Java源碼分析之ArrayList

先看私有屬性

//保存ArrayList中的內容
transient Object[] elementData; // non-private to simplify nested class access
//表示元素的數量
private int size;

transient 關鍵字，就是這部分不參與序列化

構造函數

構造函數有三個

//沒有參數時，構建一個空的
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

//當有一個參數時，就是代表開多大的空間，注意，這裏並沒有設size的值哦
private static final Object[] EMPTY_ELEMENTDATA = {};
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);
    }
}

//用集合構造，轉成數組後，返回給elementData，返回若不是Object[]將調用Arrays.copyOf方法將其轉爲Object[]
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;
    }
}

基本的方法

get方法

//正常的get方法
public E get(int index) {
    rangeCheck(index);

    return elementData(index);
}
//對index的檢測
private void rangeCheck(int index) {
    if (index >= size)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
//注意這個是沒有邊界檢測的
@SuppressWarnings("unchecked")
E elementData(int index) {
    return (E) elementData[index];
}

set方法

//有index的檢測，返回的是舊的值
public E set(int index, E element) {
    rangeCheck(index);

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

add方法

//ensureCapacityInternal稍後講，除了爲了有足夠大的空間，還有其他的作用
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

//在index位置處插入element
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++;
}
//這裏檢測是否在那個區間內，個人感覺有點兒問題，比如，初始構造給了參數，接下來在某個位置add一個數，換句話說，有了空間，但該位置沒數，此時該不該拋出越界異常
private void rangeCheckForAdd(int index) {
    if (index > size || index < 0)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

addAll

//加入集合中的全部，多開空間
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;
}

//在index位置上加入集合中的全部
public boolean addAll(int index, Collection<? extends E> c) {
    rangeCheckForAdd(index);

    Object[] a = c.toArray();
    int numNew = a.length;
    ensureCapacityInternal(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;
}

remove方法

//先看下是否越界，再移除，返回的是舊的值
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;
}

//移除第一個與o相等的對象，注意remove(int index)是移除index位置的，如果想要移除的Object本身就是int該怎麼辦，hiahia
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;
}
//和remove(int index)一樣，爲什麼不直接調用呢？
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
}

//移除fromIndex，toIndex中的元素(這個沒有判斷越界？
protected void removeRange(int fromIndex, int toIndex) {
    modCount++;
    int numMoved = size - toIndex;
    System.arraycopy(elementData, toIndex, elementData, fromIndex,
                     numMoved);

    // clear to let GC do its work
    int newSize = size - (toIndex-fromIndex);
    for (int i = newSize; i < size; i++) {
        elementData[i] = null;
    }
    size = newSize;
}

size方法

public int size() {
    return size;
}

isEmpty()方法

public boolean isEmpty() {
    return size == 0;
}

indexOf方法(O(n))

//返回o的第一次出現的下標，注意是從前往後依次遍歷的
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;
}

//返回o最後一次出現的位置，是從後往前依次遍歷的
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;
}

contains方法

//就是判斷第一次出現的位置，也是O(n)，(我還以爲會用HashSet之類的呢
public boolean contains(Object o) {
    return indexOf(o) >= 0;
}

toArray方法

public Object[] toArray() {
    return Arrays.copyOf(elementData, size);
}

@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;
}

clear

public void clear() {
    modCount++;

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

    size = 0;
}

subList

public List<E> subList(int fromIndex, int toIndex) {
   subListRangeCheck(fromIndex, toIndex, size);
      return new SubList(this, 0, fromIndex, toIndex);
}
//這裏fromIndex > toIndex並不會交換
static void subListRangeCheck(int fromIndex, int toIndex, int size) {
    if (fromIndex < 0)
        throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
    if (toIndex > size)
        throw new IndexOutOfBoundsException("toIndex = " + toIndex);
    if (fromIndex > toIndex)
        throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                           ") > toIndex(" + toIndex + ")");
}

稍高級點兒的方法

add時候的空間檢測

private static final int DEFAULT_CAPACITY = 10;
//取當前需要的空間和10的最大值
private void ensureCapacityInternal(int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }

    ensureExplicitCapacity(minCapacity);
}
//看element的已有空間，是否能滿足
private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}
//取需要的空間與原來空間1.5倍中最大值
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);
}
//所需空間非常大時候的判斷，如果是Integer.MAX_VALUE，需要84M，注意爆棧
private static int hugeCapacity(int minCapacity) {
   if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
}

求elementData與一個集合的交集和差集

//差集
public boolean removeAll(Collection<?> c) {
    Objects.requireNonNull(c);
    return batchRemove(c, false);
}
//交集
public boolean retainAll(Collection<?> c) {
   Objects.requireNonNull(c);
    return batchRemove(c, true);
}
//設一個complement變量來判斷是留或者去掉，感覺好機智啊
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;
}

modCount

對於這個變量，大家可能會好奇是什麼用，是爲了多線程中判斷是否被修改時使用，舉個例子

//可能很多人會問，明明上面剛剛賦值了expectedModCount ，只做了一個排序的操作，怎麼可能會變了呢。在這個線程1中，當然是不會變的，注意到modCount，修改這個值的函數，大概有刪除elementData中的值，修改elementData中的值，清除等，也就是說，如果其他線程2中值被修改了，modCount值就會變，那麼在線程1中modCount != expectedModCount了。但這還不是線程安全的，爲什麼呢，因爲知識拋出了異常，並沒有做出什麼操作(比如鎖等)，所以並不是線程安全的
public void sort(Comparator<? super E> c) {
    final int expectedModCount = modCount;
    Arrays.sort((E[]) elementData, 0, size, c);
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
    modCount++;
}

至此，對ArrayList有了基本的瞭解，其中很重要的Itr沒有說明，打算專門開一個說。