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 extends E> 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
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 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.
*
*
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,eadd()
/** * 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 super E> 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
private class SubList extends AbstractList
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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