Java ArrayList源码小结
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ArrayList是Java集合框架中的动态数组:
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
ArrayList实现了List接口,实现了RandomAccess(随机访问)、Serializable(序列化);
RandomAccess接口是一个标记接口,实现了RandomAccess接口的集合类,使用for(int i=0;i<list.size();i++) {list.get(i);}的效率要高于使用增强for循环遍历集合;
private static final int DEFAULT_CAPACITY = 10;
ArrayList底层由数组实现,默认容量为10;
private int size;
底层数组的长度,注意与Capacity区分;
构造函数有:public ArrayList(int initialCapacity),自定义底层数组大小;
public ArrayList(),默认数组大小;
public ArrayList(Collection<? extends E> c),复制构造方法;
public void trimToSize() { modCount++; if (size < elementData.length) { elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size); } }去除预留元素的空间,使capacity=size;
ArrayList通过grow方法实现数组的动态变容:
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); }如果数组现有长度的1.5倍大于minCapacity,则扩容至现长度的1.5倍,反之则扩容至minCapacity的大小;
public int size() { return size; }得到底层数组的长度,时间复杂度为常数级别;
public boolean isEmpty() { return size == 0; }判断数组是否为空;
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; }得到o对象第一次出现的下标,o对象可以为空,比较时采用的是equals方法,若没有找到相等的对象,返回-1;
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; }反着搜索;
public boolean contains(Object o) { return indexOf(o) >= 0; }调用indexOf方法,判断集合中是否有o对象;
public Object[] toArray() { return Arrays.copyOf(elementData, size); }拷贝ArrayList的底层数组并返回,返回数组的长度与底层数组的size一致;
public E get(int index) { rangeCheck(index); return elementData(index); }get方法先做安全性检查rangeCheck再得到index位置上的值;
public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; }set方法先rangeCheck再修改index位置上的值;
我们再来看rangeCheck方法:
private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); }注释上说,这个方法不检查index是否为负值?
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++; }第一个add方法将元素添加到数组末尾;第二个add方法调用System.arraycopy将elementData从index开始的size-index个元素复制到index+1至size+1的位置(即index开始的元素都向后移动一个位置),然后将element插入到index位置;
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; }删除某一位置元素,调用System.arraycopy将elementData从index+1开始的numMoved个元素复制到index为开头的位置;返回删除的元素的值;
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 }fastRemove方法:不做下标检查,直接删除,也没有返回值;
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; }先搜索,再调用fastRemove方法;因为已经判定index值有效,可以跳过下标检查,采用fastRemove方法;
public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; }将数组元素全部指向null,等待GC;
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; }
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; }先将集合c转换成数组,再添加或插入,并且更新size;
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; }
删除数组的一部分,并将元素指向null;
参考博客:http://blog.csdn.net/jzhf2012/article/details/8540410
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