单列集合List

1、Vector和ArrayList以及LinkedList区别和联系、应用场景

线程安全：

　　Vector：如果创建Vector时没有指定容量，则默认容量为10，底层基于数组实现，线程是安全的，底层采用synchronized同步方法进行加锁

　　　　Vector与ArrayList一样，也是通过数组实现的，不同的是它支持线程的同步，即某一时刻只有一个线程能够写Vector，避免多线程同时写而引起的不一致性，但实现同步需要很高的花费，因此，访问它比访问ArrayList慢

　　ArrayList：底层基于数组，线程不安全，查询和修改效率高，但是增加和删除效率低

　　　　ArrayList是最常用的List实现类，内部是通过数组实现的，它允许对元素进行快速随机访问。数组的缺点是每个元素之间不能有间隔，当数组大小不满足时需要增加存储能力，就要讲已经有数组的数据复制到新的存储空间中。当从ArrayList的中间位置插入或者删除元素时，需要对数组进行复制、移动、代价比较高。因此，它适合随机查找和遍历，不适合插入和删除。

　　LinkedList：底层双向链表结构，线程不安全，查询和修改效率低，但是增加和删除效率高，另外，它还提供了List接口中没有定义的方法，专门用于操作表头和表尾元素，可以当作堆栈，队列和双向队列使用；

使用场景：

• Vector很少用
• 如果需要大量的添加和删除则可以选择LinkedList
• 如果需要大量的查询和修改则可以选择ArrayList

Vector源码：

    /**
     * Adds the specified component to the end of this vector,
     * increasing its size by one. The capacity of this vector is
     * increased if its size becomes greater than its capacity.
     *
     * <p>This method is identical in functionality to the
     * {@link #add(Object) add(E)}
     * method (which is part of the {@link List} interface).
     *
     * @param   obj   the component to be added
     */
    public synchronized void addElement(E obj) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
    }

Arraylist源码：

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

LinkedList源码：

    /**
     * Appends the specified element to the end of this list.
     *
     * <p>This method is equivalent to {@link #addLast}.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

2、如果要保证ArraList线程安全，有几种方式？

2.1 自己表写一个ArrayList集合类，根据业务一般来说，add/set/remove加锁
2.2 利用List<Object> list = Collections.synchronizedList(new ArrayList<>()); //采用synchronized加锁

        public E get(int index) {
            synchronized (mutex) {return list.get(index);}
        }
        public E set(int index, E element) {
            synchronized (mutex) {return list.set(index, element);}
        }
        public void add(int index, E element) {
            synchronized (mutex) {list.add(index, element);}
        }
        public E remove(int index) {
            synchronized (mutex) {return list.remove(index);}
        }

　　

2.3 new CopyOnWriteArrayList<>().add(""); //采用 ReentrantLock加锁

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        //加锁
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            //获取原始集合
            Object[] elements = getArray();
            int len = elements.length;
            //复制一个新集合
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            newElements[len] = e;
            //替换原始集合为新集合
            setArray(newElements);
            return true;
        } finally {
            //释放锁
            lock.unlock();
        }
    }                

3、了解CopyOnWriteArrayList底层、CopyOnWriteArrayList与Collections.synchronizedList有什么区别

3.1 CopyOnWriteArrayList底层实现：

　　CopyOnWriteArrayList在执行修改操作的时候，会复制一份新的数组数据，代价昂贵，修改过后将原来的集合指向到新的集合完成操作使用ReentrantLock保证多线程环境下的集合安全

add添加：

　　在添加的时候是需要加锁的，否则多线程写的时候会Copy出N个副本出来；使用ReentrantLock保证多线程环境下的集合安全；

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        //获取了一把锁
        final ReentrantLock lock = this.lock;
        //加锁
        lock.lock();
        try {
            //获取当前数组数据，给elements
            Object[] elements = getArray();
            //记录当前数组的长度
            int len = elements.length;
            //复制一个新的数组
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            /将数据填入到新数组当中
            newElements[len] = e;
            //将当前array指针指向到新的数据
            setArray(newElements);
            return true;
        } finally {
            //释放锁
            lock.unlock();
        }
    }    

get读取：

　　读的时候没有加锁，如果读的时候有多个线程正在向CopyOnWriteArrayList添加数据，读还是会读到旧的数据，因为写的时候不会锁住旧的CopyOnWriteArrayList；

    /**
     * {@inheritDoc}
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        return get(getArray(), index);
    }

CopyOnWriteArrayList应用场景：

　　适用于读取操作远大于写操作场景（底层get读取时没有加锁，直接获取）

3.2 Collections.synchronizedList几乎底层方法都加上了synchronized的锁

        public E get(int index) {
            synchronized (mutex) {return list.get(index);}
        }
        public E set(int index, E element) {
            synchronized (mutex) {return list.set(index, element);}
        }
        public void add(int index, E element) {
            synchronized (mutex) {list.add(index, element);}
        }
        public E remove(int index) {
            synchronized (mutex) {return list.remove(index);}
        }

应用场景：

　　写操作的性能比CopyOnWriteArrayList要好，但是读取的性能不如CopyOnWriteArrayList

4、CopyOnWriteArrayList设计思想是怎么样的，有什么缺点？

设计思想：

　　读写分离，最终一致

缺点：

　　内存占用，由于写时复制，内存中就会出现两个对象占用空间，如果对象大则容易发生YongGC和FullGC

5、 ArrayList扩容机制是怎么样的

JDK1.7 以及之前版本JDK，首先从默认大小来讲，默认为10
JDK1.8 ArrayList集合大小如果创建时没有指定，则默认为0，若已经指定集合大小，则初始值为指
　　当第一次添加数据的时候，集合大小扩容为10，第二次及其后续每次按照int oldCapacity = elementData.length; newCapacity = oldCapacity+(oldCapacity>>1)

5.1 属性

默认初始值的大小：

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

默认的空对象数组：

    /**
     * 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 = {};

实际存储数据的数组：

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access

5.2 无参构造器

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

5.3 扩容机制（源码）

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    //ensureCapacityInternal方法接受了size+1作为minCapacity，并且判断如果数组是空数组，那么10和minCapacity的较大值就作为新的minCapacity。
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    //判断传入的minCapacity和elementData.length的大小，如果elementData.length大于minCapacity，说明数组容量够用，就不需要进行扩容，
    //反之，则传入minCapacity到grow()方法中，进行扩容
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        //如果其元素个数大于其容量，则进行扩容；
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        //原来的容量
        int oldCapacity = elementData.length;
        //新的容量，原来容量的1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        //如果大于ArrayList可以允许的最大容量，则设置为最大容量
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        //最终进行扩容，生成一个1.5倍元素
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

　　进入grow()方法，会将newCapacity设置为旧容量的1.5倍，这也是ArrayList每次扩容都为原来的1.5倍的由来。然后进行判断，如果newCapacity小于minCapacity，那么就将minCapacity的值赋予newCapacity。

　　然后在检查新容量是否超出了定义的容量，如果超出则调用hugeCapacity方法，比较minCapacity和MAX_ARRAY_SIZE的值；如果minCapacity大，那么新容量为Integer。MAX_VALUE,否则新容量为MAX_ARRAYSIZE。最后调用Arrays.cpoyOf传递elementData和新容量，返回新的elementData;

6、ArrayList集合框架（牢记）

    //默认初始容量
    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

    //代表默认情况下，创建集合时，是一个空数组
    /**
     * 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 = {};

    //代表集合真正存储的数据
    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access

    //当前集合大小
    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        //创建一个空数组
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 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) {
        //判断指定集合大小
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            //参数异常
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    /**
     * Replaces the element at the specified position in this list with
     * the specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

    /**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    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;
    }

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If the list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    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;
    }