public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable RandomAccess: 实现该接口可以通过下标序号快速访问 Cloneable: 能够被克隆 Serializable：这一个则是支持序列化的
成员变量
//这个是表示默认的容量大小是10 private static final int DEFAULT_CAPACITY = 10; //表示这是一个空的数组 private static final Object[] EMPTY_ELEMENTDATA = {}; //这个是和上面的那个又区别的，当添加第一个元素的时候会扩容 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; //是真正存储数据的地方 transient Object[] elementData; // non-private to simplify nested class access //这个是数组中元素的个数 private int size;
构造函数
//这个是通过给定一个具体的值(初始化长度)来进行构造，当然给定的值不能小于0，不然会抛异常 public ArrayList(int initialCapacity) { if (initialCapacity > 0) { this.elementData = new Object[initialCapacity]; } else if (initialCapacity == 0) { //如果值为0，则会构造一个空的数组 this.elementData = EMPTY_ELEMENTDATA; } else { throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); } } //没有参数传递的话，则默认初始化容量为10，并赋予一个空的数组DEFAULTCAPACITY_EMPTY_ELEMENTDATA public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } //使用已有的集合c来创建一个新的ArrayList， public ArrayList(Collection<? extends E> c) { //先将集合转成数组 elementData = c.toArray(); //如果c里面有数据 if ((size = elementData.length) != 0) { // c.toArray might (incorrectly) not return Object[] (see 6260652) //判断转换的数组是否是Object[]类型 if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); } else { // c里面没有数据 this.elementData = EMPTY_ELEMENTDATA; } }
修改容量大小
public void trimToSize() { modCount++; //也就是说数组中实际的个数小于数组的容量 if (size < elementData.length) { elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size); } } //将集合的容量增加为 minCapacity public void ensureCapacity(int minCapacity) { int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) // elementData 里面有数据 ? 0 // larger than default for default empty table. It's already // supposed to be at default size. : DEFAULT_CAPACITY; //值为10 if (minCapacity > minExpand) { ensureExplicitCapacity(minCapacity); } } 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 static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; // *增加容量以确保它至少可以容纳由minimum capacity参数指定的元素数量。 * @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; }
添加元素
//添加元素到末尾 public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } //添加元素到指定的位置 public void add(int index, E element) { //顾名思义，检查检查索引是否越界 rangeCheckForAdd(index); ensureCapacityInternal(size + 1); // Increments modCount!! //这个是重点 System.arraycopy(elementData, index, elementData, index + 1, size - index); elementData[index] = element; size++; } private void rangeCheckForAdd(int index) { if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } //将集合添加到数组里面 public boolean addAll(Collection<? extends E> c) { //集合转数组 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; } //这个是指定要将集合插入那个位置 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; }
删除元素
//删除指定位置的元素 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; } public boolean remove(Object o) { //传过来的值为null 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; } //这个是把集合清空，也就是将里面的值都值为null public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; } //删除fromIndex到toIndex之间的全部元素 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; }