jdk1.8新特性应用之Iterable

Posted 2020-11-09 不想下火车的人

　　我们继续看lambda表达式的应用：

    public void urlExcuAspect(RpcController controller, Message request, RpcCallback done)
    {

        if (Util.isEmpty(request))
        {
            return;
        }

        Descriptor descriptor = request.getDescriptorForType();

        if (Util.isEmpty(descriptor))
        {
            return;
        }

        FieldDescriptor paramMapField = descriptor.findFieldByName("paramMap");

        if (Util.isEmpty(paramMapField))
        {
            return;
        }

        List<MapEntry<String, String>> paramList = (List<MapEntry<String, String>>)request.getField(paramMapField);

        Map<String, String> paramMap = new HashMap<>();

        paramList.forEach((entry) -> {
            if (Util.isNotEmpty(entry))
            {
                paramMap.put(entry.getKey(), entry.getValue());
            }
        });

        UesServiceUtils.setPublicParamToRequest(paramMap);

    }

　　这里仍然是针对一个List接口实例paramList的操作，只不过lambda表达式所操作的函数是Collection的父接口Iterable的默认方法forEach，该方法入参是一个函数式接口Consumer：

/*
 * 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);
    }
}

　　先看下静态方法requireNonNull：

/*
 * Copyright (c) 2009, 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.Supplier;

/**
 * This class consists of {@code static} utility methods for operating
 * on objects.  These utilities include {@code null}-safe or {@code
 * null}-tolerant methods for computing the hash code of an object,
 * returning a string for an object, and comparing two objects.
 *
 * @since 1.7
 */
public final class Objects {
    private Objects() {
        throw new AssertionError("No java.util.Objects instances for you!");
    }

    /**
     * Returns {@code true} if the arguments are equal to each other
     * and {@code false} otherwise.
     * Consequently, if both arguments are {@code null}, {@code true}
     * is returned and if exactly one argument is {@code null}, {@code
     * false} is returned.  Otherwise, equality is determined by using
     * the {@link Object#equals equals} method of the first
     * argument.
     *
     * @param a an object
     * @param b an object to be compared with {@code a} for equality
     * @return {@code true} if the arguments are equal to each other
     * and {@code false} otherwise
     * @see Object#equals(Object)
     */
    public static boolean equals(Object a, Object b) {
        return (a == b) || (a != null && a.equals(b));
    }

   /**
    * Returns {@code true} if the arguments are deeply equal to each other
    * and {@code false} otherwise.
    *
    * Two {@code null} values are deeply equal.  If both arguments are
    * arrays, the algorithm in {@link Arrays#deepEquals(Object[],
    * Object[]) Arrays.deepEquals} is used to determine equality.
    * Otherwise, equality is determined by using the {@link
    * Object#equals equals} method of the first argument.
    *
    * @param a an object
    * @param b an object to be compared with {@code a} for deep equality
    * @return {@code true} if the arguments are deeply equal to each other
    * and {@code false} otherwise
    * @see Arrays#deepEquals(Object[], Object[])
    * @see Objects#equals(Object, Object)
    */
    public static boolean deepEquals(Object a, Object b) {
        if (a == b)
            return true;
        else if (a == null || b == null)
            return false;
        else
            return Arrays.deepEquals0(a, b);
    }

    /**
     * Returns the hash code of a non-{@code null} argument and 0 for
     * a {@code null} argument.
     *
     * @param o an object
     * @return the hash code of a non-{@code null} argument and 0 for
     * a {@code null} argument
     * @see Object#hashCode
     */
    public static int hashCode(Object o) {
        return o != null ? o.hashCode() : 0;
    }

   /**
    * Generates a hash code for a sequence of input values. The hash
    * code is generated as if all the input values were placed into an
    * array, and that array were hashed by calling {@link
    * Arrays#hashCode(Object[])}.
    *
    * <p>This method is useful for implementing {@link
    * Object#hashCode()} on objects containing multiple fields. For
    * example, if an object that has three fields, {@code x}, {@code
    * y}, and {@code z}, one could write:
    *
    * <blockquote><pre>
    * &#064;Override public int hashCode() {
    *     return Objects.hash(x, y, z);
    * }
    * </pre></blockquote>
    *
    * <b>Warning: When a single object reference is supplied, the returned
    * value does not equal the hash code of that object reference.</b> This
    * value can be computed by calling {@link #hashCode(Object)}.
    *
    * @param values the values to be hashed
    * @return a hash value of the sequence of input values
    * @see Arrays#hashCode(Object[])
    * @see List#hashCode
    */
    public static int hash(Object... values) {
        return Arrays.hashCode(values);
    }

    /**
     * Returns the result of calling {@code toString} for a non-{@code
     * null} argument and {@code "null"} for a {@code null} argument.
     *
     * @param o an object
     * @return the result of calling {@code toString} for a non-{@code
     * null} argument and {@code "null"} for a {@code null} argument
     * @see Object#toString
     * @see String#valueOf(Object)
     */
    public static String toString(Object o) {
        return String.valueOf(o);
    }

    /**
     * Returns the result of calling {@code toString} on the first
     * argument if the first argument is not {@code null} and returns
     * the second argument otherwise.
     *
     * @param o an object
     * @param nullDefault string to return if the first argument is
     *        {@code null}
     * @return the result of calling {@code toString} on the first
     * argument if it is not {@code null} and the second argument
     * otherwise.
     * @see Objects#toString(Object)
     */
    public static String toString(Object o, String nullDefault) {
        return (o != null) ? o.toString() : nullDefault;
    }

    /**
     * Returns 0 if the arguments are identical and {@code
     * c.compare(a, b)} otherwise.
     * Consequently, if both arguments are {@code null} 0
     * is returned.
     *
     * <p>Note that if one of the arguments is {@code null}, a {@code
     * NullPointerException} may or may not be thrown depending on
     * what ordering policy, if any, the {@link Comparator Comparator}
     * chooses to have for {@code null} values.
     *
     * @param <T> the type of the objects being compared
     * @param a an object
     * @param b an object to be compared with {@code a}
     * @param c the {@code Comparator} to compare the first two arguments
     * @return 0 if the arguments are identical and {@code
     * c.compare(a, b)} otherwise.
     * @see Comparable
     * @see Comparator
     */
    public static <T> int compare(T a, T b, Comparator<? super T> c) {
        return (a == b) ? 0 :  c.compare(a, b);
    }

    /**
     * Checks that the specified object reference is not {@code null}. This
     * method is designed primarily for doing parameter validation in methods
     * and constructors, as demonstrated below:
     * <blockquote><pre>
     * public Foo(Bar bar) {
     *     this.bar = Objects.requireNonNull(bar);
     * }
     * </pre></blockquote>
     *
     * @param obj the object reference to check for nullity
     * @param <T> the type of the reference
     * @return {@code obj} if not {@code null}
     * @throws NullPointerException if {@code obj} is {@code null}
     */
    public static <T> T requireNonNull(T obj) {
        if (obj == null)
            throw new NullPointerException();
        return obj;
    }

    /**
     * Checks that the specified object reference is not {@code null} and
     * throws a customized {@link NullPointerException} if it is. This method
     * is designed primarily for doing parameter validation in methods and
     * constructors with multiple parameters, as demonstrated below:
     * <blockquote><pre>
     * public Foo(Bar bar, Baz baz) {
     *     this.bar = Objects.requireNonNull(bar, "bar must not be null");
     *     this.baz = Objects.requireNonNull(baz, "baz must not be null");
     * }
     * </pre></blockquote>
     *
     * @param obj     the object reference to check for nullity
     * @param message detail message to be used in the event that a {@code
     *                NullPointerException} is thrown
     * @param <T> the type of the reference
     * @return {@code obj} if not {@code null}
     * @throws NullPointerException if {@code obj} is {@code null}
     */
    public static <T> T requireNonNull(T obj, String message) {
        if (obj == null)
            throw new NullPointerException(message);
        return obj;
    }

    /**
     * Returns {@code true} if the provided reference is {@code null} otherwise
     * returns {@code false}.
     *
     * @apiNote This method exists to be used as a
     * {@link java.util.function.Predicate}, {@code filter(Objects::isNull)}
     *
     * @param obj a reference to be checked against {@code null}
     * @return {@code true} if the provided reference is {@code null} otherwise
     * {@code false}
     *
     * @see java.util.function.Predicate
     * @since 1.8
     */
    public static boolean isNull(Object obj) {
        return obj == null;
    }

    /**
     * Returns {@code true} if the provided reference is non-{@code null}
     * otherwise returns {@code false}.
     *
     * @apiNote This method exists to be used as a
     * {@link java.util.function.Predicate}, {@code filter(Objects::nonNull)}
     *
     * @param obj a reference to be checked against {@code null}
     * @return {@code true} if the provided reference is non-{@code null}
     * otherwise {@code false}
     *
     * @see java.util.function.Predicate
     * @since 1.8
     */
    public static boolean nonNull(Object obj) {
        return obj != null;
    }

    /**
     * Checks that the specified object reference is not {@code null} and
     * throws a customized {@link NullPointerException} if it is.
     *
     * <p>Unlike the method {@link #requireNonNull(Object, String)},
     * this method allows creation of the message to be deferred until
     * after the null check is made. While this may confer a
     * performance advantage in the non-null case, when deciding to
     * call this method care should be taken that the costs of
     * creating the message supplier are less than the cost of just
     * creating the string message directly.
     *
     * @param obj     the object reference to check for nullity
     * @param messageSupplier supplier of the detail message to be
     * used in the event that a {@code NullPointerException} is thrown
     * @param <T> the type of the reference
     * @return {@code obj} if not {@code null}
     * @throws NullPointerException if {@code obj} is {@code null}
     * @since 1.8
     */
    public static <T> T requireNonNull(T obj, Supplier<String> messageSupplier) {
        if (obj == null)
            throw new NullPointerException(messageSupplier.get());
        return obj;
    }
}

　　再重点看Consumer接口：

/*
 * Copyright (c) 2010, 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.function;

import java.util.Objects;

/**
 * Represents an operation that accepts a single input argument and returns no
 * result. Unlike most other functional interfaces, {@code Consumer} is expected
 * to operate via side-effects.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #accept(Object)}.
 *
 * @param <T> the type of the input to the operation
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Consumer<T> {

    /**
     * Performs this operation on the given argument.
     *
     * @param t the input argument
     */
    void accept(T t);

    /**
     * Returns a composed {@code Consumer} that performs, in sequence, this
     * operation followed by the {@code after} operation. If performing either
     * operation throws an exception, it is relayed to the caller of the
     * composed operation.  If performing this operation throws an exception,
     * the {@code after} operation will not be performed.
     *
     * @param after the operation to perform after this operation
     * @return a composed {@code Consumer} that performs in sequence this
     * operation followed by the {@code after} operation
     * @throws NullPointerException if {@code after} is null
     */
    default Consumer<T> andThen(Consumer<? super T> after) {
        Objects.requireNonNull(after);
        return (T t) -> { accept(t); after.accept(t); };
    }
}

　　抽象方法accept接收一个对象，然后操作该对象，无需返回任何类型。很简单吧，拿Predicate的test对比一下，test做的是判断，accept做的是操作。举个例子：

        Consumer<String> consumer = (s) -> System.out.println("Hello, " + s.toUpperCase());
        consumer.accept("wlf");

　　这个consumer实例做的就是打印"Hello，wlf"这个操作。再回过来看我们最开始的代码：

        paramList.forEach((entry) -> {
            if (Util.isNotEmpty(entry))
            {
                paramMap.put(entry.getKey(), entry.getValue());
            }
        });

　　遍历paramList，从每个MapEntry<String, String>对象中取出key和value，放到paramMap对象中。好了，我们继续看：

        List<String> list = new ArrayList<String>();
        list.add("wlf");
        list.add("wms");
        list.stream().filter((s) -> s.startsWith("w")).forEach(System.out::println);

　　最后一行融合之前所有jdk1.8新特性，从List父接口Collection的stream默认方法取得一个Stream，通过Stream的filter进行中间操作（这里的操作就是Predicate的test，判断是否w开头），最后通过List祖父接口Iterable的forEach方法进行最终操作（这里的操作就是Consumer的accept，打印最终list对象）。