JDK源码阅读之Collection

源码版本：JDK 1.7。

集合 Collection，根据已知的内容可以知道有List、Set、Map（严格说，Map不属于Collection）等大类。

先查看 Collection，

public interface Collection<E> extends Iterable<E>

JDK说明如下：

/**
 * The root interface in the <i>collection hierarchy</i>.  A collection
 * represents a group of objects, known as its <i>elements</i>.  Some
 * collections allow duplicate elements and others do not.  Some are ordered
 * and others unordered.  The JDK does not provide any <i>direct</i>
 * implementations of this interface: it provides implementations of more
 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>.  This interface
 * is typically used to pass collections around and manipulate them where
 * maximum generality is desired.
 *
 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
 * duplicate elements) should implement this interface directly.
 *
 * <p>All general-purpose <tt>Collection</tt> implementation classes (which
 * typically implement <tt>Collection</tt> indirectly through one of its
 * subinterfaces) should provide two "standard" constructors: a void (no
 * arguments) constructor, which creates an empty collection, and a
 * constructor with a single argument of type <tt>Collection</tt>, which
 * creates a new collection with the same elements as its argument.  In
 * effect, the latter constructor allows the user to copy any collection,
 * producing an equivalent collection of the desired implementation type.
 * There is no way to enforce this convention (as interfaces cannot contain
 * constructors) but all of the general-purpose <tt>Collection</tt>
 * implementations in the Java platform libraries comply.
 *
 * <p>The "destructive" methods contained in this interface, that is, the
 * methods that modify the collection on which they operate, are specified to
 * throw <tt>UnsupportedOperationException</tt> if this collection does not
 * support the operation.  If this is the case, these methods may, but are not
 * required to, throw an <tt>UnsupportedOperationException</tt> if the
 * invocation would have no effect on the collection.  For example, invoking
 * the {@link #addAll(Collection)} method on an unmodifiable collection may,
 * but is not required to, throw the exception if the collection to be added
 * is empty.
 *
 * <p><a name="optional-restrictions"/>
 * Some collection implementations have restrictions on the elements that
 * they may contain.  For example, some implementations prohibit null elements,
 * and some have restrictions on the types of their elements.  Attempting to
 * add an ineligible element throws an unchecked exception, typically
 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.  Attempting
 * to query the presence of an ineligible element may throw an exception,
 * or it may simply return false; some implementations will exhibit the former
 * behavior and some will exhibit the latter.  More generally, attempting an
 * operation on an ineligible element whose completion would not result in
 * the insertion of an ineligible element into the collection may throw an
 * exception or it may succeed, at the option of the implementation.
 * Such exceptions are marked as "optional" in the specification for this
 * interface.
 *
 * <p>It is up to each collection to determine its own synchronization
 * policy.  In the absence of a stronger guarantee by the
 * implementation, undefined behavior may result from the invocation
 * of any method on a collection that is being mutated by another
 * thread; this includes direct invocations, passing the collection to
 * a method that might perform invocations, and using an existing
 * iterator to examine the collection.
 *
 * <p>Many methods in Collections Framework interfaces are defined in
 * terms of the {@link Object#equals(Object) equals} method.  For example,
 * the specification for the {@link #contains(Object) contains(Object o)}
 * method says: "returns <tt>true</tt> if and only if this collection
 * contains at least one element <tt>e</tt> such that
 * <tt>(o==null ? e==null : o.equals(e))</tt>."  This specification should
 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
 * invoked for any element <tt>e</tt>.  Implementations are free to implement
 * optimizations whereby the <tt>equals</tt> invocation is avoided, for
 * example, by first comparing the hash codes of the two elements.  (The
 * {@link Object#hashCode()} specification guarantees that two objects with
 * unequal hash codes cannot be equal.)  More generally, implementations of
 * the various Collections Framework interfaces are free to take advantage of
 * the specified behavior of underlying {@link Object} methods wherever the
 * implementor deems it appropriate.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements in this collection
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Set
 * @see     List
 * @see     Map
 * @see     SortedSet
 * @see     SortedMap
 * @see     HashSet
 * @see     TreeSet
 * @see     ArrayList
 * @see     LinkedList
 * @see     Vector
 * @see     Collections
 * @see     Arrays
 * @see     AbstractCollection
 * @since 1.2
 */

大意是：

Collection接口是集合框架的根接口。JDK没有为该接口提供任何直接实现，而是提供了更细化的子接口的实现，如Set、List。

所有通用目的的实现类应该提供两个标准构造方法：一个空参数的构造器、一个单参数且参数类型为Collection的构造器。没有强制措施，纯属自觉，JDK很自觉。

每个实现类的同步策略由自己决定。

集合框架接口的很多方法都是基于equals方法，例如contains(obj)。

不过，先不急于深入Collection接口，先看看其继承的Iterable接口，该接口只有一个方法：

public interface Iterable<T> {
    Iterator<T> iterator();
}

继续展开 Iterator，还是一个接口：

public interface Iterator<E> {
    boolean hasNext();
    E next();
    void remove();
}

如果使用过该接口，应该明白前两个方法的意义，一个用于判断是否还有下一个元素，另一个用于获取下一个元素同时移动游标。最后一个方法其实是删除当前指向的元素，属于可选的待实现接口方法。

事实上，该接口，Iterator取代了原有的Enumeration接口，对比一下就明白了：

public interface Enumeration<E> {
    boolean hasMoreElements();
    E nextElement();
}

Iterator 和 Enumeration 如出一辙，只不过 Iterator 更简洁，且多了一个操作。

再返回来看看 Iterable 接口，其唯一的方法是 iterator()，要求返回一个 Iterator对象；所以 Collection 接口的实现们必然实现该方法，来测试一下：

@Test
@SuppressWarnings({ "rawtypes", "unchecked" })
public void testIterator() {
    Collection collection = null;
    collection = new ArrayList();
    collection.add(1);
    collection.add(2);
    collection.add(3);

    Iterator iterator = collection.iterator(); // Iterator
    while (iterator.hasNext()) {
        System.out.println(iterator.next());
    }
}

OK，现在来看看Collection本身的接口：

public interface Collection<E> extends Iterable<E> {
    // Query Operations  - 查询操作
    int size();
    boolean isEmpty();
    boolean contains(Object o);
    Iterator<E> iterator();
    Object[] toArray();
    <T> T[] toArray(T[] a);

    // Modification Operations  - 修改操作
    boolean add(E e);
    boolean remove(Object o);

    // Bulk Operations  - 批量操作
    boolean containsAll(Collection<?> c);
    boolean addAll(Collection<? extends E> c);
    boolean removeAll(Collection<?> c);
    boolean retainAll(Collection<?> c);
    void clear();

    // Comparison and hashing  - 对比和哈希
    boolean equals(Object o);
    int hashCode();
}

不明白为什么还有 iterator() 方法，冗余了吧。

上面的方法，对我来说最需要了解的就是 toArray() 和 toArray(T[] a) ：前者就是字面意思，将集合转成数组；后者虽然也是将集合转成数字，但是，会考虑集合的size和给定数组的length，如果给定数组能容纳集合的元素，则直接返回指定的数组（填充集合元素），否则，返回一个新创建的数组！ -- 就是说，可能会省略分配内存这一步骤。

来个示例：

@Test
public void testToArray() {
    // 定义集合
    Collection<String> collection = new ArrayList<>();
    for (int i = 0; i != 5; ++i) {
        collection.add(new String(new char[] { (char) ('a' + i) }));
        collection.add(null);
    }
    System.out.println(collection); //

    // 定义目标数组
    String[] t1 = new String[9];
    Arrays.fill(t1, "xxx");
    String[] t2 = new String[10];
    Arrays.fill(t2, "xxx");
    String[] t3 = new String[11];
    Arrays.fill(t3, "xxx");
    // 转换
    String[] r1 = collection.toArray(t1);
    String[] r2 = collection.toArray(t2);
    String[] r3 = collection.toArray(t3);

    // 看看返回的数组是否原有数组
    System.out.println(r1 == t1); // false
    System.out.println(r2 == t2); // true
    System.out.println(r3 == t3); // true

    System.out.println(Arrays.toString(r1));
    System.out.println(Arrays.toString(r2));
    System.out.println(Arrays.toString(r3)); // 注意，原数组如果仍有空余空间，全部null
}

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