单列集合List
关耳er 人气:01、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 ? get(i)==null : 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;
}
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