jdk1.8新特性应用之Collection

　　之前说了jdk1.8几个新特性，现在看下实战怎么玩，直接看代码：

    public List<MSG_ConMediaInfo> getConMediaInfoList(String liveType)
    {
        if (Util.isEmpty(liveType))
        {
            return null;
        }
        List<MSG_ConMediaInfo> conMediaInfoList = getConMediaInfoList();
        if (Util.isNotEmpty(conMediaInfoList))
        {
            if (LIVE_TYPE_BEING.equals(liveType))
            {
                return conMediaInfoList.parallelStream()
                    .filter(s -> s != null)
                    .filter(s -> isLiveBeing(s))
                    .collect(Collectors.toList());
            }
            else if (LIVE_TYPE_PREVIEW.equals(liveType))
            {
                return conMediaInfoList.parallelStream()
                    .filter(s -> s != null)
                    .filter(s -> isLivePreview(s))
                    .collect(Collectors.toList());
            }
            else
            {
                return conMediaInfoList.parallelStream()
                    .filter(s -> s != null)
                    .filter(s -> liveType.equals(s.getLiveStatus()))
                    .collect(Collectors.toList());
            }
        }
        return null;
    }


    private boolean isLiveBeing(MSG_ConMediaInfo conMediaInfo)
    {
        String liveStatus = conMediaInfo.getLiveStatus();
        if (LIVE_TYPE_BEING.equals(liveStatus))
        {
            return Boolean.TRUE;
        }
        if (LIVE_TYPE_PREVIEW.equals(liveStatus))
        {
            if (!isLivePreview(conMediaInfo))
            {
                return Boolean.TRUE;
            }
        }
        return Boolean.FALSE;
    }


    private boolean isLivePreview(MSG_ConMediaInfo conMediaInfo)
    {
        if (LIVE_TYPE_PREVIEW.equals(conMediaInfo.getLiveStatus()))
        {
            String startTime =
                DateTools.timeTransform(conMediaInfo.getLiveStartTime(),
                    DateTools.DATE_PATTERN_24HOUR_16);
            int result =
                DateTools.compare(new Date(), DateTools.timeStr2Date(startTime, DateTools.DATE_PATTERN_24HOUR_16),
                    CompareDateFormate.yyyyMMddhhmmss);
            //当前时间已超过直播开始时间
            if (result != -1)
            {
                return Boolean.FALSE;
            }
            return Boolean.TRUE;
        }
        return Boolean.FALSE;
    }

　　这里3个方法，第一个方法使用了lambda表达式，这里是一个List实例conMediaInfoList，通过调用parallelStream方法得到一个Stream接口，再调用它的filter方法，该方法的参数是一个函数式接口Predicate。步步推进，终于绕到函数式接口这个jdk1.8的新特性了。我们知道，lambda表达式使用的前提就是函数式接口。

　　那么首先让我们来看下Predicate：

 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
package java.util.function;

import java.util.Objects;

/**
 * Represents a predicate (boolean-valued function) of one argument.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #test(Object)}.
 *
 * @param <T> the type of the input to the predicate
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Predicate<T> {

    /**
     * Evaluates this predicate on the given argument.
     *
     * @param t the input argument
     * @return {@code true} if the input argument matches the predicate,
     * otherwise {@code false}
     */
    boolean test(T t);

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * AND of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code false}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ANDed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * AND of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> and(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }

    /**
     * Returns a predicate that represents the logical negation of this
     * predicate.
     *
     * @return a predicate that represents the logical negation of this
     * predicate
     */
    default Predicate<T> negate() {
        return (t) -> !test(t);
    }

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * OR of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code true}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ORed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * OR of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> or(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }

    /**
     * Returns a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}.
     *
     * @param <T> the type of arguments to the predicate
     * @param targetRef the object reference with which to compare for equality,
     *               which may be {@code null}
     * @return a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}
     */
    static <T> Predicate<T> isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }
}

　　我们看到该接口只有一个抽象方法（只能有一个，否则就不能叫函数式接口），3个默认方法和一个静态方法。该接口只有一个参数t，返回一个布尔值。我们先看看能怎么用这个接口：

            // Predicate接口实例predicate指代一段判断字符串s是否长度大于0的代码
            Predicate<String> predicate = (s) -> s.length() > 0; 
            
            // predicate应用,判断字符串wlf是否长度>0
            predicate.test("wlf");
            
            // predicate应用，判断字符串wlf是否长度<=0
            predicate.negate().test("wlf"); 
            
            // 方法引用：Objects.isNull返回一个boolean，
            Predicate<Object> isNull = Objects::isNull;
            
            // 方法引用：String.isEmpty方法一个boolean
            Predicate<String> isEmpty = String::isEmpty;

　　Predicate接口做的事情就是判断参数s是否符合方法体里的判断逻辑，而方法体的逻辑是由你自己实现的。上面分别判断了一个字符串的长度大于0、不大于0，对象是否为空，字符串是否为空。

　　我们溯流而上，接下来再看下Stream的filter方法

public interface Stream<T> extends BaseStream<T, Stream<T>> {

    /**
     * Returns a stream consisting of the elements of this stream that match
     * the given predicate.
     *
     * <p>This is an <a href="package-summary.html#StreamOps">intermediate
     * operation</a>.
     *
     * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
     *                  <a href="package-summary.html#Statelessness">stateless</a>
     *                  predicate to apply to each element to determine if it
     *                  should be included
     * @return the new stream
     */
    Stream<T> filter(Predicate<? super T> predicate);
}

　　这个方法就是执行Predicate的判断逻辑，通过再返回一个Stream。再回过来看最开始的代码：

conMediaInfoList.parallelStream()
                    .filter(s -> s != null)
                    .filter(s -> isLiveBeing(s))
                    .collect(Collectors.toList());

　　我们看到lambda表达式里先判断MSG_ConMediaInfo实例是否不为null，再判断实例是否符合isLiveBeing方法里的判断逻辑，两个都返回true的话，继续调用collect方法返回一个List。

　　接下来聊下Stream接口。它表示在一组元素上一次执行的操作序列，包括中间操作或者最终操作，中间操作继续返回Stream，直到操作序列结束，执行最终操作。像上面的业务代码，filter是中间操作，collect是最终操作。那么Stream怎么创建呢？只能通过容器类来创建，有两个方法都可以返回Stream：

public interface Collection<E> extends Iterable<E> {

     /**
     * 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);
    }
}

　　Collection还有它的孩子们List、Set都可以通过parallelStream来创造一个Stream对象，然后才后面的那些lambda表达式。