Java集合-ArrayList源码

爱被打了一巴掌 2022-05-12 03:00 273阅读 0赞

## 简介 ##

>  *  ArrayList是List接口的大小可变数组的实现，ArrayList底层是用一个Object数组来存储元素，ArrayList允许存储所有不同类型元素，包括null，允许插入重复元素（底层是一个Object数组）

>  *  在用迭代器遍历ArrayList集合对象时，如果遍历过程中对集合对象的内容进行了修改（增加、删除、修改），则会抛出ConcurrentModificationException，即迭代器的快速失败（fail—fast）机制

>  *  ArrayList每次添加元素都会校验底层数组长度，数组长度不够时则扩容，扩容后，数组扩大为原来的1.5倍

>  *  The size, isEmpty, get, set,iterator, and listIterator operations run in constant time. The add operation runs in amortized constant time,that is, adding n elements requires O(n) time. All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation

ArrayList不是线程安全的，并发读写的时候，因为竞争资源而发成读写不一致的情况（ConcurrentModificationException）

解决方案：  
1、new Vector<>();  
2、Collections.synchronizedList(new ArrayList<>());  
3、new CopyOnWriteArrayList<>(); // 写时复制，读写分离

## 类图 ##

![这里写图片描述][70]

## 源码 ##

### 继承关系 ###

public class ArrayList<E> extends AbstractList<E>
            implements List<E>, RandomAccess, Cloneable, java.io.Serializable

*  RandomAccess 是一个标记接口（Marker interface），用于标明实现该接口的List支持快速随机访问，主要目的是使算法能够在随机和顺序访问的list中表现的更加高效
 *  Cloneable接口是一个空接口，仅用于标记对象，Cloneable接口里面是没有clone()方法，clone()方法是Object类里面的方法
 *  Serializable接口是启用其序列化功能的接口。实现java.io.Serializable 接口的类是可序列化的。没有实现此接口的类将不能使它们的任意状态被序列化或逆序列化

### 构造函数 ###

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);
        }
    }
    /** * Constructs an empty list with an initial capacity of ten. */
    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;
        }
    }

### 成员变量 ###

/** * Default initial capacity. */
    private static final int DEFAULT_CAPACITY = 10;
    
    /** * Shared empty array instance used for empty instances. */
    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 = { };
    
    /** * 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
    
    /** * The size of the ArrayList (the number of elements it contains). * * @serial */
    private int size;

### 成员方法 ###

*  [详见Java集合-List][Java_-List]

### 内部类ArrayListSpliterator ###

static final class ArrayListSpliterator<E> implements Spliterator<E> { 
        private final ArrayList<E> list;
        private int index; // current index, modified on advance/split
        private int fence; // -1 until used; then one past last index
        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;
        }
    
        private int getFence() {  // initialize fence to size on first use
            int hi; // (a specialized variant appears in method forEach)
            ArrayList<E> lst;
            if ((hi = fence) < 0) { 
                if ((lst = list) == null)
                    hi = fence = 0;
                else { 
                    expectedModCount = lst.modCount;
                    hi = fence = lst.size;
                }
            }
            return hi;
        }
    
        public ArrayListSpliterator<E> trySplit() { 
            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;
        }
    }

### 内部类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();
      }
    }

### 内部类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();
            }
        }
    }

### 内部类SubList ###

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) { 
            this.parent = parent;
            this.parentOffset = fromIndex;
            this.offset = offset + fromIndex;
            this.size = toIndex - fromIndex;
            this.modCount = ArrayList.this.modCount;
        }
    
        public E set(int index, E e) { 
            rangeCheck(index);
            checkForComodification();
            E oldValue = ArrayList.this.elementData(offset + index);
            ArrayList.this.elementData[offset + index] = e;
            return oldValue;
        }
    
        public E get(int index) { 
            rangeCheck(index);
            checkForComodification();
            return ArrayList.this.elementData(offset + index);
        }
    
        public int size() { 
            checkForComodification();
            return this.size;
        }
    
        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);
        }
    }

[70]: /images/20220512/8fe559ceba8e46159cecc4e210331ab3.png
[Java_-List]: https://blog.csdn.net/thebigdipperbdx/article/details/79261332