继承关系
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
常量
private static final long serialVersionUID = 8683452581122892189L; /** * Default initial capacity. * 默认初始化容量 */ private static final int DEFAULT_CAPACITY = 10; /** * Shared empty array instance used for empty instances. * 空数组共享实例 */ 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. * 缺省大小的空数组共享实例 * 与EMPTY_ELEMENTDATA区分开，以便知道第一个元素添加时如何扩容 */ private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /** * The maximum size of array to allocate. * Some VMs reserve some header words in an array. * Attempts to allocate larger arrays may result in * OutOfMemoryError: Requested array size exceeds VM limit * 最大可分配大小
存内存存储对象头信息和对象头信息 8个字节 2<sup>31</sup>-1 0111后面28个1，也就是一共31个1的整数，2^31 - 1
*/ private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
成员变量
/** * The array buffer into which the elements of the ArrayList are stored. * The capacity of the ArrayList is the length of this array buffer. Any * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA * will be expanded to DEFAULT_CAPACITY when the first element is added. */ transient Object[] elementData; // non-private to simplify nested class access 非私有，以简化嵌套类的访问
当对象存储时，它的值不需要维持，敏感信息例如密码，用transient关键字标记的成员变量不参与序列化过程
/** * The size of the ArrayList (the number of elements it contains). * * @serial */ private int size;
构造函数
/** * Constructs an empty list with the specified initial capacity. * 根据给定初始容量构造一个空的list * * @param initialCapacity the initial capacity of the list * list初始容量 * @throws IllegalArgumentException if the specified initial capacity * is negative * 当给定的初始容量非负时抛出异常 */ public ArrayList(int initialCapacity) { // 判断给定初始化容量是否合法 if (initialCapacity > 0) { // 创建数组 this.elementData = new Object[initialCapacity]; } else if (initialCapacity == 0) { // 指向EMPTY_ELEMENTDATA 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 */ // ？是“任意类” E是指定类型 public ArrayList(Collection<? extends E> c) { elementData = c.toArray(); if ((size = elementData.length) != 0) {//elementData 非空时候 // c.toArray might (incorrectly) not return Object[] (see 6260652) // c.toArray不返回 Object[]的时候，则进行数组拷贝 这边不理解
//当elementData不是Object数组时，它会使用Arrays.copyOf()方法构造一个Object数组替换elementData
if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); } else { // replace with empty array. this.elementData = EMPTY_ELEMENTDATA; } }
函数
增加
尾部增加元素
指定位置添加元素
尾部增加集合
指定位置添加集合
/** * Appends the specified element to the end of this list. * * @param e element to be appended to this list * @return <tt>true</tt> (as specified by {@link Collection#add}) */ public boolean add(E e) { // 确保内部容量 ensureCapacityInternal(size + 1); // Increments modCount!! // 尾部放入元素 容量+1 elementData[size++] = e; return true; } private static int calculateCapacity(Object[] elementData, int minCapacity) {
无参数初始化ArrayList时，每次插入元素，都需要 取 10和容量的 最大值吗？DEFAULTCAPACITY_EMPTY_ELEMENTDATA初始为空数组，之后增加元素，但地址仍不变，所以 仍 == ？ if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { return Math.max(DEFAULT_CAPACITY, minCapacity); } return minCapacity; } private void ensureCapacityInternal(int minCapacity) { ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); } private void ensureExplicitCapacity(int minCapacity) { //操作计算+1 modCount++; // overflow-conscious code // 只有当容量不够时才扩容 if (minCapacity - elementData.length > 0) grow(minCapacity); } /** * The maximum size of array to allocate. * Some VMs reserve some header words in an array. * Attempts to allocate larger arrays may result in * OutOfMemoryError: Requested array size exceeds VM limit * 最大可分配大小 */ private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; /** * Increases the capacity to ensure that it can hold at least the * number of elements specified by the minimum capacity argument. * 缓冲数组扩容以确保能够存储给定元素 * @param minCapacity the desired minimum capacity * 期望的最小容量 */ private void grow(int minCapacity) { // overflow-conscious code // 现有的元素长度 int oldCapacity = elementData.length; // 新容量 为 旧容量 + 旧容量的一半 int newCapacity = oldCapacity + (oldCapacity >> 1); // 如果计算的新容量 比 期望的最小容量 小，则采用 期望的最小容量作为扩容参数 if (newCapacity - minCapacity < 0) newCapacity = minCapacity; // 判断是否超过最大数组容量 if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); // minCapacity is usually close to size, so this is a win: // 最小扩容容量通常接近size elementData = Arrays.copyOf(elementData, newCapacity); } /** * * 取得最大容量 */ private static int hugeCapacity(int minCapacity) { // 溢出 if (minCapacity < 0) // overflow throw new OutOfMemoryError(); // 取最大容量 不懂 return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; }
/** * 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). * 指定位置添加元素 * @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) { //判断边界，可能会发生数组越界 rangeCheckForAdd(index); //确保内部容量 ensureCapacityInternal(size + 1); // Increments modCount!! //调用效率较高的System.arraycopy进行数组复制 //目的为空出指定位置 System.arraycopy(elementData, index, elementData, index + 1, size - index); //指定位置上放入指定元素 elementData[index] = element; //容量+1 size++; }
/** * A version of rangeCheck used by add and addAll. */ private void rangeCheckForAdd(int index) { if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); }
增加 集合 /** * Appends all of the elements in the specified collection to the end of * this list, in the order that they are returned by the * specified collection's Iterator. The behavior of this operation is * undefined if the specified collection is modified while the operation * is in progress. (This implies that the behavior of this call is * undefined if the specified collection is this list, and this * list is nonempty.) * * @param c collection containing elements to be added to this list * @return <tt>true</tt> if this list changed as a result of the call * @throws NullPointerException if the specified collection is null */ 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; } /** * Inserts all of the elements in the specified collection into this * list, starting at the specified position. Shifts the element * currently at that position (if any) and any subsequent elements to * the right (increases their indices). The new elements will appear * in the list in the order that they are returned by the * specified collection's iterator. * * @param index index at which to insert the first element from the * specified collection * @param c collection containing elements to be added to this list * @return <tt>true</tt> if this list changed as a result of the call * @throws IndexOutOfBoundsException {@inheritDoc} * @throws NullPointerException if the specified collection is null */ 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; }
set 修改
/** * Replaces the element at the specified position in this list with * the specified element. * * @param index index of the element to replace * @param element element to be stored at the specified position * @return the element previously at the specified position * @throws IndexOutOfBoundsException {@inheritDoc} */ public E set(int index, E element) { // 范围检查 rangeCheck(index); // 取出旧值用以返回 E oldValue = elementData(index); // 放入新的值 elementData[index] = element; return oldValue; } /** * Checks if the given index is in range. If not, throws an appropriate * runtime exception. This method does *not* check if the index is * negative: It is always used immediately prior to an array access,没看明白为什么不检查负数 * which throws an ArrayIndexOutOfBoundsException if index is negative. */ private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); }
remove 删除
根据下标移除 根据对象移除 根据集合移除 根据过滤器移除 根据范围移除(受保护的方法) /** * 删除指定位置的元素，后继元素 左移 * Removes the element at the specified position in this list. * Shifts any subsequent elements to the left (subtracts one from their * indices). * * @param index the index of the element to be removed * 下标 * @return the element that was removed from the list * 被移除的元素 * @throws IndexOutOfBoundsException {@inheritDoc} */ public E remove(int index) { // 下标检查 不检查 负数 rangeCheck(index); // 操作计数 +1 modCount++; //获取指定位置的元素，用于返回 E oldValue = elementData(index); int numMoved = size - index - 1; // index 位置后面的元素 System.arraycopy的方式 前移 if (numMoved > 0) System.arraycopy(elementData, index + 1, elementData, index, numMoved); // 最后一个数组元素引用置null 方便GC elementData[--size] = null; // clear to let GC do its work return oldValue; } /** * 当元素存在的时候，删除第一个找到的指定的元素 * 如果元素不存在，则list不会变动 * Removes the first occurrence of the specified element from this list, * if it is present. If the list does not contain the element, it is * unchanged. More formally, removes the element with the lowest index * <tt>i</tt> such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt> * (if such an element exists). Returns <tt>true</tt> if this list * contained the specified element (or equivalently, if this list * changed as a result of the call). * * @param o element to be removed from this list, if present * @return <tt>true</tt> if this list contained the specified element */ public boolean remove(Object o) { // o 是否是null元素，数组允许存储null if (o == null) { for (int index = 0; index < size; index++) // 用 == 比较 null if (elementData[index] == null) { fastRemove(index); return true; } } else { for (int index = 0; index < size; index++) // 用 equals比较 非null if (o.equals(elementData[index])) { fastRemove(index); return true; } } return false; } /** * // 根据给定的集合，将list中与集合相同的元素全部删除 * Removes from this list all of its elements that are contained in the * specified collection. * * @param c collection containing elements to be removed from 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 * (<a href="Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if this list contains a null element and the * specified collection does not permit null elements * (<a href="Collection.html#optional-restrictions">optional</a>), * or if the specified collection is null * @see Collection#contains(Object)
*/ public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c); //判断非空，空抛出异常 return batchRemove(c, false); } /** * 通过一个过滤器接口实现，来实现删除 * @param filter * @return */ @Override public boolean removeIf(Predicate<? super E> filter) { Objects.requireNonNull(filter); // figure out which elements are to be removed // any exception thrown from the filter predicate at this stage // will leave the collection unmodified // 用bitset来存储哪些下标对应的元素要删除，哪些下标对应的元素要保存 // Bitset中主要存储的是二进制位，做的也都是位运算，每一位只用来存储0，1值，主要用于对数据的标记。 int removeCount = 0; final BitSet removeSet = new BitSet(size); // 判断并发修改用 final int expectedModCount = modCount; final int size = this.size; // 按顺序遍历且没有并发修改 for (int i = 0; modCount == expectedModCount && i < size; i++) { @SuppressWarnings("unchecked") //告诉编译器忽略 unchecked 警告信息，如使用List，ArrayList等未进行参数化产生的警告信息。
final E element = (E) elementData[i];
//利用过滤器匹配元素，如果匹配，则删除计数+1，并将下标进行存储 if (filter.test(element)) {//Predicate是个断言式接口 and，or和negate &&、||和! isEqual removeSet.set(i); removeCount++; } } // 判断是否存在并发修改 if (modCount != expectedModCount) { // 抛出并发修改异常 throw new ConcurrentModificationException(); } // shift surviving elements left over the spaces left by removed elements // 判断是否有要删除的元素 final boolean anyToRemove = removeCount > 0; if (anyToRemove) { final int newSize = size - removeCount; for (int i = 0, j = 0; (i < size) && (j < newSize); i++, j++) { // 下一个要保存的元素 i = removeSet.nextClearBit(i); // 存放到新数组 elementData[j] = elementData[i]; } // 把实际要保存的元素之后的全部 置为 null ,用以GC // 实际上，上面的操作已经将要保留的元素全部前移了，后面的元素都是不保留的，所以要置为null来帮助GC for (int k = newSize; k < size; k++) { elementData[k] = null; // Let gc do its work } //设置size this.size = newSize; // 判断是否并发修改 if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } modCount++; } return anyToRemove; } /** * 删除指定范围的元素 * Removes from this list all of the elements whose index is between * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive. * Shifts any succeeding elements to the left (reduces their index). * This call shortens the list by {@code (toIndex - fromIndex)} elements. * (If {@code toIndex==fromIndex}, this operation has no effect.) * * @throws IndexOutOfBoundsException if {@code fromIndex} or * {@code toIndex} is out of range * ({@code fromIndex < 0 || * fromIndex >= size() || * toIndex > size() || * toIndex < fromIndex}) */ protected void removeRange(int fromIndex, int toIndex) { modCount++; // 1保留 - 2删除 - 3保留 将3copy到1后面，多余部分置空，帮助GC 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; } /** * 批量删除 * @param c * @param complement * @return */ 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++) // complement // removeAll 为false ,下标r的元素不在删除集合c中，存储的是 不删除的元素 // retainAll 为true ,下标r的元素在删除集合c中，存储的是 删除的元素 if (c.contains(elementData[r]) == complement) elementData[w++] = elementData[r]; } finally { // Preserve behavioral compatibility with AbstractCollection, // even if c.contains() throws. // 这里判断循环是否正常 if (r != size) { // 如果不正常的话，需要挪动元素 System.arraycopy(elementData, r, elementData, w, size - r); w += size - r; } // 如果需要删除元素的话，则证明一部分位置需要置null,帮助GC，设置修改GC 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; }
get 获取
/** * Returns the element at the specified position in this list. * * @param index index of the element to return * @return the element at the specified position in this list * @throws IndexOutOfBoundsException {@inheritDoc} */ public E get(int index) { rangeCheck(index); return elementData(index); }
contains
public boolean contains(Object o) { return indexOf(o) >= 0; } /** * 首次出现下标 * Returns the index of the first occurrence of the specified element * in this list, or -1 if this list does not contain the element. * More formally, returns the lowest index <tt>i</tt> such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, * or -1 if there is no such index. */ 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; } /** * Returns the index of the last occurrence of the specified element * in this list, or -1 if this list does not contain the element. * More formally, returns the highest index <tt>i</tt> such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, * or -1 if there is no such index. */ 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; }
clear
/** * Removes all of the elements from this list. The list will * be empty after this call returns. */ public void clear() { // 修改计数 modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; }
public Object[] toArray()和public T[] toArray(T[] a) 以后再看 List<Integer> list = new ArrayList<>(); Collections.addAll(list,1,2,3,4,5,6); // 方式1 // list.toArray(new Integer[0]); //涉及到反射，效率较低 // 方式2 // list.toArray(new Integer[list.size()]);
总结