集合类中的Collection接口实现类

今天学习一下集合包里面的内容，常见的有Collection和Map两个接口的实现类
Collection中常见的又分为两种：

1.List ，支持放入重复的对象，实现类有arraylist,linkedlist,vector,stack
           2.Set ，不支持放入重复对象，hashset,treeset
ArrayList:
    创建arraylist:提供了三种构造方式。

public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];//大于0时就直接创建object数组
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;//为0时构造一个空的
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

当插入的数据大于它的容量时就会扩容，过程如下：

  public static void main(String[] args) {
        List list = new ArrayList(2);
        list.add(1);
        list.add(2);
        list.add(3);
    }

上面定义了长度为2，但存放了三个，调用过程是这样的，在第三次时决断minCapacity-elementData.length>0就调用grow法。源码如下：

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

 private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

 private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

  private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

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

    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        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);
    }

删除元素：

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

        modCount++;
        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;
    }

public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }

private void fastRemove(int index) {
        modCount++;
        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
    }

遍历时：

public Iterator<E> iterator() {
        return new Itr();
    }

        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        Itr() {}

        public boolean hasNext() {
            return cursor != size;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();//这个异常说明在操作元素时，有其他线程对list进行了改变
            }
        }

        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) 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();
        }
    }

总结：

ArrayList基于数组方式实现，无容量限制，在插入元素时可能会扩容，在删除元素时，容量大小并不会改变，但可以通过trimToSize()来修改。在查找元素时要遍历数组，对于非null的元素，采取equals的方式查找，最后一点arrayList是非线程安全的。

LinkedList也是非线程安全的，它基于双向链表机制实现，插入元素时要新建一个Entry对象(1.8好像是Node,没骨找到Entry对象)。

Vector是基于Synchronized实现的线程安全的ArrayList，但在插入元素时容量扩充的机制和ArrayList稍有不同，并可通过capacityIncrement来控制容量的扩充。

Stack继承Vector,实现后进先出（LIFO）的弹出及压入操作，提供了push,pop,peek三个主要方法。

HashSet:

hashset是set接口的实现，set 和list区别就是set不允许有元素重复（不重复是底层用hashmap）,

总结：

HashSet基于HashMap,无容量限制，HashSet是非线程安全的。