ArrayList
java中的數組，在創建的時候需要指定數組的長度。一旦創建後，數組的大小就固定了，不能夠再變化。
但實際開發過程中，經常需要根據保存對象數量的增加擴大範圍。ArrayList 就是數組可調整大小的實現，它允許添加所有元素，也就是說，可以往數組裏面添加 null 元素。
properties
先看看所有屬性：
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
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 = {};
/**
* 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
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;
}
可以看到，在JDK8當中，數組的默認初始化容量是10：
private static final int DEFAULT_CAPACITY = 10;
而且它有兩個空數組實例。
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
它們都是空數組，區別在於，往裏面添加第一個元素的時候，DEFAULTCAPACITY_EMPTY_ELEMENTDATA 會以默認的初始化容量初始化elementData的大小，而EMPTY_ELEMENTDATA 會以給定的容量初始化大小，是這樣子嗎？我們看看代碼。
Constractor ArrayList()
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
可以看到，初始化的時候，就只是把一個static類型的空數組分給elementData.並沒有分配大小。它的大小是0.
public static void main(String args[]) {
List list = new ArrayList();
System.out.println(list.size());
}
輸出內容：
0
Process finished with exit code 0
Constractor ArrayList(int initialCapacity)
/**
* 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) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
可以看到，一旦給定了初始化容量的大小，就會根據給定的值創建對應長度的Object數組。
可以寫一個測試類測試一下。
public static void main(String args[]) {
ArrayList arrayList2 = new ArrayList(20);
System.out.println("ArrayList.size() == " + arrayList2.size());
arrayList2.add(null);
arrayList2.add(null);
arrayList2.add(null);
arrayList2.add(null);
System.out.println("ArrayList.size() == " + arrayList2.size());
}
可以看到輸出內容是：
ArrayList.size() == 0
ArrayList.size() == 4
Process finished with exit code 0
用一個給定的值構造ArrayList時。會根據這個值初始化ArrayList的成員變量。 this.elementData = new Object[initialCapacity]; 這個時候它的size沒有任何變化，還是0. 但是添加一個元素的時候。add方法會對size進行加一操作。代碼： elementData[size++] = e;
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
可以看一下ArrayList的容量擴充機制。比如現在的size是20. 添加一個元素後，他的最小容量是21. 也就是minCapacity的值是21.如果這個時候 elementData是DEFAULTCAPACITY_EMPTY_ELEMENTDATA空數組。就取minCapacity和DEFAULT_CAPACITY的最大值。然後再拿這個值和當前elementData的長度比較，如果確實要增加，新的容量，是原來的容量，向右位移一位後的值加上原來的值。大致上就是以0.5倍的大小進行增長。int newCapacity = oldCapacity + (oldCapacity >> 1); 如果容量超過了最大值：MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8； 就取最大的值MAX_ARRAY_SIZE = Integer.MAX_VALUE。MAX_VALUE 的最大值是 0x7fffffff;
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);
}
/**
* 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
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);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
通過數組下標訪問元素
可以看到，根據index獲取元素的時候，會先檢查index的範圍，index必須小於size.否則拋IndexOutOfBoundsException。 然後根據index 直接訪問elementData的元素。
/**
* 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);
}
/**
* Checks if the given index is in range. If not, throws an appropriate
* runtime exception. This method does *not* check if the index is
* negative: It is always used immediately prior to an array access,
* which throws an ArrayIndexOutOfBoundsException if index is negative.
*/
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
equals(Object o）方法
判斷邏輯是：
- 先判斷是否是同一個對象 o == this. 如果是true,就返回true.
- 判斷給的對象是不是 List 的實例，如果不是，就返回false.
- 獲取 this 和給定對象的迭代器 listIterator(). 同時遍歷，判斷相同的位置上，所包含的元素是否相等。用 equals方法判斷。
- 在判斷長度是不是相等，如果不等，返回false.如果相等，就返回true.
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof List))
return false;
ListIterator<E> e1 = listIterator();
ListIterator<?> e2 = ((List<?>) o).listIterator();
while (e1.hasNext() && e2.hasNext()) {
E o1 = e1.next();
Object o2 = e2.next();
if (!(o1==null ? o2==null : o1.equals(o2)))
return false;
}
return !(e1.hasNext() || e2.hasNext());
}
hashCode() 方法
ArrayList的hashCode()方法邏輯：
- 初始化hashCode的值爲1
- 遍歷包含的每一個元素，將當前hashCode的值乘以31. 加上下一個元素的hashCode值.下一個元素爲null,那麼它的hashCode值爲0.
/**
* Returns the hash code value for this list.
*
* <p>This implementation uses exactly the code that is used to define the
* list hash function in the documentation for the {@link List#hashCode}
* method.
*
* @return the hash code value for this list
*/
public int hashCode() {
int hashCode = 1;
for (E e : this)
hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());
return hashCode;
}
漂亮的移除中間片段
加入一個數組，只取中間一段，它的實現方法是：
- 用 listIterator(fromIndex) 找到開始的地方。
- 跳過要保留的長度。
- 遍歷剩下的元素，先移到下一個位置，再移除上一個位置的元素。
/**
* Removes from this list all of the elements whose index is between
* {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
* Shifts any succeeding elements to the left (reduces their index).
* This call shortens the list by {@code (toIndex - fromIndex)} elements.
* (If {@code toIndex==fromIndex}, this operation has no effect.)
*
* <p>This method is called by the {@code clear} operation on this list
* and its subLists. Overriding this method to take advantage of
* the internals of the list implementation can <i>substantially</i>
* improve the performance of the {@code clear} operation on this list
* and its subLists.
*
* <p>This implementation gets a list iterator positioned before
* {@code fromIndex}, and repeatedly calls {@code ListIterator.next}
* followed by {@code ListIterator.remove} until the entire range has
* been removed. <b>Note: if {@code ListIterator.remove} requires linear
* time, this implementation requires quadratic time.</b>
*
* @param fromIndex index of first element to be removed
* @param toIndex index after last element to be removed
*/
protected void removeRange(int fromIndex, int toIndex) {
ListIterator<E> it = listIterator(fromIndex);
for (int i=0, n=toIndex-fromIndex; i<n; i++) {
it.next();
it.remove();
}
}
