前言
我們在面試的時候,經常遇到面試官問的Java基礎問題,今天記錄一個經常被問到的問題————ArrayList和LinkedList的區別。
二者區別
我們先說結論,然後再從源碼角度去看具體實現。
- 前者底層是動態數組實現;後者底層是鏈表實現。
- 隨機訪問數據:前者快,後者慢。
- 插入和刪除(非末尾)數據:前者慢,後者快。
- 前者需要擴容;後者不需要擴容。
源碼分析
分析的源碼是基於jdk1.8的
ArrayList
從經常使用的方法(比如構造方法、add、get、remove)入手開始分析。
構造方法如下:
/**
* 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.
*
* 用elementData數組來存儲ArrayList中的數據,ArrayList的長度就是數組長度,當第一個元素被add進來之後,數組的長度就被擴展爲DEFAULT_CAPACITY=10
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* Shared empty array instance used for empty instances.
* 如果創建的是空list、則使用EMPTY_ELEMENTDATA數組、所有的空list指向此數組、避免代碼中過多的空數組造成資源浪費
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 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 = {};
/**
* 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) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
//空list的時候。使用EMPTY_ELEMENTDATA作爲存儲數據的數組、避免資源浪費
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*
* @param c the collection whose elements are to be placed into this list
* @throws NullPointerException if the specified collection is null
*/
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}
三個構造方法:
第一個構造方法有一個參數,用來設置初始長度的,如果明確知道list的長度,則使用此方法來構造,若傳進來的參數爲0,則直接使用EMPTY_ELEMENTDATA數組,這樣做的好處,是避免程序中,有過多的空list的情況下,造成資源浪費,jdk1.8以前的版本是,如果傳進來的參數爲0,則直接new一個空數組,這樣會造成資源的浪費;
第二個方法構造方法,也是我們經常用的構造方法,沒有參數的構造方法會使用DEFAULTCAPACITY_EMPTY_ELEMENTDATA數組作爲數據容器;
第三個構造方法是有一個Collection類型參數的構造方法,若參數中的數據不爲空,會把參數中的數據,存進elementData數組中;
下面看看如何add數據的
/**
* 在list末尾增加一個數據,
* 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) {
//擴容
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);
}
/**
* 真正的擴容的方法
* 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
//當前的容量
int oldCapacity = elementData.length;
//新容量是當前容量的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
//如果新容量比所需要的容量小,就使用所需要的容量擴容
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//如果新容量大於MAX_ARRAY_SIZE=Integer.MAX_VALUE - 8;新容量會根據所需容量minCapacity與MAX_ARRAY_SIZE大小,分別使用Integer.MAX_VALUE或者Integer.MAX_VALUE - 8
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
//將elementData擴容並將elementData中的內容複製到擴容後的數組elementData
//這裏會調用到native方法
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;
}
完成以上擴容檢測並完成擴容後,再回到add方法中,將要添加的元素添加到elementData的末尾即可。
然後簡單看一下add(int index, E element)方法:
/**
* 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++;
}
跟add(E element)方法基本一樣,唯一不同的是多了一句System.arraycopy(elementData, index, elementData, index + 1, size - index);
這句代碼就是將源數組中從位置index之後的數據整體向後移動一位,以便空出index位,爲插入element做準備、然後將element插入到index位置。
get(int index)方法很簡單不貼代碼了。
看下刪除數據的吧remove(int index)和remove(Object o)兩個方法
/**
* 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++;
//獲取要刪除的值
E oldValue = elementData(index);
//需要移動的元素的個數,被刪除的元素之後的元素都需要前移一位
int numMoved = size - index - 1;
if (numMoved > 0)
//將被刪的元素之後的元素,向前移動一位
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//將最後一位置爲空
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
這個很簡單,看註釋就好。
/**
* 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;
}
很簡單,從0開始遍歷數組,找到第一個==被刪除元素的位置,調用fastRemove(index)方法
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
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
}
what?這不就是remove(int index)方法嗎?哈哈。邏輯一樣。到此刪除的邏輯講清楚了
ArrayList的源碼就分析到這裏,其實就是對數組操作的封裝,其他方法,感興趣可以自行查看源碼。
LinkedList
我們也從構造方法,add、get和remove入手,先看構造方法:
/**
* Constructs an empty list.
*/
public LinkedList() {
}
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*
* @param c the collection whose elements are to be placed into this list
* @throws NullPointerException if the specified collection is null
*/
public LinkedList(Collection<? extends E> c) {
this();
addAll(c);
}
常用的是第一個,構造一個空的list;對於第二個會通過參數來構造一個list,其中調用到了addAll(Collection<? extends E> c)。先不看他,因爲我們還不知道LinkedList是用什麼結構存儲數據的呢,看下LinkedList都有哪些全局變量
transient int size = 0;
/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
* (first.prev == null && first.item != null)
* 指向第一個節點
*/
transient Node<E> first;
/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
* (last.next == null && last.item != null)
* 指向最後一個節點
*/
transient Node<E> last;
三個,第一個不用說,後兩個是一個Node類型。這是什麼類型呢。這是LinkedList的一個內部類:
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;
}
}
這不是一個鏈表結構的元素的結構嘛!那LinkedList就是用鏈表存儲數據的。
看下addAll(Collection<? extends E> c)方法最終調用的方法
/**
* Inserts all of the elements in the specified collection into this
* list, starting at the specified position. Shifts the element
* currently at that position (if any) and any subsequent elements to
* the right (increases their indices). The new elements will appear
* in the list in the order that they are returned by the
* specified collection's iterator.
*
* @param index index at which to insert the first element
* from the specified collection
* @param c collection containing elements to be added to this list
* @return {@code true} if this list changed as a result of the call
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index);
Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;
Node<E> pred, succ;//pred代表要插入的Collection的節點的前驅
//在末尾添加的時候,所以pred指向最後一個節點
if (index == size) {
succ = null;
pred = last;
} else {//不是在末尾添加的邏輯
//先獲取要插入位置上節點
succ = node(index);
//然後前驅指向被插入鏈表上插入位置節點的前驅
pred = succ.prev;
}
//遍歷要插入的Collection集合
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
//對每一個元素生成一個Node 、pred爲前驅
Node<E> newNode = new Node<>(pred, e, null);
if (pred == null)//如果沒有前驅,代表是第一個節點firstNode
first = newNode;
else//否則把前一個前驅的後繼指向新生成的節點
pred.next = newNode;
//然後將新生成的節點更改爲下一個節點的前驅
pred = newNode;
}
if (succ == null) {//將Collection添加到末尾的情況下,last指向最後一個節點
last = pred;
} else {
//將Collection中的最後一個節點的next指向原list中index位置上的節點succ
pred.next = succ;
//將原list中index位置上的節點succ的前驅指向Collection中的最後一個節點
succ.prev = pred;
}
//更新size
size += numNew;
modCount++;
return true;
}
這裏的邏輯有點繞,看不懂可以在紙上畫一畫,其實就是鏈表插入操作,分爲在末尾插入和中間插入兩種情況,並保存全局變量first和last。
構造方法看完了,接下來看下單個元素的添加add(E e)方法吧
/**
* Appends the specified element to the end of this list.
* 在list的末尾添加一個新的元素
* <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;
}
/**
* Links e as last element.
*/
void linkLast(E e) {
//獲取當前list的最後一個元素
final Node<E> l = last;
//生成一個新的元素,並把新元素的前驅指向last,後繼指爲null
final Node<E> newNode = new Node<>(l, e, null);
//更新last元素爲新生成的這個node
last = newNode;
//last==null表明當前是空表,所以first需要指向新生成的newNode
if (l == null)
first = newNode;
//非空表 原list的last節點的next需要指向新生成的節點newNode
else
l.next = newNode;
//更新size
size++;
modCount++;
}
在看下add(int index , E element)方法,在中間位置插入輸入
/**
* 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) {
checkPositionIndex(index);
//判斷是否是在末尾插入,若是調用linkLast方法,否則調用linkBefore方法
if (index == size)
linkLast(element);
else
linkBefore(element, node(index));
}
/**
* Inserts element e before non-null Node succ.
* succ 是要插入位置的節點
* e 是要插入的節點
*/
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
//定義一個節點pred保存要插入位置的節點的前驅節點
final Node<E> pred = succ.prev;
//新生成一個節點前驅爲上一步保存的pred,後繼爲succ節點
final Node<E> newNode = new Node<>(pred, e, succ);
//將succ的前驅指向新生成的節點
succ.prev = newNode;
//判斷插入位置是否是第一個位置,若是則滿足pred==null 將first指向newNode
if (pred == null)
first = newNode;
else//否則更新pred節點的後繼爲新生成的節點
pred.next = newNode;
//更新size
size++;
modCount++;
}
add方法分析完畢,下面看一下get方法
/**
* 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) {
checkElementIndex(index);
return node(index).item;
}
/**
* Returns the (non-null) Node at the specified element index.
*/
Node<E> node(int index) {
// assert isElementIndex(index);
//判斷index是否小於size的1/2
if (index < (size >> 1)) {
Node<E> x = first;
//從list頭開始遍歷查找數據找到第index位置上的元素
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {//否則從list尾開始遍歷,找到index位置上的元素
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}
get方法需要從list的頭或者尾開始遍歷查找,所以比起數組存儲的ArrayList來說,會比較耗時。
下面來看看remove(int index)方法
/**
* Removes the element at the specified position in this list. Shifts any
* subsequent elements to the left (subtracts one from their indices).
* Returns the element that was removed from the list.
*
* @param index the index of the element to be removed
* @return the element previously at the specified position
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
checkElementIndex(index);
//獲取要刪除位置的元素並調用unlink方法
return unlink(node(index));
}
/**
* Unlinks non-null node x.
*/
E unlink(Node<E> x) {
// assert x != null;
//獲取element及其前驅和後繼
final E element = x.item;
final Node<E> next = x.next;
final Node<E> prev = x.prev;
//前驅爲null表明是第一個元素,更新first爲此node的後繼
if (prev == null) {
first = next;
} else {
//否則不是第一個元素,將前驅的後繼指向當前node的後繼
prev.next = next;
//斷開當前node的前驅
x.prev = null;
}
//後繼爲null表明是最後一個元素,更新last指此向node的前驅
if (next == null) {
last = prev;
} else {
//否則不是最後一個元素,將後繼的前驅指向盪鞦韆node的前驅
next.prev = prev;
//斷開當前node的後繼
x.next = null;
}
//產出node的item並更新size
x.item = null;
size--;
modCount++;
return element;
}
查看源碼remove(Object o)
/**
* Removes the first occurrence of the specified element from this list,
* if it is present. If this list does not contain the element, it is
* unchanged. More formally, removes the element with the lowest index
* {@code i} such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
* (if such an element exists). Returns {@code true} 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 {@code true} if this list contained the specified element
*/
public boolean remove(Object o) {
if (o == null) {
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null) {
unlink(x);
return true;
}
}
} else {
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item)) {
unlink(x);
return true;
}
}
}
return false;
}
根據要刪除的數據是否爲null、分別遍歷查找第一個出現的o,並調用unlink方法。remove方法也講完了。
以上就是經常用到的方法的源碼分析
總結
最後來個總結吧。看前面列出的區別點,之所以出現“隨機訪問數據:前者快,後者慢。”的情況,就是因爲後者的隨機訪問,需要從頭遍歷鏈表,因爲他們存儲的位置是不連續的。而對於“插入和刪除(非末尾)數據:前者慢,後者快。”這個問題,由於數組存儲位置是連續的,從中間刪了的數據,空出來的位置,需要由後面的元素補上空位,需要移動元素,而後者不需要。所以會有此差異。