Java-Collection源碼分析（四）——ArrayList

ArrayList類

ArrayList是實現List接口的動態數組，所謂動態就是它的大小是可變的。實現了所有可選列表操作，並允許包括 null 在內的所有元素。除了實現 List 接口外，此類還提供一些方法來操作內部用來存儲列表的數組的大小。

部分源碼

     private static final long serialVersionUID = 8683452581122892189L;
     //默認初始容量。
     private static final int DEFAULT_CAPACITY = 10;
     //用於空實例的共享空數組實例。
     private static final Object[] EMPTY_ELEMENTDATA = {};
     //用於默認大小的空實例的共享空數組實例。 我們將其與EMPTY_ELEMENTDATA區別開來，以瞭解當添加第一個元素時會發生多少膨脹。
     private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
     //緩衝區，其中存儲ArrayList的元素。ArrayList的容量是此數組緩衝區的長度。 當添加第一個元素時，具有elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA的任何空的ArrayList將被擴展爲DEFAULT_CAPACITY。
    transient Object[] elementData; // non-private to simplify nested class access
    private int size;
    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);
        }
    }
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    //構造一個包含指定集合的元素的列表，按照集合的迭代器返回的順序。
    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;
        }
    }
    //將此ArrayList實例的容量修改爲列表的當前大小。 應用程序可以使用此操作來最小化ArrayList實例的存儲。
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }
    //如果需要，增加此ArrayList實例的容量，以確保它至少可以容納最小容量參數指定的元素數。minCapacity：所需的最小容量
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            ? 0 : DEFAULT_CAPACITY;
        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    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);
    }
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    //增加容量以確保它至少容納最小容量參數指定的元素數量。
    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;
    }
    public int size() {
        return size;
    }
    public boolean isEmpty() {
        return size == 0;
    }
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }
    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;
    }
    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;
}
    //返回此ArrayList實例的淺拷貝。
    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(e);
        }
    }

    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }
    //以正確的順序（從第一個到最後一個元素）返回一個包含此列表中所有元素的數組。返回的數組將是“安全的”，因爲該列表不保留對它的引用。 （換句話說，這個方法必須分配一個新的數組）。 因此，調用者可以自由地修改返回的數組。此方法作爲基於陣列和基於集合的API之間的橋樑。
    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;
    }
    E elementData(int index) {
        return (E) elementData[index];
    }
    public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    }

    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    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++;
    }
    //刪除此列表中指定位置的元素。 將任何後續元素移動到左側（從其索引中減去一個元素）。
    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;
    }
    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 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
    }
    //從此列表中刪除所有元素。 此通話返回後，列表將爲空。
    public void clear() {
        modCount++;

        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }
    //追加所有指定集合中的元素到此列表的結尾，它們是由指定集合的迭代器返回的順序。 如果指定的集合在操作進行中被修改，則此操作的行爲是未定義的。 （這意味着如果指定的集合是此列表，並且此列表是非空的，則此調用的行爲是未定義的。）

    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);
        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;
    }

    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }

    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }
    //僅保留此列表中包含在指定集合中的元素。 換句話說，從此列表中刪除其中不包含在指定集合中的所有元素。
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            if (r != size) {
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
            if (w != size) {
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                modCount += size - w;
                size = w;
                modified = true;
            }
        }
        return modified;
    }
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        int expectedModCount = modCount;
        s.defaultWriteObject();
        s.writeInt(size);
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;
        s.defaultReadObject();
        s.readInt(); // ignored
        if (size > 0) {
            ensureCapacityInternal(size);
            Object[] a = elementData;
            for (int i=0; i<size; i++) {
                a[i] = s.readObject();
            }
        }
    }

圖解：

不說廢話，直接上圖解：

add()

add(int index, E element)

remove(intindex)

clear()

addAll(Collection<?extends E> c)

addAll(intindex, Collection<? extends E> c)

removeRange(intfromIndex, int toIndex)