    }
    public synchronized boolean addAll(int index, Collection<? extends E> c) {
        modCount++;
        if (index < 0 || index > elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityHelper(elementCount + numNew);
        int numMoved = elementCount - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);
        System.arraycopy(a, 0, elementData, index, numNew);
        elementCount += numNew;
        return numNew != 0;
    }
    public synchronized boolean equals(Object o) {
        return super.equals(o);
    }
    public synchronized int hashCode() {
        return super.hashCode();
    }
    public synchronized String toString() {
        return super.toString();
    }
    public synchronized List<E> subList(int fromIndex, int toIndex) {
        return Collections.synchronizedList(super.subList(fromIndex, toIndex),
                                            this);
    }
    protected synchronized void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = elementCount - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);
        // Let gc do its work
        int newElementCount = elementCount - (toIndex-fromIndex);
        while (elementCount != newElementCount)
            elementData[--elementCount] = null;
    }
    private void readObject(ObjectInputStream in)
            throws IOException, ClassNotFoundException {
        ObjectInputStream.GetField gfields = in.readFields();
        int count = gfields.get("elementCount", 0);
        Object[] data = (Object[])gfields.get("elementData", null);
        if (count < 0 || data == null || count > data.length) {
            throw new StreamCorruptedException("Inconsistent vector internals");
        }
        elementCount = count;
        elementData = data.clone();
    }
    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException {
        final java.io.ObjectOutputStream.PutField fields = s.putFields();
        final Object[] data;
        synchronized (this) {
            fields.put("capacityIncrement", capacityIncrement);
            fields.put("elementCount", elementCount);
            data = elementData.clone();
        }
        fields.put("elementData", data);
        s.writeFields();
    }
    public synchronized ListIterator<E> listIterator(int index) {
        if (index < 0 || index > elementCount)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }
    public synchronized ListIterator<E> listIterator() {
        return new ListItr(0);
    }
    public synchronized Iterator<E> iterator() {
        return new Itr();
    }
    private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;
        public boolean hasNext() {
            // Racy but within spec, since modifications are checked
            // within or after synchronization in next/previous
            return cursor != elementCount;
        }
        public E next() {
            synchronized (Vector.this) {
                checkForComodification();
                int i = cursor;
                if (i >= elementCount)
                    throw new NoSuchElementException();
                cursor = i + 1;
                return elementData(lastRet = i);
            }
        }
        public void remove() {
            if (lastRet == -1)
                throw new IllegalStateException();
            synchronized (Vector.this) {
                checkForComodification();
                Vector.this.remove(lastRet);
                expectedModCount = modCount;
            }
            cursor = lastRet;
            lastRet = -1;
        }
        @Override
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            synchronized (Vector.this) {
                final int size = elementCount;
                int i = cursor;
                if (i >= size) {
                    return;
                }
        @SuppressWarnings("unchecked")
                final E[] elementData = (E[]) Vector.this.elementData;
                if (i >= elementData.length) {
                    throw new ConcurrentModificationException();
                }
                while (i != size && modCount == expectedModCount) {
                    action.accept(elementData[i++]);
                }
                // update once at end of iteration to reduce heap write traffic
                cursor = i;
                lastRet = i - 1;
                checkForComodification();
            }
        }
        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }
    /**
     * An optimized version of AbstractList.ListItr
     */
    final class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }
        public boolean hasPrevious() {
            return cursor != 0;
        }
        public int nextIndex() {
            return cursor;
        }
        public int previousIndex() {
            return cursor - 1;
        }
        public E previous() {
            synchronized (Vector.this) {
                checkForComodification();
                int i = cursor - 1;
                if (i < 0)
                    throw new NoSuchElementException();
                cursor = i;
                return elementData(lastRet = i);
            }
        }
        public void set(E e) {
            if (lastRet == -1)
                throw new IllegalStateException();
            synchronized (Vector.this) {
                checkForComodification();
                Vector.this.set(lastRet, e);
            }
        }
        public void add(E e) {
            int i = cursor;
            synchronized (Vector.this) {
                checkForComodification();
                Vector.this.add(i, e);
                expectedModCount = modCount;
            }
            cursor = i + 1;
            lastRet = -1;
        }
    }
    @Override
    public synchronized void forEach(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        final int expectedModCount = modCount;
        @SuppressWarnings("unchecked")
        final E[] elementData = (E[]) this.elementData;
        final int elementCount = this.elementCount;
        for (int i=0; modCount == expectedModCount && i < elementCount; i++) {
            action.accept(elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }
    @Override
    @SuppressWarnings("unchecked")
    public synchronized boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        // figure out which elements are to be removed
        // any exception thrown from the filter predicate at this stage
        // will leave the collection unmodified
        int removeCount = 0;
        final int size = elementCount;
        final BitSet removeSet = new BitSet(size);
        final int expectedModCount = modCount;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            @SuppressWarnings("unchecked")
            final E element = (E) elementData[i];
            if (filter.test(element)) {
                removeSet.set(i);
                removeCount++;
            }
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        // shift surviving elements left over the spaces left by removed elements
        final boolean anyToRemove = removeCount > 0;
        if (anyToRemove) {
            final int newSize = size - removeCount;
            for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
                i = removeSet.nextClearBit(i);
                elementData[j] = elementData[i];
            }
            for (int k=newSize; k < size; k++) {
                elementData[k] = null;  // Let gc do its work
            }
            elementCount = newSize;
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            modCount++;
        }
        return anyToRemove;
    }
    @Override
    @SuppressWarnings("unchecked")
    public synchronized void replaceAll(UnaryOperator<E> operator) {
        Objects.requireNonNull(operator);
        final int expectedModCount = modCount;
        final int size = elementCount;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            elementData[i] = operator.apply((E) elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }
    @SuppressWarnings("unchecked")
    @Override
    public synchronized void sort(Comparator<? super E> c) {
        final int expectedModCount = modCount;
        Arrays.sort((E[]) elementData, 0, elementCount, c);
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }
    @Override
    public Spliterator<E> spliterator() {
        return new VectorSpliterator<>(this, null, 0, -1, 0);
    }
    /** Similar to ArrayList Spliterator */
    static final class VectorSpliterator<E> implements Spliterator<E> {
        private final Vector<E> list;
        private Object[] array;
        private int index; // current index, modified on advance/split
        private int fence; // -1 until used; then one past last index
        private int expectedModCount; // initialized when fence set
        /** Create new spliterator covering the given  range */
        VectorSpliterator(Vector<E> list, Object[] array, int origin, int fence,
                          int expectedModCount) {
            this.list = list;
            this.array = array;
            this.index = origin;
            this.fence = fence;
            this.expectedModCount = expectedModCount;
        }
        private int getFence() { // initialize on first use
            int hi;
            if ((hi = fence) < 0) {
                synchronized(list) {
                    array = list.elementData;
                    expectedModCount = list.modCount;
                    hi = fence = list.elementCount;
                }
            }
            return hi;
        }
        public Spliterator<E> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null :
                new VectorSpliterator<E>(list, array, lo, index = mid,
                                         expectedModCount);
        }
        @SuppressWarnings("unchecked")
        public boolean tryAdvance(Consumer<? super E> action) {
            int i;
            if (action == null)
                throw new NullPointerException();
            if (getFence() > (i = index)) {
                index = i + 1;
                action.accept((E)array[i]);
                if (list.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                return true;
            }
            return false;
        }
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> action) {
            int i, hi; // hoist accesses and checks from loop
            Vector<E> lst; Object[] a;
            if (action == null)
                throw new NullPointerException();
            if ((lst = list) != null) {
                if ((hi = fence) < 0) {
                    synchronized(lst) {
                        expectedModCount = lst.modCount;
                        a = array = lst.elementData;
                        hi = fence = lst.elementCount;
                    }
                }
                else
                    a = array;
                if (a != null && (i = index) >= 0 && (index = hi) <= a.length) {
                    while (i < hi)
                        action.accept((E) a[i++]);
                    if (lst.modCount == expectedModCount)
                        return;
                }
            }
            throw new ConcurrentModificationException();
        }
        public long estimateSize() {
            return (long) (getFence() - index);
        }
        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }
}

HashSet简介

HashSet的特点

• 无序性（存储元素无序）
• 唯一性（允许使用null）
• 本质上，HashSet底层是通过HashMap来保证唯一性
• HashSet没有提供get()方法，同HashMap一样，因为Set内部是无序的，所以只能通过迭代的方式获得

HashSet的继承体系

HashSet源码分析

1. 属性(成员变量)

// HashSet内部使用HashMap来存储元素，因此本质上是HashMap
private transient HashMap<E,Object> map;
// 虚拟对象，用来作为value放到map中(在HashSet底层的HashMap中，key为要存储的元素，value统一为PRESENT)
private static final Object PRESENT = new Object();

2. 构造方法

public HashSet() {
    map = new HashMap<>();
}
public HashSet(Collection<? extends E> c) {
    map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
    addAll(c);
}
public HashSet(int initialCapacity, float loadFactor) {
    map = new HashMap<>(initialCapacity, loadFactor);
}
public HashSet(int initialCapacity) {
    map = new HashMap<>(initialCapacity);
}
// 注意：这里未用public修饰，主要是给LinkedHashSet使用的
HashSet(int initialCapacity, float loadFactor, boolean dummy) {
    map = new LinkedHashMap<>(initialCapacity, loadFactor);
}

构造方法都是调用HashMap对应的构造方法。最后一个构造方法有点特殊，它不是public的，意味着它只能被同一个包或者子类调用，这是LinkedHashSet专属的方法。

3. 成员方法

3.1 添加元素add(E e)

// HashSet添加元素的时候，直接调用的是HashMap中的put()方法，
// 把元素本身作为key，把PRESENT作为value，也就是这个map中所有的value都是一样的。
public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

3.2 删除元素remove(Object o)

// HashSet删除元素，直接调用HashMap的remove方法
public boolean remove(Object o) {
    // 注意：map的remove返回是删除元素的value，而Set的remov返回的是boolean类型
    // 如果是null的话说明没有该元素，如果不是null肯定等于PRESENT
    return map.remove(o)==PRESENT;
}

3.3 查找元素contains(Object o)

// Set中没有get()方法，不像List那样可以按index获取元素
public boolean contains(Object o) {
    return map.containsKey(o);
}

4. 完整代码

HashSet是基于HashMap的，所以其源码较少：

package java.util;
import java.io.InvalidObjectException;
import sun.misc.SharedSecrets;

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    static final long serialVersionUID = -5024744406713321676L;
    // 内部元素存储在HashMap中
    private transient HashMap<E,Object> map;
    // 虚拟元素，用来存到map元素的value中的，没有实际意义
    private static final Object PRESENT = new Object();
    // 空构造方法
    public HashSet() {
        map = new HashMap<>();
    }
    // 把另一个集合的元素全都添加到当前Set中
    // 注意，这里初始化map的时候是计算了它的初始容量的
    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }
    // 指定初始容量和装载因子
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }
    // 只指定初始容量
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }
    // LinkedHashSet专用的方法
    // dummy是没有实际意义的, 只是为了跟上上面那个操持方法签名不同而已
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }
    // 迭代器
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }
    // 元素个数
    public int size() {
        return map.size();
    }
    // 检查是否为空
    public boolean isEmpty() {
        return map.isEmpty();
    }
    // 检查是否包含某个元素
    public boolean contains(Object o) {
        return map.containsKey(o);
    }
    // 添加元素
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }
    // 删除元素
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }
    // 清空所有元素
    public void clear() {
        map.clear();
    }
    // 克隆方法
    @SuppressWarnings("unchecked")
    public Object clone() {
        try {
            HashSet<E> newSet = (HashSet<E>) super.clone();
            newSet.map = (HashMap<E, Object>) map.clone();
            return newSet;
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }
    // 序列化写出方法
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // 写出非static非transient属性
        s.defaultWriteObject();
        // 写出map的容量和装载因子
        s.writeInt(map.capacity());
        s.writeFloat(map.loadFactor());
        // 写出元素个数
        s.writeInt(map.size());
        // 遍历写出所有元素
        for (E e : map.keySet())
            s.writeObject(e);
    }
    // 序列化读入方法
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // 读入非static非transient属性
        s.defaultReadObject();
        // 读入容量, 并检查不能小于0
        int capacity = s.readInt();
        if (capacity < 0) {
            throw new InvalidObjectException("Illegal capacity: " +
                                             capacity);
        }
        // 读入装载因子, 并检查不能小于等于0或者是NaN(Not a Number)
        // java.lang.Float.NaN = 0.0f / 0.0f;
        float loadFactor = s.readFloat();
        if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
            throw new InvalidObjectException("Illegal load factor: " +
                                             loadFactor);
        }
        // 读入元素个数并检查不能小于0
        int size = s.readInt();
        if (size < 0) {
            throw new InvalidObjectException("Illegal size: " +
                                             size);
        }
        // 根据元素个数重新设置容量
        // 这是为了保证map有足够的容量容纳所有元素, 防止无意义的扩容
        capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
                HashMap.MAXIMUM_CAPACITY);
        // 再次检查某些东西, 不重要的代码忽视掉
        SharedSecrets.getJavaOISAccess()
                     .checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));
        // 创建map, 检查是不是LinkedHashSet类型
        map = (((HashSet<?>)this) instanceof LinkedHashSet ?
               new LinkedHashMap<E,Object>(capacity, loadFactor) :
               new HashMap<E,Object>(capacity, loadFactor));
        // 读入所有元素, 并放入map中
        for (int i=0; i<size; i++) {
            @SuppressWarnings("unchecked")
                E e = (E) s.readObject();
            map.put(e, PRESENT);
        }
    }
    // 可分割的迭代器, 主要用于多线程并行迭代处理时使用
    public Spliterator<E> spliterator() {
        return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
    }
}

小结

• HashSet内部使用HashMap的key存储元素，以此来保证元素不重复；
• HashSet是无序的，因为HashMap的key是无序的；
• HashSet中允许有一个null元素，因为HashMap允许key为null；
• HashSet是非线程安全的；HashSet是没有get()方法的；

扩展：

当向HashMap中存储n个元素时，它的初始化容量应指定为：((n/0.75f) + 1)，如果这个值小于16，就直接使用16为容量。初始化时指定容量是为了减少扩容的次数，提高效率。

LinkedHashSet分析

package java.util;
// LinkedHashSet继承自HashSet
public class LinkedHashSet<E>
    extends HashSet<E>
    implements Set<E>, Cloneable, java.io.Serializable {
    private static final long serialVersionUID = -2851667679971038690L;
    // 传入容量和装载因子
    public LinkedHashSet(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor, true);
    }
    // 只传入容量, 装载因子默认为0.75
    public LinkedHashSet(int initialCapacity) {
        super(initialCapacity, .75f, true);
    }
    // 使用默认容量16, 默认装载因子0.75
    public LinkedHashSet() {
        super(16, .75f, true);
    }
    // 将集合c中的所有元素添加到LinkedHashSet中
    // 好奇怪, 这里计算容量的方式又变了
    // HashSet中使用的是Math.max((int) (c.size()/.75f) + 1, 16)
    // 这一点有点不得其解, 是作者偷懒？
    public LinkedHashSet(Collection<? extends E> c) {
        super(Math.max(2*c.size(), 11), .75f, true);
        addAll(c);
    }
    // 可分割的迭代器, 主要用于多线程并行迭代处理时使用
    @Override
    public Spliterator<E> spliterator() {
        return Spliterators.spliterator(this, Spliterator.DISTINCT | Spliterator.ORDERED);
    }
}

• LinkedHashSet继承自HashSet，它的添加、删除、查询等方法都是直接用的HashSet的，唯一的不同就是它使用LinkedHashMap存储元素。
• LinkedHashSet是有序的，它是按照插入的顺序排序的。
• LinkedHashSet是不支持按访问顺序对元素排序的，只能按插入顺序排序。

因为，LinkedHashSet所有的构造方法都是调用HashSet的同一个构造方法，如下：

	// HashSet的构造方法
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

通过调用LinkedHashMap的构造方法初始化map，如下所示：

    public LinkedHashMap(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor);
        accessOrder = false;
    }

总结

这样可以看到，这里把accessOrder写死为false了，所以，LinkedHashSet是不支持按访问顺序对元素排序的，只能按插入顺序排序。还请大家多多关注我们的其他文章！