ArrayList方法初探

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1、继承的类以及实现的接口：
继承自：AbstractList
实现了：List，RandomAccess，Cloneable，java.io.Serializable；
定义的成员变量：

transient Object[] elementData;
是一个数组缓冲区，从下面可以看到所有的操作似乎都与之有关。（transient 表示其不可序列化）
size: The size of the ArrayList (the number of elements it contains).
我们可以在创建ArrayList的时候指定其大小，相应的其实是实例化了前面提到的对象Object类型的数组。

public ArrayList(int initialCapacity) {
    super();
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
    this.elementData = new Object[initialCapacity];
}
但如果我们没有指定其大小的话，他就会默认创建一个容量为10的数组。
public ArrayList() {
    this(10);
}
        /**
     * 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 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);
    }
上面的这个构造方法允许你传入一个Collection或者其子类。
仔细看他的处理，首先将这个传进来的Collection转换成数组，并指向ArrayList中定义的Object类型的数组。然后获取其长度; 
注：6260652

public static  T[] copyOf(U[] original, int newLength, Class 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;
}

/**
  * Trims the capacity of this ArrayList instance to be the
  * list's current size.  An application can use this operation to minimize
  * the storage of an ArrayList instance.
  */
  public void trimToSize() {
     modCount++;
     int oldCapacity = elementData.length;
     if (size < oldCapacity) {
        elementData = Arrays.copyOf(elementData, size);
     }
  }
注：modCount是其父类AbstractList中的一个成员变量, 其代表的含义是: 
The number of times this list has been structurally modified, Structural modifications are those that 
change the size of the list, or otherwise perturb it in such a fashion that iterations in progress may 
yield incorrect results.
该方法的作用是修整此ArrayList实例的是列表的当前大小的容量。应用程序可以使用此操作，以尽量减少一个ArrayList实例的存储。
    /**
     * Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     * @param minCapacity the desired minimum capacity
     */
    public void ensureCapacity(int minCapacity) {
        modCount++;
        int oldCapacity = elementData.length;
        if (minCapacity > oldCapacity) {
            Object oldData[] = elementData;
            int newCapacity = (oldCapacity * 3) / 2 + 1;
            if (newCapacity < minCapacity)
                newCapacity = minCapacity;
            // minCapacity is usually close to size, so this is a win:
            elementData = Arrays.copyOf(elementData, newCapacity);
        }
    }
这个方法可以实现手动扩大容量，如果你申请的容量大于原来容量的1.5倍+1，那么就使用你申请的容量大小作为之后的List的容量。
return true if this list contains no elements.
public boolean isEmpty() {
        return size == 0;
}

    /**
     * Returns true if this list contains the specified element. More formally, returns true if and
     * only if this list contains at least one element e such that
     * (o==null ? e==null : o.equals(e)).
     */
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }
    /**
     * 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 i such that
     * (o==null ? get(i)==null : o.equals(get(i))), 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;
    }
他会遍历List中的元素，然后将每个元素都与源元素比较，如果相同，就返回这个元素的下标。否则返回-1;
   /**
     * 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 i such that
     * (o==null ? get(i)==null : o.equals(get(i))), 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;
    }
从最后一个元素遍历，逐个比较，如果相等，则返回元素的下标，否则返回-1;
   /**
     * Returns a shallow copy of this ArrayList instance. (The elements themselves are not copied.)
     * 
     * @return a clone of this ArrayList instance
     */
    public Object clone() {
        try {
            ArrayList v = (ArrayList) 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();
        }
    }
加粗标红的那个语句表名，在进行克隆的时候会将所有的元素都拷贝过去，两个对象各自独立，不会相互影响。注意和使用"="进行传值的方式的区别，使用"="传递的是对象的引用，他们指向同一块内存地址，即指向同一个对象，因此他们修改的也是同一个对象。
ArrayList list = new ArrayList(5);
        list.add("welcome");
        list.add("to");
        list.add("Java");

        @SuppressWarnings("unchecked")
        ArrayList listCopy = (ArrayList) list.clone();
        listCopy.add("world");
        System.out.println(list.toString());
        System.out.println(listCopy.toString());

        ArrayList listCopy2 = list;
        listCopy2.add("Hello");
        System.out.println(list.toString());
        System.out.println(listCopy2.toString());
输出结果：
[welcome, to, Java]
[welcome, to, Java, world]
[welcome, to, Java, Hello]
[welcome, to, Java, Hello]

   /**
     * Returns an array containing all of the elements in this list in proper sequence (from first to last element).
     * 

     * 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.
     * 

     * 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);
    }
注：请看下面一段代码：
List list = new ArrayList();
// ...省略添加元素的细节
// 意将list转换为数组
String[] str = list.toArray();// 会报类型不匹配的错误
String[] str = (String[]) list.toArray();// 会报ClassCastException异常
主要是因为上面这个方法返回的是一个Object类型的一个数组，就像数组的初始化一样，必须一个一个的赋值，而不能将整个数组转换为另外一种类型。
下面这个方法为上面的问题提供了解决方案：
可以改为：
String[] str = (String[]) list.toArray(new String[list.size()]);

    /**
     * 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.
     * 

     * 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 null. (This is useful in
     * determining the length of the list only 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
     */
    public  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;【疑问：为何此处还要置null？】
        return a;
    }


    /**
     * Appends the specified element to the end of this list.
     * @return true (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacity(size + 1); // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
如果只是普通的增加一个元素，则会直接增加到下一个元素的位置。但是如果想是替换某个位置的元素，则需要指定要替换元素的位置，并给出待替换值，下面是这个功能的实现：
    /**
     * 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
     */
    public void add(int index, E element) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);

        ensureCapacity(size + 1); // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1, size - index);
        elementData[index] = element;
        size++;
    }

下面这个方法可以指定待移除的元素的位置。
    /**
     * 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 = (E) 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;
    }

    /**
     * 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 i such
     * that (o==null ? get(i)==null : o.equals(get(i))) (if such an element exists).
     * Returns 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 true 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
    }

    /**
     * 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;
    }

    /**
     * Appends all of the elements in the specified collection to the end of this list, in the order that they are
     * returned by the specified collection's Iterator. The behavior of this operation is undefined if the specified
     * collection is modified while the operation is in progress. (This implies that the behavior of this call is
     * undefined if the specified collection is this list, and this list is nonempty.)
     * 
     * @param c collection containing elements to be added to this list
     * @return true if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacity(size + numNew); // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }

    /**
     * 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 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 c) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);

        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacity(size + numNew); // Increments modCount

        int numMoved = size - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew, numMoved);

        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

    /**
     * Removes from this list all of the elements whose index is between fromIndex, inclusive, and
     * toIndex, exclusive. Shifts any succeeding elements to the left (reduces their index). This call shortens
     * the list by (toIndex - fromIndex) elements. (If toIndex==fromIndex, this operation has no
     * effect.)
     * 
     * @param fromIndex index of first element to be removed
     * @param toIndex index after last element to be removed
     * @throws IndexOutOfBoundsException if fromIndex or toIndex out of range (fromIndex < 0 || fromIndex >=
     *             size() || toIndex > size() || toIndex < fromIndex)
     */
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved);

        // Let gc do its work
        int newSize = size - (toIndex - fromIndex);
        while (size != newSize)
            elementData[--size] = null;
    }

    /**
     * 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("Index: " + index + ", Size: " + size);
    }

    /**
     * Save the state of the ArrayList instance to a stream (that is, serialize it). 
     * @serialData The length of the array backing the ArrayList instance is emitted (int), followed by all of
     *             its elements (each an Object) 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 array length
        s.writeInt(elementData.length);

        // 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 ArrayList instance from a stream (that is, deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
        // Read in size, and any hidden stuff
        s.defaultReadObject();

        // Read in array length and allocate array
        int arrayLength = s.readInt();
        Object[] a = elementData = new Object[arrayLength];

        // Read in all elements in the proper order.
        for (int i = 0; i < size; i++)
            a[i] = s.readObject();
    }