jdk1.8新特性之接口default方法

Posted 2020-11-08 不想下火车的人

　　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。