前言
为什么会想写这东西呢？这是我们都非常常用的数据结构，然而平时除了添加，遍历操作，很少使用其他功能，即使是这样，我们也存在万般理由搞清楚他们的 API，这对于我们在编程时如何选择二者具有良好的指导意义。所以，我觉得好好看看这二者的区别，之前一直以为只是 ArrayList 和 LinkedList 只是数组和链表的区别，我已经大错特错了， Java8 的实现远远不止于此，LinkedList 还有双端队列的功能，之前一直没注意到，最近发现 ArrayDeque，却没有看到 LinkedDeque，恍然想起 LinkedList 实现了 Deque 接口，这才恍然大悟～～
ArrayList & LinkedList
他们的区别从实现的接口上也可以看出来， LinkedList 多实现了一个接口，下次面试官再问到，我们可以谈谈这个问题，面试官会另眼相看的，面试官就喜欢深入研究的
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
ArrayList 原理
初始大小为 10，transient Object[] elementData 用来存储元素
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
transient Object[] elementData; // non-private to simplify nested class access
private int size;
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);
}
}
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
add 方法，首先检查容量，检查的时候首先在是否是空的，获取一个最小容量，才去执行扩容，为 elementData 赋值
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
扩容策略，每次增长 1/2
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);
}
如果你想在数组中确定某一个元素的话，那么需要遍历，有两种遍历方式，可根据场景自行选择
public int indexOf(Object o) {
if (o == null) {
for (int i = 0; i < size; i++)
if (elementData[i]==null)
return i;
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
public int lastIndexOf(Object o) {
if (o == null) {
for (int i = size-1; i >= 0; i--)
if (elementData[i]==null)
return i;
} else {
for (int i = size-1; i >= 0; i--)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
toArray 通常需要进行类型强转
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
获取某个索引的元素，我们看到也是强转类型
E elementData(int index) {
return (E) elementData[index];
}
/**
* 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);
}
还有 set 方法，覆盖之前的值，返回旧值
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
插入，需要移动之后的所有元素
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++;
}
删除某个索引的元素，同样需要移动元素
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;
}
删除某个 object，看到了 fastRemove，只不过是去除了边界验证
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;
}
LinkedList
由于是链表，很多首尾节点的插入删除操作，方便了很多
属性
很简单的三个属性
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;
node 静态内部类，根据 Effective Java 的描述，这种设计是比较好的。
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;
}
}
我们简单看一些方法即可，由于实现了 Deque，大部分操作非常相似
add 在尾部添加元素，无需边界检查
public boolean add(E e) {
linkLast(e);
return true;
}
void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
if (l == null)
first = newNode;
else
l.next = newNode;
size++;
modCount++;
}
获取元素，需要注意的是为了加快速度，会判断更接近后边还是前边，这样能省一般时间
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}
Node<E> node(int index) {
// assert isElementIndex(index);
if (index < (size >> 1)) {
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}
而 indexOf(Object o) 就没这个幸运了，必须老老实实从头开始
public int indexOf(Object o) {
int index = 0;
if (o == null) {
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null)
return index;
index++;
}
} else {
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item))
return index;
index++;
}
}
return -1;
}
一些关于队列的操作，注意方法的返回值，如果出现为空是否会抛出异常呢，像 element pop remove 会抛异常，而 peek，poll 则不会，这些都要注意，否则一不小心就会引发惨案！！！
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}
/**
* Retrieves, but does not remove, the head (first element) of this list.
*
* @return the head of this list
* @throws NoSuchElementException if this list is empty
* @since 1.5
*/
public E element() {
return getFirst();
}
public E getFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return f.item;
}
/**
* Retrieves and removes the head (first element) of this list.
*
* @return the head of this list, or {@code null} if this list is empty
* @since 1.5
*/
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}
public E pop() {
return removeFirst();
}
其它的便不贴代码了，都很好理解了
小结
总的来说，对这两个东西的用法更加清晰了，来龙去脉摸得更准确了～
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