1. ArrayList的设计思路。
直接看源码
/** <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @author Josh Bloch
* @author Neal Gafter
* @see Collection
* @see List
* @see LinkedList
* @see Vector
* @since 1.2
*/
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;可以看出ArrayList实现List<E>, RandomAccess, Cloneable, java.io.Serializable。
定义serialVersionUID
AbstractList封装了List操作的核心抽象方法
/**
* This class provides a skeletal implementation of the {@link List}
* interface to minimize the effort required to implement this interface
* backed by a "random access" data store (such as an array). For sequential
* access data (such as a linked list), {@link AbstractSequentialList} should
* be used in preference to this class.
*
* <p>To implement an unmodifiable list, the programmer needs only to extend
* this class and provide implementations for the {@link #get(int)} and
* {@link List#size() size()} methods.
*
* <p>To implement a modifiable list, the programmer must additionally
* override the {@link #set(int, Object) set(int, E)} method (which otherwise
* throws an {@code UnsupportedOperationException}). If the list is
* variable-size the programmer must additionally override the
* {@link #add(int, Object) add(int, E)} and {@link #remove(int)} methods.
*
* <p>The programmer should generally provide a void (no argument) and collection
* constructor, as per the recommendation in the {@link Collection} interface
* specification.
*
* <p>Unlike the other abstract collection implementations, the programmer does
* <i>not</i> have to provide an iterator implementation; the iterator and
* list iterator are implemented by this class, on top of the "random access"
* methods:
* {@link #get(int)},
* {@link #set(int, Object) set(int, E)},
* {@link #add(int, Object) add(int, E)} and
* {@link #remove(int)}.
*
* <p>The documentation for each non-abstract method in this class describes its
* implementation in detail. Each of these methods may be overridden if the
* collection being implemented admits a more efficient implementation.
*
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @author Josh Bloch
* @author Neal Gafter
* @since 1.2
*/
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {AbstractCollection 实现 Collection接口
public interface Collection<E> extends Iterable<E>Collection定义了迭代器的接口
下面来看ArrayList的本质
2. ArrayList结构分析
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;DEFAULT_CAPACITY 定义了ArrayList的初始化容量10.
size 定义存储节点的大小
transient Object[] elementData; // non-private to simplify nested class access
定义了ArrayList的本质Object数组。使用了transient修饰，不能序列化。
但ArrayList实现了 java.io.Serializable，数组如何序列化呢
ArrayList使用了自定义序列化模式
/**
* Save the state of the <tt>ArrayList</tt> instance to a stream (that
* is, serialize it).
*
* @serialData The length of the array backing the <tt>ArrayList</tt>
* instance is emitted (int), followed by all of its elements
* (each an <tt>Object</tt>) in the proper order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException{
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();
// Write out size as capacity for behavioural compatibility with clone()
s.writeInt(size);
// Write out all elements in the proper order.
for (int i=0; i<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
/**
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
elementData = EMPTY_ELEMENTDATA;
// Read in size, and any hidden stuff
s.defaultReadObject();
// Read in capacity
s.readInt(); // ignored
if (size > 0) {
// be like clone(), allocate array based upon size not capacity
int capacity = calculateCapacity(elementData, size);
SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);
ensureCapacityInternal(size);
Object[] a = elementData;
// Read in all elements in the proper order.
for (int i=0; i<size; i++) {
a[i] = s.readObject();
}
}
}3种序列化方式
1. 默认序列化，实现了 java.io.Serializable或者Externalizable即可
2. Externalizable继承于Serializable，该接口序列化，反序列化可自定义writeExternal()与readExternal()方法，且必须实现，否则序列化，反序列化无结果，所有字段的值均为空。另外反射调用的是无参构造函数，需要序列化对象有公共无参构造函数。
3. 自定义序列化是由java.io.ObjectInputStream/java.io.ObjectOutputStream在序列化/反序列化时候，通过反射检查该类是否存在以下方法（0或多个）：序列化调用write方法，反序列调用read方法；当某个字段被声明为transient后，默认序列化机制就会忽略该字段。
//修改序列化的对象
Object writeReplace() throws ObjectStreamException;
//out.defaultWriteObject() 默认的序列化方式
//out.writeObject("XX") 自定义序列化方式
void writeObject(java.io.ObjectOutputStream out) throws IOException;
//in.defaultReadObject() 默认的反序列化方式
//in.readObject() 自定义读取
void readObject(java.io.ObjectInputStream in) throws Exception;
//修改返回的对象
Object readResolve() throws ObjectStreamException;ArrayList的数组使用自定义序列化，从源码上看，是为了节省内存空间，提高效率。因为ArrayList的数组容量大小是比size大或者相等，在序列化的时候只会序列化size大小的容量，节省空间和内存开销，传输开销（远程传输的时候），序列化反序列化开销。这点设计很赞啊！
3.核心方法介绍
3.1 构造函数
/**
* 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);
}
}
/**
* 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;
}
}3种构造，默认构造函数是创建空数组{}，如果传入有值的集合就是数组的拷贝。Arrays.copyOf(elementData, size, Object[].class);
/**
* Copies the specified array, truncating or padding with nulls (if necessary)
* so the copy has the specified length. For all indices that are
* valid in both the original array and the copy, the two arrays will
* contain identical values. For any indices that are valid in the
* copy but not the original, the copy will contain <tt>null</tt>.
* Such indices will exist if and only if the specified length
* is greater than that of the original array.
* The resulting array is of the class <tt>newType</tt>.
*
* @param <U> the class of the objects in the original array
* @param <T> the class of the objects in the returned array
* @param original the array to be copied
* @param newLength the length of the copy to be returned
* @param newType the class of the copy to be returned
* @return a copy of the original array, truncated or padded with nulls
* to obtain the specified length
* @throws NegativeArraySizeException if <tt>newLength</tt> is negative
* @throws NullPointerException if <tt>original</tt> is null
* @throws ArrayStoreException if an element copied from
* <tt>original</tt> is not of a runtime type that can be stored in
* an array of class <tt>newType</tt>
* @since 1.6
*/
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
@SuppressWarnings("unchecked")
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}
public static native void arraycopy(Object src, int srcPos,
Object dest, int destPos,
int length);System的arraycopy方法，native方法，性能较强，推荐数组拷贝的时候使用
3.2 add方法
/**
* 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) {
//检测容量，不够就扩容
ensureCapacityInternal(size + 1); // Increments modCount!!
//写入赋值
elementData[size++] = e;
return true;
}ensureCapacityInternal
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return minCapacity;
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private void ensureExplicitCapacity(int minCapacity) {
//修改计数
modCount++;
// overflow-conscious code
//如果，最小容量比数组容量大，扩容
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}grow代码
/**
* 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;
//扩容的核心本质oldCapacity >> 1，即50%的数组容量
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:
//扩容数组拷贝至新容量数组
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;
}容量不足时（size不是容量，容量是数组的length），扩容50%的容量 oldCapacity >> 1，然后new 新数组，拷贝旧数组至新数组。
3.3 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) {
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));
}很简单，检查index下标是否>= size，下标从0开始，一定小于size，然后数组取数。
3.4 remove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++;
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;
}
/*
* 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
}逻辑很简单，判断index或者object合法性，size-1，数组index向前拷贝一位，数组末尾置为null。
4. ArrayList迭代
/**
* Returns an iterator over the elements in this list in proper sequence.
*
* <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
*
* @return an iterator over the elements in this list in proper sequence
*/
public Iterator<E> iterator() {
return new Itr();
}
/**
* An optimized version of AbstractList.Itr
*/
private class Itr implements Iterator<E> {
int cursor; // index of next element to return
int lastRet = -1; // index of last element returned; -1 if no such
int expectedModCount = modCount;
Itr() {}
public boolean hasNext() {
return cursor != size;
}
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
Objects.requireNonNull(consumer);
final int size = ArrayList.this.size;
int i = cursor;
if (i >= size) {
return;
}
final Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length) {
throw new ConcurrentModificationException();
}
while (i != size && modCount == expectedModCount) {
consumer.accept((E) elementData[i++]);
}
// update once at end of iteration to reduce heap write traffic
cursor = i;
lastRet = i - 1;
checkForComodification();
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}内部类迭代器，从0开始维护内部类变量cursor，逐一取值。
5. 总结
ArrayList核心就是数组，默认容量10，容量不足时以50%容量扩增。本质就是数组的拷贝。
优势：1. index下标取值方便；2. 数组空间连续，访问速度快。
缺点：1. 数组连续空间，分配内存连续，可能会频繁GC；2. 非末尾元素的修改或删除时，数组需要复制，效率低。