ArrayList的重要方法
grow(int minCapacity)
方法来增加list容量
/** * 增加list的容量,确保它至少可以存放指定的最小数量的元素. * @param minCapacity the desired minimum capacity */ private void grow(int minCapacity) { // overflow-conscious code int oldCapacity = elementData.length; // 新的容量=旧的容量*1.5 int newCapacity = oldCapacity + (oldCapacity >> 1); // 如果新的容量小于指定的最小容量,那么以指定的最小容量作为新的容量 if (newCapacity - minCapacity < 0) newCapacity = minCapacity; // 如果新的的容量大于最大限制容量,为了防止溢出, // 根据指定的最小容量来设置新的容量 if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); elementData = Arrays.copyOf(elementData, newCapacity); } 复制代码
add(E e)添加元素
/** * list末尾添加指定元素 * @param e element to be appended to this list * @return <tt>true</tt> (as specified by {@link Collection#add}) */ public boolean add(E e) { //增加一个元素前,确保list空间足够 ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } 复制代码
get(int index)获取元素
/** * 返回指定下标元素 * @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);//下标检查 return elementData(index); } 复制代码
删除元素remove(int index)
/** * 删除指定位置的元素.将该位置及后边的元素左移.返回旧的该位置的元素 * @param index the index of the element to be removed * @return the element that was removed from the list * @throws IndexOutOfBoundsException {@inheritDoc} */ public E remove(int index) { rangeCheck(index);//检查index合法性 modCount++; E oldValue = elementData(index); int numMoved = size - index - 1; if (numMoved > 0) //该位置及后面的元素全部往前移 System.arraycopy(elementData, index+1, elementData, index, numMoved); //list元素个数-1,全部元素左移后,将最后一个元素置为null elementData[--size] = null; // clear to let GC do its work return oldValue; } 复制代码
过滤器过滤元素removeIf(Predicate<? super E> filter)
@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 //删除的元素数量 int removeCount = 0; //BitSet是一个用来表示各个数值是否存在的类. //它将多个数值放在一个long[]中,long[]中的每一个值只能是0或者1. //0表示该index的数不存在,1表示该index的数存在. //例如三个数1,6,7存在long[]中,因为一个long值长度为64bit,因此,只需一个long值即可存 //存起来的结果是: [000...76000010],第1位,6位,7位的值置为1 final BitSet removeSet = new BitSet(size); //期望修改次数 final int expectedModCount = modCount; final int size = this.size; //遍历所有元素 for (int i=0; modCount == expectedModCount && i < size; i++) { @SuppressWarnings("unchecked") final E element = (E) elementData[i]; //如果该元素符合过滤条件 if (filter.test(element)) { //BitSet中标记 removeSet.set(i); 删除元素数量+1 removeCount++; } } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } // shift surviving elements left over the spaces left by removed elements final boolean anyToRemove = removeCount > 0; if (anyToRemove) { final int newSize = size - removeCount; //遍历所有元素 for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) { //i为下一个未删除的元素 i = removeSet.nextClearBit(i); //将未删除的元素集中挪到数组的前面 elementData[j] = elementData[i]; } //遍历未被删除的元素后面的所有元素,将所有元素置为null,等待回收 for (int k=newSize; k < size; k++) { elementData[k] = null; // Let gc do its work } //设置list中元素数量 this.size = newSize; if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } modCount++; } //返回是否有元素被删除 return anyToRemove; } 复制代码
源码全解读
package java.util; import java.util.function.Consumer; import java.util.function.Predicate; import java.util.function.UnaryOperator; /** * * @author Josh Bloch * @author Neal Gafter * @see Collection * @see List * @see LinkedList * @see Vector * @since 1.2 */ public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable { private static final long serialVersionUID = 8683452581122892189L; /** * DEFAULT_CAPACITY: 默认初始容量 */ private static final int DEFAULT_CAPACITY = 10; /** * 空的共享示例 */ 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. */ private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /** * 存储ArrayList的元素的数组缓冲区。ArrayList的容量是此数组缓冲区的长度。 */ transient Object[] elementData; * 非私有化为了简化类的嵌套访问 /** * ArrayList拥有的元素个数 * * @serial */ private int size; /** * 指定初始容量,构造一个空的list * @param initialCapacity the initial capacity of the list * @throws IllegalArgumentException if the specified initial capacity * is negative */ 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); } } /** * 构造一个空的list,默认初始容量为10 */ public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } /** * 使用集合来构造一个list * * @param c the collection whose elements are to be placed into this list * @throws NullPointerException if the specified collection is null */ public ArrayList(Collection<? extends E> c) { elementData = c.toArray(); if ((size = elementData.length) != 0) { // 如果返回的不是Object类型,那么新建一个数组,复制原来的元素并转换为Object类型 if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); } else { // 空数组 this.elementData = EMPTY_ELEMENTDATA; } } /** * 将list数组的大小设置到和size一样大,来节省存储空间 */ public void trimToSize() { modCount++; if (size < elementData.length) { elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size); } } /** * Increases the capacity of this <tt>ArrayList</tt> instance, if * necessary, to ensure that it can hold at least the number of elements * specified by the minimum capacity argument. * * @param minCapacity the desired minimum capacity */ public void ensureCapacity(int minCapacity) { // list不等于空,最小扩容就是0,否则最小容量是10 int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) // any size if not default element table ? 0 // larger than default for default empty table. It's already // supposed to be at default size. : DEFAULT_CAPACITY; // 如果最小容量大于最小扩容,那么确保最小容量可用,将list扩容到最小容量 if (minCapacity > minExpand) { ensureExplicitCapacity(minCapacity); } } private void ensureCapacityInternal(int minCapacity) { // 如果list为空,最小容量是10或者最小容量中较大的值 if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } // 确保最小容量可用,将list扩容到最小容量 ensureExplicitCapacity(minCapacity); } // 保证最小容量: 防止内存溢出 private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } /** * list的最大可分配大小.一些虚拟机需要在数组里保留一些头信息,获取更大的空间可 * 能会导致内存溢出. */ private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; /** * 增加list的容量,确保它至少可以存放指定的最小数量的元素. * @param minCapacity the desired minimum capacity */ private void grow(int minCapacity) { // overflow-conscious code int oldCapacity = elementData.length; // 新的容量=旧的容量*1.5 int newCapacity = oldCapacity + (oldCapacity >> 1); // 如果新的容量小于指定的最小容量,那么以指定的最小容量作为新的容量 if (newCapacity - minCapacity < 0) newCapacity = minCapacity; // 如果新的的容量大于最大限制容量,为了防止溢出, // 根据指定的最小容量来设置新的容量 if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); elementData = Arrays.copyOf(elementData, newCapacity); } private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) * 如果最小容量数字已经溢出了,抛异常 throw new OutOfMemoryError(); // 如果最小容量>Integer.MAX_VALUE - 8,那么最小容量 // 设为Integer.MAX_VALUE,否则最小容量设为Integer.MAX_VALUE-8 return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; } /** * 查询list中元素个数 */ public int size() { return size; } /** * 查询list是否为空 */ public boolean isEmpty() { return size == 0; } /** * 查询是否包含指定元素 */ public boolean contains(Object o) { return indexOf(o) >= 0; } /** * 遍历list查询指定元素的位置.如果没有返回-1 */ 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; } /** * 查询某个元素最后出现的位置,没有的话返回-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; } /** * 复制一个list,元素本身并不会被复制,相当于浅复制 */ public Object clone() { try { ArrayList<?> v = (ArrayList<?>) super.clone(); v.elementData = Arrays.copyOf(elementData, size); v.modCount = 0; return v; } catch (CloneNotSupportedException e) { * this shouldn't happen, since we are Cloneable throw new InternalError(e); } } /** * 返回一个包含所有元素并且顺序正确的数组. */ public Object[] toArray() { return Arrays.copyOf(elementData, size); } /** * 将list中的全部元素放到数组a中,并覆盖. * 如果a的空间足够,那么直接覆盖a,多余的空间用null填充, * 如果a中没有足够的空间,那么新生成一个数组, * 并将list中的元素全部放进去,而a不变. * @throws NullPointerException if the specified array is null */ @SuppressWarnings("unchecked") public <T> T[] toArray(T[] a) { if (a.length < size) // Make a new array of a's runtime type, but my contents: return (T[]) Arrays.copyOf(elementData, size, a.getClass()); System.arraycopy(elementData, 0, a, 0, size); if (a.length > size) a[size] = null; return a; } //下标查找 @SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; } /** * 返回指定下标元素 * @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);//下标检查 return elementData(index); } /** * 根据下标来设置元素,返回旧的该位置的元素 * @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) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; } /** * list末尾添加指定元素 * @param e element to be appended to this list * @return <tt>true</tt> (as specified by {@link Collection#add}) */ public boolean add(E e) { //增加一个元素前,确保list空间足够 ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } /** * 在制定位置增加新的元素,原来该位置的元素以及后边的元素全部往后挪一个位置. * @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) { rangeCheckForAdd(index);//检查index是否越界. //确保空间足够 ensureCapacityInternal(size + 1); // Increments modCount!! //将该位置及后边的元素右移 System.arraycopy(elementData, index, elementData, index + 1, size - index); //添加该元素 elementData[index] = element; //元素个数+1 size++; } /** * 删除指定位置的元素.将该位置及后边的元素左移.返回旧的该位置的元素 * @param index the index of the element to be removed * @return the element that was removed from the list * @throws IndexOutOfBoundsException {@inheritDoc} */ public E remove(int index) { rangeCheck(index);//检查index合法性 modCount++; E oldValue = elementData(index); int numMoved = size - index - 1; if (numMoved > 0) //该位置及后面的元素全部往前移 System.arraycopy(elementData, index+1, elementData, index, numMoved); //list元素个数-1,全部元素左移后,将最后一个元素置为null elementData[--size] = null; // clear to let GC do its work return oldValue; } /** * 如果存在某元素,遍历删除第一个. * @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) { if (o == null) { for (int index = 0; index < size; index++) if (elementData[index] == null) { //该位置及后面的元素全部往前移,并把最后一个元素置 //为null,list元素个数-1 fastRemove(index); return true; } } else { for (int index = 0; index < size; index++) if (o.equals(elementData[index])) { fastRemove(index); return true; } } return false; } /* * 删除指定位置的元素,不检查index合法性,不返回被删除的元素的值 */ 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 } /** * 删除所有元素 */ public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) //设置元素为空 elementData[i] = null; //list的元素个数置为0 size = 0; } /** * 将集合c中的全部元素添加到list的尾部. * @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) { //集合转换为数组 Object[] a = c.toArray(); int numNew = a.length; //如果容量足够,那么扩容 ensureCapacityInternal(size + numNew); // Increments modCount //复制集合中的元素到list中 System.arraycopy(a, 0, elementData, size, numNew); size += numNew; //如果集合长度不为0,则添加成功 return numNew != 0; } /** * 将集合c中的全部元素添加到list的指定位置,原来位置以及后面的元素往后移. * @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);//检查index合法性 Object[] a = c.toArray(); int numNew = a.length; //如果新的容量合法,那么扩容 ensureCapacityInternal(size + numNew); // Increments modCount int numMoved = size - index; if (numMoved > 0) //将原来位置的元素以及后面的元素后移 System.arraycopy(elementData, index, elementData, index + numNew, numMoved); //将集合中的全部元素放进去 System.arraycopy(a, 0, elementData, index, numNew); size += numNew; return numNew != 0; } /** * 将list中下标从fromIndex到toIndex的元素删除 * @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++; int numMoved = size - toIndex; //将下标从toIndex开始的全部元素往前移至fromIndex的位置. System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved); // clear to let GC do its work //新的list元素个数 int newSize = size - (toIndex-fromIndex); for (int i = newSize; i < size; i++) { //将超出下标范围的元素的值设为null elementData[i] = null; } size = newSize; } /** * 检查index合法性,越界抛出异常.这个方法并不检查index是否为负数. * 当index为负数时,数组本身也会抛出越界的错误,所以这个方法没必要 * 检查index是否为负数. */ private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } /** * add和addAll方法检查index是否合法 */ private void rangeCheckForAdd(int index) { if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } /** * 构造下标越界的错误提醒 */ private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+size; } /** * 从list中删除与集合c重复的元素 * * @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); //批量删除 return batchRemove(c, false); } /** * list中只保留与集合c相同的元素 * * @param c collection containing elements to be retained in 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 retainAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, true); } /** * list批量删除/保留与集合c相同的元素.complement: 删除为false,保留为true */ private boolean batchRemove(Collection<?> c, boolean complement) { final Object[] elementData = this.elementData; //r: 遍历元素最后一个元素的下标,w: 剩下元素最后一个元素的下标 int r = 0, w = 0; //是否删除元素 boolean modified = false; try { for (; r < size; r++) if (c.contains(elementData[r]) == complement) //把要保留的元素按照顺序还放到list里,将原来的元素覆盖. elementData[w++] = elementData[r]; } finally { //原方法有可能会报错,这样r就没走到r.size if (r != size) { //将r到size-1的元素移到保留的元素的后面 System.arraycopy(elementData, r, elementData, w, size - r); //w的下标增加r到size-1个元素 w += size - r; } if (w != size) { // clear to let GC do its work //将保留的元素的后面的元素的值置为null for (int i = w; i < size; i++) elementData[i] = null; //修改的次数就是删除的元素个数 modCount += size - w; size = w; //w和size不相等,说明有元素被删除 modified = true; } } return modified; } /** * Save the state of the <tt>ArrayList</tt> instance to a stream (that * is, serialize it). * * @serialData The length of the array backing the <tt>ArrayList</tt> * instance is emitted (int), followed by all of its elements * (each an <tt>Object</tt>) in the proper order. */ private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException{ // Write out element count, and any hidden stuff int expectedModCount = modCount; s.defaultWriteObject(); // Write out size as capacity for behavioural compatibility with clone() s.writeInt(size); // Write out all elements in the proper order. for (int i=0; i<size; i++) { s.writeObject(elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } /** * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is, * deserialize it). */ private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { elementData = EMPTY_ELEMENTDATA; // Read in size, and any hidden stuff s.defaultReadObject(); // Read in capacity s.readInt(); * ignored if (size > 0) { // be like clone(), allocate array based upon size not capacity ensureCapacityInternal(size); Object[] a = elementData; // Read in all elements in the proper order. for (int i=0; i<size; i++) { a[i] = s.readObject(); } } } /** * Returns a list iterator over the elements in this list (in proper * sequence), starting at the specified position in the list. * The specified index indicates the first element that would be * returned by an initial call to {@link ListIterator#next next}. * An initial call to {@link ListIterator#previous previous} would * return the element with the specified index minus one. * * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>. * * @throws IndexOutOfBoundsException {@inheritDoc} */ public ListIterator<E> listIterator(int index) { if (index < 0 || index > size) throw new IndexOutOfBoundsException("Index: "+index); return new ListItr(index); } /** * Returns a list iterator over the elements in this list (in proper * sequence). * * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>. * * @see #listIterator(int) */ public ListIterator<E> listIterator() { return new ListItr(0); } /** * Returns an iterator over the elements in this list in proper sequence. * * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>. * * @return an iterator over the elements in this list in proper sequence */ public Iterator<E> iterator() { return new Itr(); } /** * 内部类,优化版本的AbstractList.Itr,迭代器 * 迭代器的方法类似于链表,但实际实现还是以数组的方式来实现的 */ private class Itr implements Iterator<E> { //游标,指向下一个元素 int cursor; // index of next element to return //最后一个返回的元素的下标 int lastRet = -1; // index of last element returned; -1 if no such //期待的版本号: 如果跟list中的保持一致,说明中间没有被修改. int expectedModCount = modCount; //如果游标不等于index,那么有下一个元素 public boolean hasNext() { return cursor != size; } @SuppressWarnings("unchecked") public E next() { //检查有没有其他线程修改list内容 checkForComodification(); int i = cursor; if (i >= size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) throw new ConcurrentModificationException(); //修改游标 cursor = i + 1; //返回下一个元素.修改最后返回元素的下标 return (E) elementData[lastRet = i]; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { //删除当前元素 ArrayList.this.remove(lastRet); //删除元素后游标指向上一个元素 cursor = lastRet; //最后修改元素设为初始值 lastRet = -1; //将期望版本号改为同list的版本号保持一致 expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } @Override @SuppressWarnings("unchecked") //消费剩下的元素 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(); } //遍历消费从i开始的元素 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(); } } /** * 优化版本的AbstractList.ListItr */ private class ListItr extends Itr implements ListIterator<E> { 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list,从原list的fromIndex(包含),到原list的toIndex(不包含). * 对新的list的修改会反映在原list上. * * @throws IndexOutOfBoundsException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public List<E> subList(int fromIndex, int toIndex) { subListRangeCheck(fromIndex, toIndex, size); return new SubList(this, 0, fromIndex, toIndex); } //参数检查 static void subListRangeCheck(int fromIndex, int toIndex, int size) { if (fromIndex < 0) throw new IndexOutOfBoundsException("fromIndex = " + fromIndex); if (toIndex > size) throw new IndexOutOfBoundsException("toIndex = " + toIndex); if (fromIndex > toIndex) throw new IllegalArgumentException("fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")"); } private class SubList extends AbstractList<E> implements RandomAccess { private final AbstractList<E> parent; private final int parentOffset; private final int offset; int size; SubList(AbstractList<E> parent, int offset, int fromIndex, int toIndex) { //原list this.parent = parent; //子list与原list的偏移量就是fromIndex this.parentOffset = fromIndex; //子list的偏移量(即子list的游标)是原list的偏移量+fromIndex this.offset = offset + fromIndex; //子list的长度是toIndex - fromIndex this.size = toIndex - fromIndex; //子list版本号与原list版本号一直 this.modCount = ArrayList.this.modCount; } public E set(int index, E e) { //检查index合法性 rangeCheck(index); //检查版本号是否一致,看看操作期间是否被修改 checkForComodification(); E oldValue = ArrayList.this.elementData(offset + index); //设置值 ArrayList.this.elementData[offset + index] = e; return oldValue; } //取值 public E get(int index) { rangeCheck(index); checkForComodification(); return ArrayList.this.elementData(offset + index); } //长度 public int size() { checkForComodification(); return this.size; } //调用原list方法添加元素 public void add(int index, E e) { rangeCheckForAdd(index); checkForComodification(); parent.add(parentOffset + index, e); this.modCount = parent.modCount; this.size++; } public E remove(int index) { rangeCheck(index); checkForComodification(); E result = parent.remove(parentOffset + index); this.modCount = parent.modCount; this.size--; return result; } protected void removeRange(int fromIndex, int toIndex) { checkForComodification(); parent.removeRange(parentOffset + fromIndex, parentOffset + toIndex); this.modCount = parent.modCount; this.size -= toIndex - fromIndex; } public boolean addAll(Collection<? extends E> c) { return addAll(this.size, c); } public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); int cSize = c.size(); if (cSize==0) return false; checkForComodification(); parent.addAll(parentOffset + index, c); this.modCount = parent.modCount; this.size += cSize; return true; } public Iterator<E> iterator() { return listIterator(); } public ListIterator<E> listIterator(final int index) { checkForComodification(); rangeCheckForAdd(index); final int offset = this.offset; return new ListIterator<E>() { int cursor = index; int lastRet = -1; int expectedModCount = ArrayList.this.modCount; public boolean hasNext() { return cursor != SubList.this.size; } @SuppressWarnings("unchecked") public E next() { checkForComodification(); int i = cursor; if (i >= SubList.this.size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (offset + i >= elementData.length) throw new ConcurrentModificationException(); cursor = i + 1; return (E) elementData[offset + (lastRet = i)]; } public boolean hasPrevious() { return cursor != 0; } @SuppressWarnings("unchecked") public E previous() { checkForComodification(); int i = cursor - 1; if (i < 0) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (offset + i >= elementData.length) throw new ConcurrentModificationException(); cursor = i; return (E) elementData[offset + (lastRet = i)]; } @SuppressWarnings("unchecked") public void forEachRemaining(Consumer<? super E> consumer) { Objects.requireNonNull(consumer); final int size = SubList.this.size; int i = cursor; if (i >= size) { return; } final Object[] elementData = ArrayList.this.elementData; if (offset + i >= elementData.length) { throw new ConcurrentModificationException(); } while (i != size && modCount == expectedModCount) { consumer.accept((E) elementData[offset + (i++)]); } * update once at end of iteration to reduce heap write traffic lastRet = cursor = i; checkForComodification(); } public int nextIndex() { return cursor; } public int previousIndex() { return cursor - 1; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { SubList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = ArrayList.this.modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void set(E e) { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.set(offset + lastRet, e); } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void add(E e) { checkForComodification(); try { int i = cursor; SubList.this.add(i, e); cursor = i + 1; lastRet = -1; expectedModCount = ArrayList.this.modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } final void checkForComodification() { if (expectedModCount != ArrayList.this.modCount) throw new ConcurrentModificationException(); } }; } public List<E> subList(int fromIndex, int toIndex) { subListRangeCheck(fromIndex, toIndex, size); return new SubList(this, offset, fromIndex, toIndex); } private void rangeCheck(int index) { if (index < 0 || index >= this.size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private void rangeCheckForAdd(int index) { if (index < 0 || index > this.size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+this.size; } private void checkForComodification() { if (ArrayList.this.modCount != this.modCount) throw new ConcurrentModificationException(); } public Spliterator<E> spliterator() { checkForComodification(); return new ArrayListSpliterator<E>(ArrayList.this, offset, offset + this.size, this.modCount); } } @Override public void forEach(Consumer<? super E> action) { Objects.requireNonNull(action); final int expectedModCount = modCount; @SuppressWarnings("unchecked") final E[] elementData = (E[]) this.elementData; final int size = this.size; //遍历消费元素 for (int i=0; modCount == expectedModCount && i < size; i++) { action.accept(elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } /** * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em> * and <em>fail-fast</em> {@link Spliterator} over the elements in this * list. * * <p>The {@code Spliterator} reports {@link Spliterator#SIZED}, * {@link Spliterator#SUBSIZED}, and {@link Spliterator#ORDERED}. * Overriding implementations should document the reporting of additional * characteristic values. * * @return a {@code Spliterator} over the elements in this list * @since 1.8 */ @Override public Spliterator<E> spliterator() { return new ArrayListSpliterator<>(this, 0, -1, 0); } /** Index-based split-by-two, lazily initialized Spliterator */ static final class ArrayListSpliterator<E> implements Spliterator<E> { /* * If ArrayLists were immutable, or structurally immutable (no * adds, removes, etc), we could implement their spliterators * with Arrays.spliterator. Instead we detect as much * interference during traversal as practical without * sacrificing much performance. We rely primarily on * modCounts. These are not guaranteed to detect concurrency * violations, and are sometimes overly conservative about * within-thread interference, but detect enough problems to * be worthwhile in practice. To carry this out, we (1) lazily * initialize fence and expectedModCount until the latest * point that we need to commit to the state we are checking * against; thus improving precision. (This doesn't apply to * SubLists, that create spliterators with current non-lazy * values). (2) We perform only a single * ConcurrentModificationException check at the end of forEach * (the most performance-sensitive method). When using forEach * (as opposed to iterators), we can normally only detect * interference after actions, not before. Further * CME-triggering checks apply to all other possible * violations of assumptions for example null or too-small * elementData array given its size(), that could only have * occurred due to interference. This allows the inner loop * of forEach to run without any further checks, and * simplifies lambda-resolution. While this does entail a * number of checks, note that in the common case of * list.stream().forEach(a), no checks or other computation * occur anywhere other than inside forEach itself. The other * less-often-used methods cannot take advantage of most of * these streamlinings. */ private final ArrayList<E> list; //下标: 默认为0 private int index; // current index, modified on advance/split //分割次数,默认-1 private int fence; // -1 until used; then one past last index //期望版本号: 默认0 private int expectedModCount; // initialized when fence set /** Create new spliterator covering the given range */ ArrayListSpliterator(ArrayList<E> list, int origin, int fence, int expectedModCount) { this.list = list; * OK if null unless traversed this.index = origin; this.fence = fence; this.expectedModCount = expectedModCount; } //当fence为获取分割次数 private int getFence() { // initialize fence to size on first use //每一个分割里的终止位置的下标 int hi; // (a specialized variant appears in method forEach) ArrayList<E> lst; //此处会给hi赋值为fence. if ((hi = fence) < 0) { if ((lst = list) == null) hi = fence = 0; else { expectedModCount = lst.modCount; //分割次数是元素的个数 hi = fence = lst.size; } } return hi; } public ArrayListSpliterator<E> trySplit() { //获取分割次数,lo为起始下标,mid为起始下标+终止下标/2 int hi = getFence(), lo = index, mid = (lo + hi) >>> 1; return (lo >= mid) ? null : // divide range in half unless too small new ArrayListSpliterator<E>(list, lo, index = mid, expectedModCount); } public boolean tryAdvance(Consumer<? super E> action) { if (action == null) throw new NullPointerException(); int hi = getFence(), i = index; if (i < hi) { index = i + 1; @SuppressWarnings("unchecked") E e = (E)list.elementData[i]; action.accept(e); if (list.modCount != expectedModCount) throw new ConcurrentModificationException(); return true; } return false; } public void forEachRemaining(Consumer<? super E> action) { int i, hi, mc; * hoist accesses and checks from loop ArrayList<E> lst; Object[] a; if (action == null) throw new NullPointerException(); if ((lst = list) != null && (a = lst.elementData) != null) { if ((hi = fence) < 0) { mc = lst.modCount; hi = lst.size; } else mc = expectedModCount; if ((i = index) >= 0 && (index = hi) <= a.length) { for (; i < hi; ++i) { @SuppressWarnings("unchecked") E e = (E) a[i]; action.accept(e); } if (lst.modCount == mc) return; } } throw new ConcurrentModificationException(); } public long estimateSize() { return (long) (getFence() - index); } public int characteristics() { return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED; } } @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 //删除的元素数量 int removeCount = 0; //BitSet是一个用来表示各个数值是否存在的类. //它将多个数值放在一个long[]中,long[]中的每一个值只能是0或者1. //0表示该index的数不存在,1表示该index的数存在. //例如三个数1,6,7存在long[]中,因为一个long值长度为64bit,因此,只需一个long值即可存 //存起来的结果是: [000...76000010],第1位,6位,7位的值置为1 final BitSet removeSet = new BitSet(size); //期望修改次数 final int expectedModCount = modCount; final int size = this.size; //遍历所有元素 for (int i=0; modCount == expectedModCount && i < size; i++) { @SuppressWarnings("unchecked") final E element = (E) elementData[i]; //如果该元素符合过滤条件 if (filter.test(element)) { //BitSet中标记 removeSet.set(i); 删除元素数量+1 removeCount++; } } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } // shift surviving elements left over the spaces left by removed elements final boolean anyToRemove = removeCount > 0; if (anyToRemove) { final int newSize = size - removeCount; //遍历所有元素 for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) { //i为下一个未删除的元素 i = removeSet.nextClearBit(i); //将未删除的元素集中挪到数组的前面 elementData[j] = elementData[i]; } //遍历未被删除的元素后面的所有元素,将所有元素置为null,等待回收 for (int k=newSize; k < size; k++) { elementData[k] = null; // Let gc do its work } //设置list中元素数量 this.size = newSize; if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } modCount++; } //返回是否有元素被删除 return anyToRemove; } @Override @SuppressWarnings("unchecked") public void replaceAll(UnaryOperator<E> operator) { Objects.requireNonNull(operator); final int expectedModCount = modCount; final int size = this.size; for (int i=0; modCount == expectedModCount && i < size; i++) { elementData[i] = operator.apply((E) elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } modCount++; } @Override @SuppressWarnings("unchecked") public void sort(Comparator<? super E> c) { final int expectedModCount = modCount; //调用Arrays.sort方法来对list进行排序 Arrays.sort((E[]) elementData, 0, size, c); if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } modCount++; } } 复制代码
原文
https://juejin.im/post/5ef63ac3e51d45349f7b05ab
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