Java源码学习--ArrayList源码解析

ArrayList类中全局变量的意义

/**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer.
*/
private transient Object[] elementData; //存放值的数组

/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size; //数组中元素的个数

/**
* The maximum size of array to allocate.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; //数组的临界点-8

ArrayList的构造方法

创建一个ArrayList对象，不带参数的时候 是创建了一个长度为10的Object数组 带int类型参数的时候就是创建指定长度的Object数组 带Collection类型的构造方法，指明该ArrayList是什么类型的数组

/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this(10);
}

/**
* 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) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}

/**
* 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();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}

常用的方法

1). Arrays.copyOf(T[] original, int newLength) 此方法是创建一个新数组返回，并且将原有数组中的值复制到新数组中

public static <T> T[] copyOf(T[] original, int newLength) {
return (T[]) copyOf(original, newLength, original.getClass());
}

public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
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;
}

2). System.arraycopy(Object src, int srcPos,Object dest, int destPos, int length); 该方法被标记了native，调用了系统的C/C++代码，在JDK中是看不到的，但在openJDK中可以看到其源码。该函数实际上最终调用了C语言的memmove()函数，因此它可以保证同一个数组内元素的正确复制和移动，比一般的复制方法的实现效率要高很多，很适合用来批量处理数组。Java强烈推荐在复制大量数组元素时用该方法，以取得更高的效率。 方法的参数的含义

第一个是要复制的数组，第二个是从要复制的数组的第几个开始，第三个是复制到那，四个是复制到的数组第几个开始，最后一个是复制长度

public static native void arraycopy(Object src,  int  srcPos,
Object dest, int destPos,
int length);

trimToSize()方法

此方法是将数组中的元素个数做为数组的长度生成一个新的数组，并将新数组内存地址指向当前集合，将多余的空间释放

/**
* Trims the capacity of this <tt>ArrayList</tt> instance to be the
* list's current size.  An application can use this operation to minimize
* the storage of an <tt>ArrayList</tt> instance.
*/
public void trimToSize() {
modCount++;
int oldCapacity = elementData.length;
if (size < oldCapacity) {
elementData = Arrays.copyOf(elementData, size);
}
}

size()方法

int返回值 返回数组中元素的个数

/**
* Returns the number of elements in this list.
*
* @return the number of elements in this list
*/
public int size() {
return size;
}

isEmpty()方法

boolean返回值，返回当前数组的的个数是否等于0 等于0为true 否则为false

/**
* Returns <tt>true</tt> if this list contains no elements.
*
* @return <tt>true</tt> if this list contains no elements
*/
public boolean isEmpty() {
return size == 0;
}

contains(Object o)方法

boolean返回值 此方法的原理是调用indexOf(Object o)之后判断返回值是否>=0,具体的后面将indexOf时详细讲解

/**
* Returns <tt>true</tt> if this list contains the specified element.
* More formally, returns <tt>true</tt> if and only if this list contains
* at least one element <tt>e</tt> such that
* <tt>(o==null ? e==null : o.equals(e))</tt>.
*
* @param o element whose presence in this list is to be tested
* @return <tt>true</tt> if this list contains the specified element
*/
public boolean contains(Object o) {
return indexOf(o) >= 0;
}

indexOf(Object o)方法 正序比较

int返回值 此方法是将数组中的每个元素都取出来与传入的对象进行对比，如果相等就返回对象所在下标，否则返回-1

/**
* Returns the index of the first occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the lowest index <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
* or -1 if there is no such index.
*/
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;
}

lastIndexOf(Object o)方法 倒序比较

int返回值 实现原理跟indexOf一样，只是一个是从前到后的比较，一个是从后到前的比较

/**
* Returns the index of the last occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the highest index <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
* or -1 if there is no such index.
*/
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;
}

clone()方法

Object返回值 官方解释“浅表复制”，我的理解就是创建一个List对象，但是对象内的元素内存指向没变，也就是说但修改这个集合中元素时，另一个集合中的元素也会发生变化

，也就是List集合对象，通过创建一个ArrayList 集合来接收数组

/**
* Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The
* elements themselves are not copied.)
*
* @return a clone of this <tt>ArrayList</tt> instance
*/
public Object clone() {
try {
@SuppressWarnings("unchecked")
ArrayList<E> v = (ArrayList<E>) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError();
}
}

toArray()方法

Object[] 返回值，此方法就是将底层存放的数组复制一个返回回去

/**
* Returns an array containing all of the elements in this list
* in proper sequence (from first to last element).
*
* <p>The returned array will be "safe" in that no references to it are
* maintained by this list.  (In other words, this method must allocate
* a new array).  The caller is thus free to modify the returned array.
*
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this list in
*         proper sequence
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}

toArray(T[] a)方法

Object[] 返回值，如果传入数组长度比当前集合中元素个数小，则创建一个新的数组返回，大小为集合中元素的个数，类型为传入数组的类型

传入数组长度等于集合中元素个数 则将集合中的值复制进入则返回传入数组，并返回传入数组

如果长度大于元素数组个数 除复制集合外 还将传入数组的第size个数组置为空

/**
* Returns an array containing all of the elements in this list in proper
* sequence (from first to last element); the runtime type of the returned
* array is that of the specified array.  If the list fits in the
* specified array, it is returned therein.  Otherwise, a new array is
* allocated with the runtime type of the specified array and the size of
* this list.
*
* <p>If the list fits in the specified array with room to spare
* (i.e., the array has more elements than the list), the element in
* the array immediately following the end of the collection is set to
* <tt>null</tt>.  (This is useful in determining the length of the
* list <i>only</i> if the caller knows that the list does not contain
* any null elements.)
*
* @param a the array into which the elements of the list are to
*          be stored, if it is big enough; otherwise, a new array of the
*          same runtime type is allocated for this purpose.
* @return an array containing the elements of the list
* @throws ArrayStoreException if the runtime type of the specified array
*         is not a supertype of the runtime type of every element in
*         this list
* @throws NullPointerException if the specified array is null
*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}

get(int index)方法

Object 返回值 此方法先验证传入的下标是否在数组中，如果存在则返回对应下标的值，否则则跑出异常

/**
* 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));
}

/**
* 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);
}

set(int index, E element)方法

Object返回值，修改指定下标的值，并且将原来的值返回回来

/**
* 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;
}

add(E e)方法 插入数组最后一位

当传入的一个参数的时候 先调用ensureCapacityInternal()方法，ensureCapacityInternal是判断底层生成的那个Object数组是否越界，如果越界，则新生成一个数组，并存入值，否则则将值存入数组中

判断规则为 原来值的长度的1.5倍比传入的值大则创建一个新的数组，数组长度为原来的1.5倍，如果小的话 就创建一个长度为传入参数的数组，最后还有个判断是判断数组最大长度，如果新生成的那个数组长度值比系统定义的数组最大长度还大，那么将创建一个数组，此数组长度为系统默认的最大长度

/**
* 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;
}

/**
* 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 void ensureCapacityInternal(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}

private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}

ArrayList add(int index, E element) 指定插入位置

先验证要插入的位置是否在数组中，然后用跟上面原理一样的方式创建一个新数组，并且新数组，然后调用系统最底层方法System.arraycopy复制一个数组出来，将当前值存入复制出来的集合中的index位置，因为这里多了一个元素，所以要将AarrayList中的size+1

/**
* 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++;
}

remove(int index)方法 删除指定下标的元素

Object 返回值 先验证下标是否可用，然后得到当前对象，得到System.arraycopy要复制长度的值，因为下面是要从传入数组的下一个值开始复制，因此这里要减1否则取值时要越界，因为原有的值从新数组中删除，所以原有的值后面所有值都会前进一位，导致数组最后一位无值，因此需要将最后一位赋值为空

/**
* 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; // Let gc do its work

return oldValue;
}

remove(Object o)方法 根据对象删除集合中元素

boolean 返回值 判断传入的对象是否在数组中，存在则执行remove(int index)的原理，并返回true,否则返回false

/**
* 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; // Let gc do its work
}

clear()方法

此方法为情况当前集合，也就是清空数组，但需要注意的是，该方法不会改变数组的长度，只会将数组的值赋为空

/**
* Removes all of the elements from this list.  The list will
* be empty after this call returns.
*/
public void clear() {
modCount++;

// Let gc do its work
for (int i = 0; i < size; i++)
elementData[i] = null;

size = 0;
}

注意事项

集合当中只能放置对象的引用，无法放置原生数据类型，我们需要使用原生数据类型的包装类才能加入到集合当中；

集合当中放置的都是Object类型，因此取出来的也是Object类型，那么我们必须要使用强制类型转换将其转换为真正的类型（放置进去的类型）；

总结

通过阅读ArrayList的源码才知道，原来ArrayList的底层是用数组实现的，并且add方法是用创建数组的方式来增长边界的。