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  • jdk1.8新特性之接口default方法

      众所周知,default是java的关键字之一,使用场景是配合switch关键字用于条件分支的默认项。但自从java的jdk1.8横空出世以后,它就被赋予了另一项很酷的能力——在接口中定义非抽象方法。

      众所周知,java的接口只能定义静态且不可变的常量或者公共抽象方法,不可能定义非抽象的具体方法。但自从jdk1.8横空出世以后,它就被default关键字赋予了另一项很酷的能力——在接口中定义非抽象方法。好了不废话了,看具体例子吧:

      1、父接口Iterable,定义了两个default方法forEach和spliterator:

    /*
     * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
     * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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    package java.lang;
    
    import java.util.Iterator;
    import java.util.Objects;
    import java.util.Spliterator;
    import java.util.Spliterators;
    import java.util.function.Consumer;
    
    /**
     * Implementing this interface allows an object to be the target of
     * the "for-each loop" statement. See
     * <strong>
     * <a href="{@docRoot}/../technotes/guides/language/foreach.html">For-each Loop</a>
     * </strong>
     *
     * @param <T> the type of elements returned by the iterator
     *
     * @since 1.5
     * @jls 14.14.2 The enhanced for statement
     */
    public interface Iterable<T> {
        /**
         * Returns an iterator over elements of type {@code T}.
         *
         * @return an Iterator.
         */
        Iterator<T> iterator();
    
        /**
         * Performs the given action for each element of the {@code Iterable}
         * until all elements have been processed or the action throws an
         * exception.  Unless otherwise specified by the implementing class,
         * actions are performed in the order of iteration (if an iteration order
         * is specified).  Exceptions thrown by the action are relayed to the
         * caller.
         *
         * @implSpec
         * <p>The default implementation behaves as if:
         * <pre>{@code
         *     for (T t : this)
         *         action.accept(t);
         * }</pre>
         *
         * @param action The action to be performed for each element
         * @throws NullPointerException if the specified action is null
         * @since 1.8
         */
        default void forEach(Consumer<? super T> action) {
            Objects.requireNonNull(action);
            for (T t : this) {
                action.accept(t);
            }
        }
    
        /**
         * Creates a {@link Spliterator} over the elements described by this
         * {@code Iterable}.
         *
         * @implSpec
         * The default implementation creates an
         * <em><a href="Spliterator.html#binding">early-binding</a></em>
         * spliterator from the iterable's {@code Iterator}.  The spliterator
         * inherits the <em>fail-fast</em> properties of the iterable's iterator.
         *
         * @implNote
         * The default implementation should usually be overridden.  The
         * spliterator returned by the default implementation has poor splitting
         * capabilities, is unsized, and does not report any spliterator
         * characteristics. Implementing classes can nearly always provide a
         * better implementation.
         *
         * @return a {@code Spliterator} over the elements described by this
         * {@code Iterable}.
         * @since 1.8
         */
        default Spliterator<T> spliterator() {
            return Spliterators.spliteratorUnknownSize(iterator(), 0);
        }
    }

      2、子接口复写了spliterator方法

    /*
     * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
     * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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    package java.util;
    
    import java.util.function.Predicate;
    import java.util.stream.Stream;
    import java.util.stream.StreamSupport;
    
    /**
     * 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.</a>  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>Some collection operations which perform recursive traversal of the
     * collection may fail with an exception for self-referential instances where
     * the collection directly or indirectly contains itself. This includes the
     * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()}
     * methods. Implementations may optionally handle the self-referential scenario,
     * however most current implementations do not do so.
     *
     * <p>This interface is a member of the
     * <a href="{@docRoot}/../technotes/guides/collections/index.html">
     * Java Collections Framework</a>.
     *
     * @implSpec
     * The default method implementations (inherited or otherwise) do not apply any
     * synchronization protocol.  If a {@code Collection} implementation has a
     * specific synchronization protocol, then it must override default
     * implementations to apply that protocol.
     *
     * @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
     */
    
    public interface Collection<E> extends Iterable<E> {
        // Query Operations
    
        /**
         * Returns the number of elements in this collection.  If this collection
         * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
         * <tt>Integer.MAX_VALUE</tt>.
         *
         * @return the number of elements in this collection
         */
        int size();
    
        /**
         * Returns <tt>true</tt> if this collection contains no elements.
         *
         * @return <tt>true</tt> if this collection contains no elements
         */
        boolean isEmpty();
    
        /**
         * Returns <tt>true</tt> if this collection contains the specified element.
         * More formally, returns <tt>true</tt> if and only if this collection
         * contains at least one element <tt>e</tt> such that
         * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
         *
         * @param o element whose presence in this collection is to be tested
         * @return <tt>true</tt> if this collection contains the specified
         *         element
         * @throws ClassCastException if the type of the specified element
         *         is incompatible with this collection
         *         (<a href="#optional-restrictions">optional</a>)
         * @throws NullPointerException if the specified element is null and this
         *         collection does not permit null elements
         *         (<a href="#optional-restrictions">optional</a>)
         */
        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 <tt>Iterator</tt> over the elements in this collection
         */
        Iterator<E> 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.
         *
         * <p>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.
         *
         * <p>This method acts as bridge between array-based and collection-based
         * APIs.
         *
         * @return an array containing all of the elements in this collection
         */
        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.
         *
         * <p>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
         * <tt>null</tt>.  (This is useful in determining the length of this
         * collection <i>only</i> if the caller knows that this collection does
         * not contain any <tt>null</tt> elements.)
         *
         * <p>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.
         *
         * <p>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.
         *
         * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
         * The following code can be used to dump the collection into a newly
         * allocated array of <tt>String</tt>:
         *
         * <pre>
         *     String[] y = x.toArray(new String[0]);</pre>
         *
         * Note that <tt>toArray(new Object[0])</tt> is identical in function to
         * <tt>toArray()</tt>.
         *
         * @param <T> the runtime type of the array to contain the collection
         * @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
         */
        <T> T[] toArray(T[] a);
    
        // Modification Operations
    
        /**
         * Ensures that this collection contains the specified element (optional
         * operation).  Returns <tt>true</tt> if this collection changed as a
         * result of the call.  (Returns <tt>false</tt> if this collection does
         * not permit duplicates and already contains the specified element.)<p>
         *
         * 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 <tt>null</tt> 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.<p>
         *
         * If a collection refuses to add a particular element for any reason
         * other than that it already contains the element, it <i>must</i> throw
         * an exception (rather than returning <tt>false</tt>).  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 <tt>true</tt> if this collection changed as a result of the
         *         call
         * @throws UnsupportedOperationException if the <tt>add</tt> 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
         */
        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 <tt>e</tt> such that
         * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if
         * this collection contains one or more such elements.  Returns
         * <tt>true</tt> 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 <tt>true</tt> 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
         *         (<a href="#optional-restrictions">optional</a>)
         * @throws NullPointerException if the specified element is null and this
         *         collection does not permit null elements
         *         (<a href="#optional-restrictions">optional</a>)
         * @throws UnsupportedOperationException if the <tt>remove</tt> operation
         *         is not supported by this collection
         */
        boolean remove(Object o);
    
    
        // Bulk Operations
    
        /**
         * Returns <tt>true</tt> if this collection contains all of the elements
         * in the specified collection.
         *
         * @param  c collection to be checked for containment in this collection
         * @return <tt>true</tt> 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
         *         (<a href="#optional-restrictions">optional</a>)
         * @throws NullPointerException if the specified collection contains one
         *         or more null elements and this collection does not permit null
         *         elements
         *         (<a href="#optional-restrictions">optional</a>),
         *         or if the specified collection is null.
         * @see    #contains(Object)
         */
        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 <tt>true</tt> if this collection changed as a result of the call
         * @throws UnsupportedOperationException if the <tt>addAll</tt> 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)
         */
        boolean addAll(Collection<? extends E> 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 <tt>true</tt> if this collection changed as a result of the
         *         call
         * @throws UnsupportedOperationException if the <tt>removeAll</tt> 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
         *         (<a href="#optional-restrictions">optional</a>)
         * @throws NullPointerException if this collection contains one or more
         *         null elements and the specified collection does not support
         *         null elements
         *         (<a href="#optional-restrictions">optional</a>),
         *         or if the specified collection is null
         * @see #remove(Object)
         * @see #contains(Object)
         */
        boolean removeAll(Collection<?> c);
    
        /**
         * Removes all of the elements of this collection that satisfy the given
         * predicate.  Errors or runtime exceptions thrown during iteration or by
         * the predicate are relayed to the caller.
         *
         * @implSpec
         * The default implementation traverses all elements of the collection using
         * its {@link #iterator}.  Each matching element is removed using
         * {@link Iterator#remove()}.  If the collection's iterator does not
         * support removal then an {@code UnsupportedOperationException} will be
         * thrown on the first matching element.
         *
         * @param filter a predicate which returns {@code true} for elements to be
         *        removed
         * @return {@code true} if any elements were removed
         * @throws NullPointerException if the specified filter is null
         * @throws UnsupportedOperationException if elements cannot be removed
         *         from this collection.  Implementations may throw this exception if a
         *         matching element cannot be removed or if, in general, removal is not
         *         supported.
         * @since 1.8
         */
        default boolean removeIf(Predicate<? super E> filter) {
            Objects.requireNonNull(filter);
            boolean removed = false;
            final Iterator<E> each = iterator();
            while (each.hasNext()) {
                if (filter.test(each.next())) {
                    each.remove();
                    removed = true;
                }
            }
            return removed;
        }
    
        /**
         * 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 <tt>true</tt> if this collection changed as a result of the call
         * @throws UnsupportedOperationException if the <tt>retainAll</tt> 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
         *         (<a href="#optional-restrictions">optional</a>)
         * @throws NullPointerException if this collection contains one or more
         *         null elements and the specified collection does not permit null
         *         elements
         *         (<a href="#optional-restrictions">optional</a>),
         *         or if the specified collection is null
         * @see #remove(Object)
         * @see #contains(Object)
         */
        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 <tt>clear</tt> operation
         *         is not supported by this collection
         */
        void clear();
    
    
        // Comparison and hashing
    
        /**
         * Compares the specified object with this collection for equality. <p>
         *
         * While the <tt>Collection</tt> interface adds no stipulations to the
         * general contract for the <tt>Object.equals</tt>, programmers who
         * implement the <tt>Collection</tt> interface "directly" (in other words,
         * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
         * or a <tt>List</tt>) must exercise care if they choose to override the
         * <tt>Object.equals</tt>.  It is not necessary to do so, and the simplest
         * course of action is to rely on <tt>Object</tt>'s implementation, but
         * the implementor may wish to implement a "value comparison" in place of
         * the default "reference comparison."  (The <tt>List</tt> and
         * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
         *
         * The general contract for the <tt>Object.equals</tt> method states that
         * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
         * only if <tt>b.equals(a)</tt>).  The contracts for <tt>List.equals</tt>
         * and <tt>Set.equals</tt> state that lists are only equal to other lists,
         * and sets to other sets.  Thus, a custom <tt>equals</tt> method for a
         * collection class that implements neither the <tt>List</tt> nor
         * <tt>Set</tt> interface must return <tt>false</tt> 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 <tt>Set</tt> and
         * <tt>List</tt> interfaces.)
         *
         * @param o object to be compared for equality with this collection
         * @return <tt>true</tt> 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
         * <tt>Collection</tt> interface adds no stipulations to the general
         * contract for the <tt>Object.hashCode</tt> method, programmers should
         * take note that any class that overrides the <tt>Object.equals</tt>
         * method must also override the <tt>Object.hashCode</tt> method in order
         * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
         * In particular, <tt>c1.equals(c2)</tt> implies that
         * <tt>c1.hashCode()==c2.hashCode()</tt>.
         *
         * @return the hash code value for this collection
         *
         * @see Object#hashCode()
         * @see Object#equals(Object)
         */
        int hashCode();
    
        /**
         * Creates a {@link Spliterator} over the elements in this collection.
         *
         * Implementations should document characteristic values reported by the
         * spliterator.  Such characteristic values are not required to be reported
         * if the spliterator reports {@link Spliterator#SIZED} and this collection
         * contains no elements.
         *
         * <p>The default implementation should be overridden by subclasses that
         * can return a more efficient spliterator.  In order to
         * preserve expected laziness behavior for the {@link #stream()} and
         * {@link #parallelStream()}} methods, spliterators should either have the
         * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
         * <em><a href="Spliterator.html#binding">late-binding</a></em>.
         * If none of these is practical, the overriding class should describe the
         * spliterator's documented policy of binding and structural interference,
         * and should override the {@link #stream()} and {@link #parallelStream()}
         * methods to create streams using a {@code Supplier} of the spliterator,
         * as in:
         * <pre>{@code
         *     Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
         * }</pre>
         * <p>These requirements ensure that streams produced by the
         * {@link #stream()} and {@link #parallelStream()} methods will reflect the
         * contents of the collection as of initiation of the terminal stream
         * operation.
         *
         * @implSpec
         * The default implementation creates a
         * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
         * from the collections's {@code Iterator}.  The spliterator inherits the
         * <em>fail-fast</em> properties of the collection's iterator.
         * <p>
         * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
         *
         * @implNote
         * The created {@code Spliterator} additionally reports
         * {@link Spliterator#SUBSIZED}.
         *
         * <p>If a spliterator covers no elements then the reporting of additional
         * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
         * does not aid clients to control, specialize or simplify computation.
         * However, this does enable shared use of an immutable and empty
         * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
         * empty collections, and enables clients to determine if such a spliterator
         * covers no elements.
         *
         * @return a {@code Spliterator} over the elements in this collection
         * @since 1.8
         */
        @Override
        default Spliterator<E> spliterator() {
            return Spliterators.spliterator(this, 0);
        }
    
        /**
         * Returns a sequential {@code Stream} with this collection as its source.
         *
         * <p>This method should be overridden when the {@link #spliterator()}
         * method cannot return a spliterator that is {@code IMMUTABLE},
         * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
         * for details.)
         *
         * @implSpec
         * The default implementation creates a sequential {@code Stream} from the
         * collection's {@code Spliterator}.
         *
         * @return a sequential {@code Stream} over the elements in this collection
         * @since 1.8
         */
        default Stream<E> stream() {
            return StreamSupport.stream(spliterator(), false);
        }
    
        /**
         * Returns a possibly parallel {@code Stream} with this collection as its
         * source.  It is allowable for this method to return a sequential stream.
         *
         * <p>This method should be overridden when the {@link #spliterator()}
         * method cannot return a spliterator that is {@code IMMUTABLE},
         * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
         * for details.)
         *
         * @implSpec
         * The default implementation creates a parallel {@code Stream} from the
         * collection's {@code Spliterator}.
         *
         * @return a possibly parallel {@code Stream} over the elements in this
         * collection
         * @since 1.8
         */
        default Stream<E> parallelStream() {
            return StreamSupport.stream(spliterator(), true);
        }
    }

      3、实现类UnmodifiableCollection复写了父类的forEach、spliterator方法

        /**
         * @serial include
         */
        static class UnmodifiableCollection<E> implements Collection<E>, Serializable {
            private static final long serialVersionUID = 1820017752578914078L;
            final Collection<? extends E> c;
            UnmodifiableCollection(Collection<? extends E> c) {
                if (c==null)
                    throw new NullPointerException();
                this.c = c;
            }
            public int size()                   {return c.size();}
            public boolean isEmpty()            {return c.isEmpty();}
            public boolean contains(Object o)   {return c.contains(o);}
            public Object[] toArray()           {return c.toArray();}
            public <T> T[] toArray(T[] a)       {return c.toArray(a);}
            public String toString()            {return c.toString();}
            public Iterator<E> iterator() {
                return new Iterator<E>() {
                    private final Iterator<? extends E> i = c.iterator();
                    public boolean hasNext() {return i.hasNext();}
                    public E next()          {return i.next();}
                    public void remove() {
                        throw new UnsupportedOperationException();
                    }
                    @Override
                    public void forEachRemaining(Consumer<? super E> action) {
                        // Use backing collection version
                        i.forEachRemaining(action);
                    }
                };
            }
            public boolean add(E e) {
                throw new UnsupportedOperationException();
            }
            public boolean remove(Object o) {
                throw new UnsupportedOperationException();
            }
            public boolean containsAll(Collection<?> coll) {
                return c.containsAll(coll);
            }
            public boolean addAll(Collection<? extends E> coll) {
                throw new UnsupportedOperationException();
            }
            public boolean removeAll(Collection<?> coll) {
                throw new UnsupportedOperationException();
            }
            public boolean retainAll(Collection<?> coll) {
                throw new UnsupportedOperationException();
            }
            public void clear() {
                throw new UnsupportedOperationException();
            }
            // Override default methods in Collection
            @Override
            public void forEach(Consumer<? super E> action) {
                c.forEach(action);
            }
            @Override
            public boolean removeIf(Predicate<? super E> filter) {
                throw new UnsupportedOperationException();
            }
            @SuppressWarnings("unchecked")
            @Override
            public Spliterator<E> spliterator() {
                return (Spliterator<E>)c.spliterator();
            }
            @SuppressWarnings("unchecked")
            @Override
            public Stream<E> stream() {
                return (Stream<E>)c.stream();
            }
            @SuppressWarnings("unchecked")
            @Override
            public Stream<E> parallelStream() {
                return (Stream<E>)c.parallelStream();
            }
        }

      从上面祖父孙三代可以看到,default就是给接口赋予了原来抽象类的能力,实现类可以像使用抽象类的方法一样,直接使用接口里的方法。

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  • 原文地址:https://www.cnblogs.com/wuxun1997/p/9079291.html
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