上篇我们分析了Java集合的总体类图，下面我们接着来具体探究一下相关的源码。

Collection 简介

Collection 的定义

public interface Collection<E> extends Iterable<E>{}

Collection是一个接口，是高度抽象的集合，其包含了集合的基本操作，如添加，删除，遍历，是否为空，集合转为数组，集合大小，集合之间的合并。

其API如下：

// Collection的API
abstract boolean         add(E object)     //添加元素
abstract boolean         addAll(Collection<? extends E> collection)  //添加集合
abstract void            clear()  //删除元素
abstract boolean         contains(Object object) //是否包含特定元素
abstract boolean         containsAll(Collection<?> collection)  //是否包含某个集合
abstract boolean         equals(Object object)  //
abstract int             hashCode()  //获取集合的hashCode值
abstract boolean         isEmpty()  //判断集合是否为空
abstract Iterator<E>     iterator()  //获取集合的迭代器
abstract boolean         remove(Object object)  //移除元素
abstract boolean         removeAll(Collection<?> collection)
abstract boolean         retainAll(Collection<?> collection)
abstract int             size()  //获取集合的大小
abstract <T> T[]         toArray(T[] array)   //集合转化成数组
abstract Object[]        toArray()  //集合转化成数组

List 简介

List的定义

public interface List<E> extends Collection<E>{}

List 是一个继承于Collection的接口，即List是集合中的一种接口。所以，Collection 接口有的方法List中也有。

List 是一个有序队列，集合的初始索引是0，以后每增加一个元素索引+1。List中允许有重复元素。

其增加的API如下：(与Collection相同的API不重复列出)

abstract void                add(int location, E object)  //在指定位置添加元素
abstract boolean             addAll(int location, Collection<? extends E> collection) //在指定位置添加集合
abstract E                   get(int location)  //获取指定位置的元素
abstract int                 indexOf(Object object)//获取元素的索引值
abstract int                 lastIndexOf(Object object)  
abstract ListIterator<E>     listIterator(int location)
abstract ListIterator<E>     listIterator()
abstract E                   remove(int location)  //移除指定位置的元素
abstract E                   set(int location, E object)   
abstract List<E>             subList(int start, int end)  //获取子集合

Set 简介

Set的定义

public interface Set<E> extends Collection<E>{}

Set是一个继承于Collection的接口，即List是集合中的一种接口。不允许有重复元素。其API与Collection的API完全一样。再次不再赘述。

Queue 简介

AbstractCollection

AbstractCollection的定义是

public abstract class AbstractCollection<E> implements Collection<E>{}

AbstractCollection是一个实现了Collection的抽象类，其实现了Collection中除了iterator和size方法之外的方法。所以，继承与AbstractCollection的其他实现类，只需要实现AbstractCollection类没有实现的方法以及按照需求重写相关方法。

public abstract Iterator<E> iterator();
    public abstract int size();

具体分析下AbstractCollection类下的相关方法的实现。

public boolean isEmpty() {
        return size() == 0;
    }
    //检查集合是否包含特定元素
    public boolean contains(Object o) {
        Iterator<E> it = iterator();
        if (o==null) {     //任何非空集合都包含null
            while (it.hasNext())
                if (it.next()==null)
                    return true;
        } else {
            while (it.hasNext())
                if (o.equals(it.next()))
                    return true;
        }
        return false;
    }
  //将集合转化成数组
    public Object[] toArray() {
        // Estimate size of array; be prepared to see more or fewer elements
        Object[] r = new Object[size()];  //定义一个与集合等大的数组
        Iterator<E> it = iterator();
      //循环将集合中的元素copy到数组r中
        for (int i = 0; i < r.length; i++) {
            if (! it.hasNext()) // fewer elements than expected
                return Arrays.copyOf(r, i);   
            r[i] = it.next();
        }
        return it.hasNext() ? finishToArray(r, it) : r;
    }
 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    private static <T> T[] finishToArray(T[] r, Iterator<?> it) {
        int i = r.length;
        while (it.hasNext()) {
            int cap = r.length;
            if (i == cap) {
                int newCap = cap + (cap >> 1) + 1;
                // overflow-conscious code
                if (newCap - MAX_ARRAY_SIZE > 0)
                    newCap = hugeCapacity(cap + 1);
                r = Arrays.copyOf(r, newCap);
            }
            r[i++] = (T)it.next();
        }
        // trim if overallocated
        return (i == r.length) ? r : Arrays.copyOf(r, i);
    }
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError
                ("Required array size too large");
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }
    public <T> T[] toArray(T[] a) {
        // Estimate size of array; be prepared to see more or fewer elements
        int size = size();
        T[] r = a.length >= size ? a :
                  (T[])java.lang.reflect.Array
                  .newInstance(a.getClass().getComponentType(), size);
        Iterator<E> it = iterator();
        for (int i = 0; i < r.length; i++) {
            if (! it.hasNext()) { // fewer elements than expected
                if (a == r) {
                    r[i] = null; // null-terminate
                } else if (a.length < i) {
                    return Arrays.copyOf(r, i);
                } else {
                    System.arraycopy(r, 0, a, 0, i);
                    if (a.length > i) {
                        a[i] = null;
                    }
                }
                return a;
            }
            r[i] = (T)it.next();
        }
        // more elements than expected
        return it.hasNext() ? finishToArray(r, it) : r;
    }
    // Modification Operations
    public boolean add(E e) {
        throw new UnsupportedOperationException();
    }
    //删除元素o  
    public boolean remove(Object o) {
        Iterator<E> it = iterator();
        if (o==null) {
            while (it.hasNext()) {
                if (it.next()==null) {
                    it.remove();
                    return true;
                }
            }
        } else {
            while (it.hasNext()) {
                if (o.equals(it.next())) {
                    it.remove();
                    return true;
                }
            }
        }
        return false;
    }
    // Bulk Operations
    public boolean containsAll(Collection<?> c) {
        for (Object e : c)
            if (!contains(e))
                return false;
        return true;
    }
    public boolean addAll(Collection<? extends E> c) {
        boolean modified = false;
        for (E e : c)
            if (add(e))
                modified = true;
        return modified;
    }
    //删除集合c中所有元素（如果存在的话）  
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;
        Iterator<?> it = iterator();
        while (it.hasNext()) {
            if (c.contains(it.next())) {
                it.remove();
                modified = true;
            }
        }
        return modified;
    }
    //删除不在集合c中的所有元素（如果存在的话）  
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;
        Iterator<E> it = iterator();
        while (it.hasNext()) {
            if (!c.contains(it.next())) {
                it.remove();
                modified = true;
            }
        }
        return modified;
    }
// 清空
    public void clear() {
        Iterator<E> it = iterator();
        while (it.hasNext()) {
            it.next();
            it.remove();
        }
    }
    //  String conversion
// 将List 转成[String] biao'shi
    public String toString() {
        Iterator<E> it = iterator();
        if (! it.hasNext())
            return "[]";
        StringBuilder sb = new StringBuilder();
        sb.append('[');
        for (;;) {
            E e = it.next();
            sb.append(e == this ? "(this Collection)" : e);
            if (! it.hasNext())
                return sb.append(']').toString();
            sb.append(',').append(' ');
        }
    }

AbstractList

AbstractList的定义

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E>{}

从定义看出AbstractList继承了AbstractCollection类。实现了List接口。与AbstractCollection类相比其实现了iterator方法。

AbstractList部分源码分析

// Search Operations
    //如果找到则返回该元素的前一个索引值，没有就返回-1
    public int indexOf(Object o) {
        ListIterator<E> it = listIterator();
        if (o==null) {
            while (it.hasNext())
                if (it.next()==null)
                    return it.previousIndex();
        } else {
            while (it.hasNext())
                if (o.equals(it.next()))
                    return it.previousIndex();
        }
        return -1;
    }
    //如果找到则返回该元素的后一个索引值，没有就返回-1
    public int lastIndexOf(Object o) {
        ListIterator<E> it = listIterator(size());
        if (o==null) {
            while (it.hasPrevious())
                if (it.previous()==null)
                    return it.nextIndex();
        } else {
            while (it.hasPrevious())
                if (o.equals(it.previous()))
                    return it.nextIndex();
        }
        return -1;
    }
    // Bulk Operations
   //清除集合中的元素
    public void clear() {
        removeRange(0, size());
    }
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);  //检查索引值是否合法
        boolean modified = false;
        for (E e : c) {
            add(index++, e);
            modified = true;
        }
        return modified;
    }
      private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size())
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    protected void removeRange(int fromIndex, int toIndex) {
        ListIterator<E> it = listIterator(fromIndex);
        for (int i=0, n=toIndex-fromIndex; i<n; i++) {
            it.next();
            it.remove();
        }
    }
--------------------------------------------------------------------------------
   public Iterator<E> iterator() {
        return new Itr();
    }
    public ListIterator<E> listIterator() {
        return listIterator(0);
    }
    public ListIterator<E> listIterator(final int index) {
        rangeCheckForAdd(index);
        return new ListItr(index);
    }
    private class Itr implements Iterator<E> {
        /**
         * Index of element to be returned by subsequent call to next.
         */
        int cursor = 0;
        /**
         * Index of element returned by most recent call to next or
         * previous.  Reset to -1 if this element is deleted by a call
         * to remove.
         */
        int lastRet = -1;
        /**
         * The modCount value that the iterator believes that the backing
         * List should have.  If this expectation is violated, the iterator
         * has detected concurrent modification.（源代码的注释）
         *  译：iterator 每次遍历都会检测modCount的值是否与List应有的长度相等，如果不相等的话，则会抛出一个ConcurrentModificationException的异常
           这种场景常见于多线程的情况下。当一个线程正在遍历集合list时，如果在其中另外一个线程删除了list中的某个元素，则会出现该异常
         */
        int expectedModCount = modCount;
        public boolean hasNext() {
            return cursor != size();
        }
        public E next() {
            checkForComodification();
            try {
                int i = cursor;
                E next = get(i);
                lastRet = i;
                cursor = i + 1;
                return next;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }
        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();
            try {
                AbstractList.this.remove(lastRet);
                if (lastRet < cursor)
                    cursor--;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException e) {
                throw new ConcurrentModificationException();
            }
        }
        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }
    private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            cursor = index;
        }
        public boolean hasPrevious() {
            return cursor != 0;
        }
        public E previous() {
            checkForComodification();
            try {
                int i = cursor - 1;
                E previous = get(i);
                lastRet = cursor = i;
                return previous;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }
        public int nextIndex() {
            return cursor;
        }
        public int previousIndex() {
            return cursor-1;
        }
        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();
            try {
                AbstractList.this.set(lastRet, e);
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
        public void add(E e) {
            checkForComodification();
            try {
                int i = cursor;
                AbstractList.this.add(i, e);
                lastRet = -1;
                cursor = i + 1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }
--------------------------------------------------------------------------------
// 获取List的子集subList，指定开始索引和结束索引
 public List<E> subList(int fromIndex, int toIndex) {
        return (this instanceof RandomAccess ?
                new RandomAccessSubList<>(this, fromIndex, toIndex) :
                new SubList<>(this, fromIndex, toIndex));
    }
class SubList<E> extends AbstractList<E> {
    private final AbstractList<E> l;
    private final int offset;
    private int size;
    /** 从`SubList`的构造器我们可以看出，其并不是返回一个真正的子集，还是原有的集合List，只是偏移量改成了fromIndex，大小改成了toIndex - fromIndex，所以对子集增加，删除元素会直接作用域原有的List*/
    SubList(AbstractList<E> list, int fromIndex, int toIndex) {
        if (fromIndex < 0)
            throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
        if (toIndex > list.size())
            throw new IndexOutOfBoundsException("toIndex = " + toIndex);
        if (fromIndex > toIndex)
            throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                               ") > toIndex(" + toIndex + ")");
        l = list;
        offset = fromIndex;
        size = toIndex - fromIndex;
        this.modCount = l.modCount;
    }
    public E set(int index, E element) {
        rangeCheck(index);
        checkForComodification();
        return l.set(index+offset, element);
    }
    public E get(int index) {
        rangeCheck(index);
        checkForComodification();
        return l.get(index+offset);
    }
    public int size() {
        checkForComodification();
        return size;
    }
    public void add(int index, E element) {
        rangeCheckForAdd(index);
        checkForComodification();
        l.add(index+offset, element);
        this.modCount = l.modCount;
        size++;
    }
    public E remove(int index) {
        rangeCheck(index);
        checkForComodification();
        E result = l.remove(index+offset);
        this.modCount = l.modCount;
        size--;
        return result;
    }
    protected void removeRange(int fromIndex, int toIndex) {
        checkForComodification();
        l.removeRange(fromIndex+offset, toIndex+offset);
        this.modCount = l.modCount;
        size -= (toIndex-fromIndex);
    }
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        int cSize = c.size();
        if (cSize==0)
            return false;
        checkForComodification();
        l.addAll(offset+index, c);
        this.modCount = l.modCount;
        size += cSize;
        return true;
    }
    public Iterator<E> iterator() {
        return listIterator();
    }
    public ListIterator<E> listIterator(final int index) {
        checkForComodification();
        rangeCheckForAdd(index);
        return new ListIterator<E>() {
            private final ListIterator<E> i = l.listIterator(index+offset);
            public boolean hasNext() {
                return nextIndex() < size;
            }
            public E next() {
                if (hasNext())
                    return i.next();
                else
                    throw new NoSuchElementException();
            }
            public boolean hasPrevious() {
                return previousIndex() >= 0;
            }
            public E previous() {
                if (hasPrevious())
                    return i.previous();
                else
                    throw new NoSuchElementException();
            }
            public int nextIndex() {
                return i.nextIndex() - offset;
            }
            public int previousIndex() {
                return i.previousIndex() - offset;
            }
            public void remove() {
                i.remove();
                SubList.this.modCount = l.modCount;
                size--;
            }
            public void set(E e) {
                i.set(e);
            }
            public void add(E e) {
                i.add(e);
                SubList.this.modCount = l.modCount;
                size++;
            }
        };
    }
    public List<E> subList(int fromIndex, int toIndex) {
        return new SubList<>(this, fromIndex, toIndex);
    }
    private void rangeCheck(int index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }
    private void checkForComodification() {
        if (this.modCount != l.modCount)
            throw new ConcurrentModificationException();
    }
}

从上述源码中，我理解的两条重要信息是：

1. AbstractList是非线程安全的，当一个线程在遍历List时，另外一个线程对该List进行修改则会直接抛出ConcurrentModificationException的异常

2. 从SubList的构造器我们可以看出，其并不是返回一个真正的子集，还是原有的集合List，只是偏移量改成了fromIndex，大小改成了toIndex - fromIndex，所以对子集增加，删除元素会直接作用域原有的List

AbstractSet

AbstractSet的定义是

public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E>{}

AbstractSet的部分源码分析

protected AbstractSet() {
    }
    // Comparison and hashing
    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof Set))
            return false;
        Collection<?> c = (Collection<?>) o;
        if (c.size() != size())
            return false;
        try {
            return containsAll(c);
        } catch (ClassCastException unused)   {
            return false;
        } catch (NullPointerException unused) {
            return false;
        }
    }
    public int hashCode() {
        int h = 0;
        Iterator<E> i = iterator();
        while (i.hasNext()) {
            E obj = i.next();
            if (obj != null)
                h += obj.hashCode();
        }
        return h;
    }
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;
        if (size() > c.size()) {
            for (Iterator<?> i = c.iterator(); i.hasNext(); )
                modified |= remove(i.next());
        } else {
            for (Iterator<?> i = iterator(); i.hasNext(); ) {
                if (c.contains(i.next())) {
                    i.remove();
                    modified = true;
                }
            }
        }
        return modified;
    }

从上述源码中可以看出AbstractSet 只是重写了AbstractCollection中的removeAll方法。

Iterator 简介

Iterator的定义

public interface Iterator<E>{}

Iterator是一个接口，Iterator 是Collection分支下的集合的最重要的遍历工具，所有的List和Set都需要依赖它。

从API中看出，其包含的方法有，是否有下一个元素，获取下一个元素，删除当前元素。

Iterator遍历集合时采取的是 fast-fail 机制，即，当某一个线程A通过iterator去遍历某集合的过程中，若该集合的内容被其他线程所改变了，那么线程A访问集合时，就会抛出CurrentModificationException异常，产生fail-fast事件。

Iterator的API是

abstract boolean hasNext()  
abstract E next()  
abstract void remove()

ListIterator 简介

ListIterator的定义

public interface ListIterator<E> extends Iterator<E> {}

ListIterator是一个继承Iterator的接口，它是队列迭代器。专门用于遍历List，能提供向前和向后遍历。相比于Iterator，它新增了添加、是否存在上一个元素、获取上一个元素等API接口：

// 继承于Iterator的接口  
abstract boolean hasNext()  
abstract E next()  
abstract void remove()  
// 新增API接口  
abstract void add(E object)  
abstract boolean hasPrevious()  
abstract int nextIndex()  
abstract E previous()  
abstract int previousIndex()  
abstract void set(E object)

Java 集合框架02---Collection的源码分析