collection 接口是所有集合（list，map.set）的 父接口，集合都是对此接口的实现。

/* * @(#)Collection.java    1.55 06/04/21 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */package java.util;/** * The root interface in the collection hierarchy.  A collection * represents a group of objects, known as its elements.  Some * collections allow duplicate elements and others do not.  Some are ordered * and others unordered.  The JDK does not provide any direct * implementations of this interface: it provides implementations of more * specific subinterfaces like Set and List.  This interface * is typically used to pass collections around and manipulate them where * maximum generality is desired. * * 
Bags or multisets (unordered collections that may contain * duplicate elements) should implement this interface directly. * * 
All general-purpose Collection implementation classes (which * typically implement Collection 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 Collection, 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 Collection * implementations in the Java platform libraries comply. * * 
The "destructive" methods contained in this interface, that is, the * methods that modify the collection on which they operate, are specified to * throw UnsupportedOperationException if this collection does not * support the operation.  If this is the case, these methods may, but are not * required to, throw an UnsupportedOperationException 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. * * 
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 * NullPointerException or ClassCastException.  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. * * 
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. * * 
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 true if and only if this collection * contains at least one element e such that * (o==null ? e==null : o.equals(e))."  This specification should * not be construed to imply that invoking Collection.contains * with a non-null argument o will cause o.equals(e) to be * invoked for any element e.  Implementations are free to implement * optimizations whereby the equals 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. * * 
This interface is a member of the *  * Java Collections Framework. * * @author  Josh Bloch * @author  Neal Gafter * @version 1.55, 04/21/06 * @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 */ 继承了iterable，则在遍历集合的时候可以创建迭代器对象，使用迭代器遍历集合，，可以使用语法糖foreach语句直接遍历集合。public interface Collection
      
        extends Iterable
       
         {    // Query Operations    /**     * Returns the number of elements in this collection.  If this collection     * contains more than 
        Integer.MAX_VALUE elements, returns     * 
        Integer.MAX_VALUE.     *     * @return the number of elements in this collection     */ 集合的长度     int size();    /**     * Returns 
        true if this collection contains no elements.     *     * @return 
        true if this collection contains no elements     */ 集合是否为空。     boolean isEmpty();    /**     * Returns 
        true if this collection contains the specified element.     * More formally, returns 
        true if and only if this collection     * contains at least one element 
        e such that     * 
        (o==null ? e==null : o.equals(e)).     *     * @param o element whose presence in this collection is to be tested     * @return 
        true if this collection contains the specified     *         element     * @throws ClassCastException if the type of the specified element     *         is incompatible with this collection (optional)     * @throws NullPointerException if the specified element is null and this     *         collection does not permit null elements (optional)     */    boolean contains(Object o);    /**     * Returns an iterator over the elements in this collection.  There are no     * guarantees concerning the order in which the elements are returned     * (unless this collection is an instance of some class that provides a     * guarantee).     *     * @return an 
        Iterator over the elements in this collection     */    Iterator
        
          iterator();    /**     * Returns an array containing all of the elements in this collection.     * If this collection makes any guarantees as to what order its elements     * are returned by its iterator, this method must return the elements in     * the same order.     *     * 
        
       
      
The returned array will be "safe" in that no references to it are     * maintained by this collection.  (In other words, this method must     * allocate a new array even if this collection is backed by an array).     * The caller is thus free to modify the returned array.     *     * 
This method acts as bridge between array-based and collection-based     * APIs.     *     * @return an array containing all of the elements in this collection     */ 将集合转为object对象的数组。     Object[] toArray();    /**     * Returns an array containing all of the elements in this collection;     * the runtime type of the returned array is that of the specified array.     * If the collection fits in the specified array, it is returned therein.     * Otherwise, a new array is allocated with the runtime type of the     * specified array and the size of this collection.     *     * 
If this collection fits in the specified array with room to spare     * (i.e., the array has more elements than this collection), the element     * in the array immediately following the end of the collection is set to     * null.  (This is useful in determining the length of this     * collection only if the caller knows that this collection does     * not contain any null elements.)     *     * 
If this collection makes any guarantees as to what order its elements     * are returned by its iterator, this method must return the elements in     * the same order.     *     * 
Like the {
    @link #toArray()} method, this method acts as bridge between     * array-based and collection-based APIs.  Further, this method allows     * precise control over the runtime type of the output array, and may,     * under certain circumstances, be used to save allocation costs.     *     * 
Suppose x is a collection known to contain only strings.     * The following code can be used to dump the collection into a newly     * allocated array of String:     *     * 
     *     String[] y = x.toArray(new String[0]);
     *     * Note that toArray(new Object[0]) is identical in function to     * toArray().     *     * @param a the array into which the elements of this collection are to be     *        stored, if it is big enough; otherwise, a new array of the same     *        runtime type is allocated for this purpose.     * @return an array containing all of the elements in this collection     * @throws ArrayStoreException if the runtime type of the specified array     *         is not a supertype of the runtime type of every element in     *         this collection     * @throws NullPointerException if the specified array is null     */ 使用到泛型。     
     
       T[] toArray(T[] a);    // Modification Operations    /**     * Ensures that this collection contains the specified element (optional     * operation).  Returns 
      true if this collection changed as a     * result of the call.  (Returns 
      false if this collection does     * not permit duplicates and already contains the specified element.)
      
     *     * Collections that support this operation may place limitations on what     * elements may be added to this collection.  In particular, some     * collections will refuse to add null elements, and others will     * impose restrictions on the type of elements that may be added.     * Collection classes should clearly specify in their documentation any     * restrictions on what elements may be added.
      
     *     * If a collection refuses to add a particular element for any reason     * other than that it already contains the element, it must throw     * an exception (rather than returning false).  This preserves     * the invariant that a collection always contains the specified element     * after this call returns.     *     * @param e element whose presence in this collection is to be ensured     * @return true if this collection changed as a result of the     *         call     * @throws UnsupportedOperationException if the add operation     *         is not supported by this collection     * @throws ClassCastException if the class of the specified element     *         prevents it from being added to this collection     * @throws NullPointerException if the specified element is null and this     *         collection does not permit null elements     * @throws IllegalArgumentException if some property of the element     *         prevents it from being added to this collection     * @throws IllegalStateException if the element cannot be added at this     *         time due to insertion restrictions     */ 向集合中添加元素 参数为泛型。   我的理解是：泛型使用后，这个参数可以使java中的任意类型。。     boolean add(E e);    /**     * Removes a single instance of the specified element from this     * collection, if it is present (optional operation).  More formally,     * removes an element e such that     * (o==null ? e==null : o.equals(e)), if     * this collection contains one or more such elements.  Returns     * true if this collection contained the specified element (or     * equivalently, if this collection changed as a result of the call).     *     * @param o element to be removed from this collection, if present     * @return true if an element was removed as a result of this call     * @throws ClassCastException if the type of the specified element     *            is incompatible with this collection (optional)     * @throws NullPointerException if the specified element is null and this     *         collection does not permit null elements (optional)     * @throws UnsupportedOperationException if the remove operation     *         is not supported by this collection     */ 移除集合中的元素。     boolean remove(Object o);    // Bulk Operations    /**     * Returns true if this collection contains all of the elements     * in the specified collection.     *     * @param  c collection to be checked for containment in this collection     * @return true if this collection contains all of the elements     *           in the specified collection     * @throws ClassCastException if the types of one or more elements     *         in the specified collection are incompatible with this     *         collection (optional)     * @throws NullPointerException if the specified collection contains one     *         or more null elements and this collection does not permit null     *         elements (optional), or if the specified collection is null     * @see    #contains(Object)     */ 当前集合是否包含另一个集合的所有元素。     boolean containsAll(Collection
        c);    /**     * Adds all of the elements in the specified collection to this collection     * (optional operation).  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 collection, and this collection is     * nonempty.)     *     * @param c collection containing elements to be added to this collection     * @return true if this collection changed as a result of the call     * @throws UnsupportedOperationException if the addAll operation     *         is not supported by this collection     * @throws ClassCastException if the class of an element of the specified     *         collection prevents it from being added to this collection     * @throws NullPointerException if the specified collection contains a     *         null element and this collection does not permit null elements,     *         or if the specified collection is null     * @throws IllegalArgumentException if some property of an element of the     *         specified collection prevents it from being added to this     *         collection     * @throws IllegalStateException if not all the elements can be added at     *         this time due to insertion restrictions     * @see #add(Object)     */ 将另一个集合的所有元素添加到当前结合中。     boolean addAll(Collection
        c);    /**     * Removes all of this collection's elements that are also contained in the     * specified collection (optional operation).  After this call returns,     * this collection will contain no elements in common with the specified     * collection.     *     * @param c collection containing elements to be removed from this collection     * @return true if this collection changed as a result of the     *         call     * @throws UnsupportedOperationException if the removeAll method     *         is not supported by this collection     * @throws ClassCastException if the types of one or more elements     *         in this collection are incompatible with the specified     *         collection (optional)     * @throws NullPointerException if this collection contains one or more     *         null elements and the specified collection does not support     *         null elements (optional), or if the specified collection is null     * @see #remove(Object)     * @see #contains(Object)     */     boolean removeAll(Collection
        c);    /**     * Retains only the elements in this collection that are contained in the     * specified collection (optional operation).  In other words, removes from     * this collection all of its elements that are not contained in the     * specified collection.     *     * @param c collection containing elements to be retained in this collection     * @return true if this collection changed as a result of the call     * @throws UnsupportedOperationException if the retainAll operation     *         is not supported by this collection     * @throws ClassCastException if the types of one or more elements     *         in this collection are incompatible with the specified     *         collection (optional)     * @throws NullPointerException if this collection contains one or more     *         null elements and the specified collection does not permit null     *         elements (optional), or if the specified collection is null     * @see #remove(Object)     * @see #contains(Object)     */ 和移除相反。保留当前集合中这个集合的元素，，其他的删除。     boolean retainAll(Collection
        c);    /**     * Removes all of the elements from this collection (optional operation).     * The collection will be empty after this method returns.     *     * @throws UnsupportedOperationException if the clear operation     *         is not supported by this collection     */ 清空集合中的元素     void clear();    // Comparison and hashing    /**     * Compares the specified object with this collection for equality. 
      
     *     * While the Collection interface adds no stipulations to the     * general contract for the Object.equals, programmers who     * implement the Collection interface "directly" (in other words,     * create a class that is a Collection but is not a Set     * or a List) must exercise care if they choose to override the     * Object.equals.  It is not necessary to do so, and the simplest     * course of action is to rely on Object's implementation, but     * the implementor may wish to implement a "value comparison" in place of     * the default "reference comparison."  (The List and     * Set interfaces mandate such value comparisons.)
      
     *     * The general contract for the Object.equals method states that     * equals must be symmetric (in other words, a.equals(b) if and     * only if b.equals(a)).  The contracts for List.equals     * and Set.equals state that lists are only equal to other lists,     * and sets to other sets.  Thus, a custom equals method for a     * collection class that implements neither the List nor     * Set interface must return false when this collection     * is compared to any list or set.  (By the same logic, it is not possible     * to write a class that correctly implements both the Set and     * List interfaces.)     *     * @param o object to be compared for equality with this collection     * @return true if the specified object is equal to this     * collection     *     * @see Object#equals(Object)     * @see Set#equals(Object)     * @see List#equals(Object)     */    boolean equals(Object o);    /**     * Returns the hash code value for this collection.  While the     * Collection interface adds no stipulations to the general     * contract for the Object.hashCode method, programmers should     * take note that any class that overrides the Object.equals     * method must also override the Object.hashCode method in order     * to satisfy the general contract for the Object.hashCodemethod.     * In particular, c1.equals(c2) implies that     * c1.hashCode()==c2.hashCode().     *     * @return the hash code value for this collection     *     * @see Object#hashCode()     * @see Object#equals(Object)     */    int hashCode();}