public class ArrayListextends AbstractList implements List , RandomAccess, Cloneable, java.io.Serializable
可变数组大小的List实现，允许所有的元素，包括null。（该类可粗略地看作是Vector，除了它不是同步化的）
size、isEmpty、get、set、iterator和listIterator操作的运行时间是常量。add操作对于添加n个元素，需要O(n)的时间。其他的操作需要线性时间。
每个ArrayList对象有一个capacity变量，它总是比list的size大一点，当一个元素添加到ArrayList时，它的capacity会自动地增大，以保证足够大的数组来存放元素。
ArrayList不是线程安全的。
3个实例变量
//默认的容器大小private static final int DEFAULT_CAPACITY = 10;//空数组,用于表示一个空的ArrayListprivate static final Object[] EMPTY_ELEMENTDATA = {};//装元素的容器transient Object[] elementData;//ArrayList的大小private int size;
3个构造器
//指定容器大小的构造器public ArrayList(int initialCapacity) { super(); if (initialCapacity < 0) throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); this.elementData = new Object[initialCapacity];}//使用默认容器大小public ArrayList() { super(); this.elementData = EMPTY_ELEMENTDATA;}//通过Collection实例构造ArrayListpublic 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);}
扩容方式
//确保容器大小,作为导出APIpublic void ensureCapacity(int minCapacity) { int minExpand = (elementData != EMPTY_ELEMENTDATA) // any size if real element table ? 0 // larger than default for empty table. It's already supposed to be // at default size. : DEFAULT_CAPACITY; if (minCapacity > minExpand) { ensureExplicitCapacity(minCapacity); }}//确保容器大小,类私有的private void ensureCapacityInternal(int minCapacity) { if (elementData == 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);}//扩容的方式是原容量的1.5倍 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;}
迭代器：
//返回普通的迭代器public Iteratoriterator() { return new Itr();}//普通的迭代器实现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; public boolean hasNext() { return cursor != size; } @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]; } 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(); }}
//返回列表迭代器,index指定初始位置public ListIteratorlistIterator(int index) { if (index < 0 || index > size) throw new IndexOutOfBoundsException("Index: "+index); return new ListItr(index);}//返回列表迭代器,初始位置为0public ListIterator listIterator() { return new ListItr(0);}//这是一个列表的迭代器,可以往前迭代,也可以往后迭代private class ListItr extends Itr implements ListIterator { ListItr(int index) { super(); cursor = index; } 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]; } 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(); } }}
//返回一个子列表,对该子列表的操作会影响原链表,如subList(f,t).clear()会把原列表f到t之间的元素删除public ListsubList(int fromIndex, int toIndex) { subListRangeCheck(fromIndex, toIndex, size); return new SubList(this, 0, fromIndex, toIndex);}