ArrayList
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ArrayList
ArrayList是基于底层数组的实现List。故对于LinkedList,ArrayListy善于随机访问数组,插入和删除则比较慢。
ArrayList的定义
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
- 从ArrayList<\E>可以看出它是支持泛型的,它继承自AbstractList,实现了List、RandomAccess、Cloneable、java.io.Serializable接口。
常量
private static final long serialVersionUID = 8683452581122892189L;private static final int DEFAULT_CAPACITY = 10;private static final Object[] EMPTY_ELEMENTDATA = {};private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};transient Object[] elementData; private int size;
- serialVersionUID :Java的序列化机制是通过serialVersionUID来验证版本一致性。
- DEFAULT_CAPACITY :数组初始容量。
- EMPTY_ELEMENTDATA:空数组,用以区别DEFAULTCAPACITY_EMPTY_ELEMENTDATA 。
- DEFAULTCAPACITY_EMPTY_ELEMENTDATA :空数组,无参构造器被使用。
- size:表示数组的大小。
构造器
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;} 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; } }
- ArrayList(int initialCapacity):有参构造器,创建容量为initialCapacity的数组。
- ArrayList() :无参构造器,elementData指向DEFAULTCAPACITY_EMPTY_ELEMENTDATA数组(容量为0但第一次对elementData使用add()方法时,会对其扩容,扩容默认大小为10)
- ArrayList(Collection< ? extends E> c):创建的数组所含元素与c中的顺序、大小一致。
添加
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++;}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 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); }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; }
调用add(E e)或add(int index, E element)方法,首先会调用ensureCapacityInternal(int minCapacity)方法,确保数组有空间保存新添加的数组。
ensureCapacityInternal(int minCapacity)方法,先判断数组elementData 是否由无参构造器创建,若是则在该数组第一次创建时把minCapacity赋值为10;
ensureExplicitCapacity(int minCapacity)方法,会比较最小容量minCapacity与数组实际大小。若小则说明数组有足够空间保存新值,若大则说明数组没有足够空间保存新值且需要对数组扩容即调用grow(int minCapacity)方法。
grow(int minCapacity)方法中newCapacity = oldCapacity + (oldCapacity >> 1);即newCapacity =1.5倍elementData.length;也就是在newCapacity - minCapacity < 0为假且不越界的情况下会把数组扩大至1.5倍elementData.length;
删除
public E remove(int index) { rangeCheck(index);//检查是否越界 modCount++;//Fail-Fast 机制 E oldValue = elementData(index); int numMoved = size - index - 1;// 确认边界并把index后的元素向前移一位; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved);// 把数组最后一位设置为null;原本元素即在GC时被回收 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 } public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; }public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, false); }protected void removeRange(int fromIndex, int toIndex) { modCount++; int numMoved = size - toIndex;// 把toIndex后的元素移动到fromIndex后 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; } public static void arraycopy(Object srsc,//源数组 int srcPos,//源数组要复制的起始位置 Object dest,//目的数组 int destPos,//目的数组放置的起始位置 int length) //复制的长度
System.arraycopy()方法把srsc第srcPos后的元素复制到dest数组第destpos位置;
remove(int index)方法,先检查是否越界后把index后的元素向前移一位,从而完成删除功能;
- remove(Object o)方法,先找到o元素后由于o元素下标在范围内不会造成越界,故使用fastRemove(int index)方法快速删除;
- fastRemove(int index)方法,跳过了越界检查故remove(int index)快;
- Clear()方法把elementData数组中每个元素设为null;GC时会适当的回收;
- removeRange(int fromIndex, int toIndex)方法把elementData数组下标fromIndex到toIndex的元素删除,运用到了System.arraycopy()方法;
Fail-Fast机制–modCount
- 在多线程下,由于增加、删除操作会造成线程不安全,故用modCount来保障,若modCount与expectedModCount不一致则会造成异常,从而保障数据安全;
public ListIterator<E> listIterator() { return new ListItr(0); }public Iterator<E> iterator() { return new Itr();} private class Itr implements Iterator<E> { 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<? 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(); } }
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