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【java源码一带一路系列】之ArrayList

RebeccaZhong / 1779人阅读

摘要:一路至此,风景过半。与虽然名字各异,源码实现基本相同,除了增加了线程安全。同时注意溢出情况处理。同时增加了考虑并发问题。此外,源码中出现了大量泛型如。允许为非线程安全有序。

一路至此,风景过半。ArrayList与Vector虽然名字各异,源码实现基本相同,除了Vector增加了线程安全。所以作者建议我们在不需要线程安全的情况下尽量使用ArrayList。下面看看在ArrayList源码中遇到什么有趣的事情。

DEFAULTCAPACITY_EMPTY_ELEMENTDATA与EMPTY_ELEMENTDATA
/**
 * 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 = {};

/**
 * 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 c) {
    elementData = c.toArray();
    if ((size = elementData.length) != 0) {
        // c.toArray might (incorrectly) not return Object[] (see 6260652) ①
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    } else {
        // replace with empty array.
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

区别:DEFAULTCAPACITY_EMPTY_ELEMENTDATA用于无参初始化;EMPTY_ELEMENTDATA用于指定容量为0时的初始化。

trimToSize()
/**
 * 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++;
    if (size < elementData.length) {
        elementData = (size == 0)
          ? EMPTY_ELEMENTDATA
          : Arrays.copyOf(elementData, size);
    }
}

去除扩容后未存放元素的预留空间,以size为基准。

ensureCapacity() --> ensureExplicitCapacity() --> grow() --> hugeCapacity()
/**
 * 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) {
    int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
        // any size if not default element table
        ? 0
        // larger than default for default empty table. It"s already
        // supposed to be at default size.
        : DEFAULT_CAPACITY;

    if (minCapacity > minExpand) {
        ensureExplicitCapacity(minCapacity);
    }
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

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

预设容量(提前扩容),可提高初始化效率。扩容后比扩容前多了“oldCapacity >> 1”(即多了原来的50%)。同时注意溢出情况处理。(overflow-conscious code)。即“a-b<0”而不是"a

int a = Integer.MAX_VALUE;
int b = Integer.MAX_VALUE + 1;    
System.out.println(a < b); // false    
System.out.println(a - b < 0); // true
toArray()
/**
 * 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 */ @SuppressWarnings("unchecked") 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; return a; }

当传入数组长度大于ArrayList的size时,将a[size]置空作为调用者判断标志。根据这段代码写了个demo帮助理解:(扩展知识见②)

ArrayList al = new ArrayList();
al.add("s");

String[] s = {"c","h","e"};
String[] sal = (String[]) al.toArray(s);
System.out.println(sal[0] + "," + sal[1] + "," + sal[2]); // s,null,e
add()
/**
 * 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++;
}

新增、删除都用到了System.arraycopy(Object src, int srcPos, Object dest, int destPos, int length);下面举例加深理解。

String[] arr ={"r", "e", "b", "e", "y", "."}; 
System.arraycopy(arr, 0, arr, 2, 2);
for(String i : arr) {
    System.out.println(i);
}

即将arr下标从0开始的2个元素拷贝到arr下标从2开始的位置。

retainAll()
/**
 * Retains only the elements in this list that are contained in the
 * specified collection.  In other words, removes from this list all
 * of its elements that are not contained in the specified collection.
 *
 * @param c collection containing elements to be retained in this list
 * @return {@code true} if this list changed as a result of the call
 * @throws ClassCastException if the class of an element of this list
 *         is incompatible with the specified collection
 * (optional)
 * @throws NullPointerException if this list contains a null element and the
 *         specified collection does not permit null elements
 * (optional),
 *         or if the specified collection is null
 * @see Collection#contains(Object)
 */
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.
        // 保证异常时,未比较元素不丢失
        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;
}

a.retainAll(c)可以看成取a与c交集,a非c子集时,返回true。a中只留在c中存在的元素,其余删除。否则,返回false。

“elementData[w++] = elementData[r];”w永远小于等于r,因此可以将找到的相等元素大胆的放在elementData[w++]中(elementData[w++]是先放后加)。

iterator()
/**
 * Returns an iterator over the elements in this list in proper sequence.
 *
 * 

The returned iterator is fail-fast. * * @return an iterator over the elements in this list in proper sequence */ public Iterator iterator() { return new Itr(); } /** * An optimized version of AbstractList.Itr */ private class Itr implements Iterator { int cursor; // index of next element to return int lastRet = -1; // index of last element returned; -1 if no such int expectedModCount = modCount; // 是否有下一个元素 public boolean hasNext() { return cursor != size; } // 游标移动 @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]; //第一次cursor=1,lastRet=0 } // lastRet不等于-1时才能进行删除,即next()后才能使用remove() 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 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(); } }

Iterator()所指类似数据库的游标,不了解的同学可参看以下解释:

当使用语句Iterator it=List.Iterator()时,迭代器it指向的位置是Iterator1指向的位置,当执行语句it.next()之后,迭代器指向的位置后移到Iterator2指向的位置。[1]

由源码可见ArrayList的迭代器基于Itr子类实现。该类实现了Iterator接口,并重写了它的全部方法(4种)。同时增加了checkForComodification()考虑并发问题。

listIterator()
/**
 * Returns a list iterator over the elements in this list (in proper
 * sequence), starting at the specified position in the list.
 * The specified index indicates the first element that would be
 * returned by an initial call to {@link ListIterator#next next}.
 * An initial call to {@link ListIterator#previous previous} would
 * return the element with the specified index minus one.
 *
 * 

The returned list iterator is fail-fast. * * @throws IndexOutOfBoundsException {@inheritDoc} */ public ListIterator listIterator(int index) { if (index < 0 || index > size) throw new IndexOutOfBoundsException("Index: "+index); return new ListItr(index); } /** * Returns a list iterator over the elements in this list (in proper * sequence). * *

The returned list iterator is fail-fast. * * @see #listIterator(int) */ public ListIterator listIterator() { return new ListItr(0); } /** * An optimized version of AbstractList.ListItr */ private class ListItr extends Itr implements ListIterator { ListItr(int index) { super(); cursor = index; } // 向前 public boolean hasPrevious() { return cursor != 0; } public int nextIndex() { return cursor; } public int previousIndex() { return cursor - 1; } @SuppressWarnings("unchecked") public E previous() { checkForComodification(); int i = cursor - 1; if (i < 0) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) throw new ConcurrentModificationException(); cursor = i; return (E) elementData[lastRet = i]; } // 替换 public void set(E e) { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.set(lastRet, e); } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void add(E e) { checkForComodification(); try { int i = cursor; ArrayList.this.add(i, e); cursor = i + 1; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } }

listIterator有2中构造方法,即它多了指定游标的功能。它实现了ListIterator extends Iterator接口,比Iterator多了一些方法。Iterator只能从前往后删除,而listIterator可实现从后往前删除/替换。同时也提供了获取前后下标的方法。

说点什么

private class SubList extends AbstractList implements RandomAccess {...} 源码太长就不贴出来了。它是ArrayList的子类,表示ArrayList的子集,需要注意的是,对它的数据进行更改会影响原数据。

此外,源码中出现了大量泛型(如T、E...)。希望顺便巩固泛型知识。

ArrayList允许为null;非线程安全;有序。

更多有意思的内容,欢迎访问笔者小站: rebey.cn

知识点

1.[ArrayList c.toArray might (incorrectly) not return Object[] (see 6260652);](http://www.cnblogs.com/cmdra/...

2.[为什么 Java ArrayList.toArray(T[]) 方法的参数类型是 T 而不是 E ?](http://www.cnblogs.com/xiaomi...;2016.04.07;

[1]JAVA中ListIterator和Iterator详解与辨析;2014.11.27;

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