jdk1.8新特性之lambda表達式

　　lambda表達式其實就是指一個匿名函數，應用最普遍的就是匿名內部類的簡化。在jdk1.8以前，咱們定義一個匿名內部類可能須要寫一大坨代碼，如今有了lambda以後，能夠寫的很簡潔了。但不是說lambda只能用來簡化匿名內部類，從lambda的實際做用和表現上來看，它就是一個變量指代了一個代碼塊。而可以使用lambda表達式的一個前提要求是，該變量必須實現某個函數式接口。啥是函數式接口？參考jdk1.8新特性之函數式接口。看一個匿名內部類的例子：

    FutureTask<String> coverTask = new FutureTask<String>(new Callable()
    {
        public String call()
        {
            return PortalBookCoverCacheService.getInstance().getBookCover(bookItem,
                PropertiesConfig.getProperty("cdnUrl"));
        }
    });

　　上面在實例化FutureTask對象的時候，使用了Callable接口實例的匿名內部類，實際上FutureTask構造器裏就是一個Callable的實例：

Callable callable = new Callable()
    {
        public String call()
        {
            return PortalBookCoverCacheService.getInstance().getBookCover(bookItem,
                PropertiesConfig.getProperty("cdnUrl"));
        }
    };

　　若是咱們使用lambda表達式，上面的callable變量實際上就是

    Callable callable = () ->
        {
            return PortalBookCoverCacheService.getInstance().getBookCover(bookItem,
                PropertiesConfig.getProperty("cdnUrl"));
        }
    ;

　　callable變量指代的就是一段代碼塊，把這個變量放入FutureTask的構造器裏，匿名內部類能夠簡化爲：

                    FutureTask<String> coverTask = new FutureTask<String>(() -> {
                        return PortalBookCoverCacheService.getInstance()
                            .getBookCover(bookItem, PropertiesConfig.getProperty("cdnUrl"));
                    });

　　爲啥能這麼簡化呢？由於它符合lambda表達式規範，即Callable是一個函數式接口：

/** <a href="http://www.cpupk.com/decompiler">Eclipse Class Decompiler</a> plugin, Copyright (c) 2017 Chen Chao. */
/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */
/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

/**
 * A task that returns a result and may throw an exception.
 * Implementors define a single method with no arguments called
 * {@code call}.
 *
 * <p>The {@code Callable} interface is similar to {@link
 * java.lang.Runnable}, in that both are designed for classes whose
 * instances are potentially executed by another thread.  A
 * {@code Runnable}, however, does not return a result and cannot
 * throw a checked exception.
 *
 * <p>The {@link Executors} class contains utility methods to
 * convert from other common forms to {@code Callable} classes.
 *
 * @see Executor
 * @since 1.5
 * @author Doug Lea
 * @param <V> the result type of method {@code call}
 */
@FunctionalInterface
public interface Callable<V> {
    /**
     * Computes a result, or throws an exception if unable to do so.
     *
     * @return computed result
     * @throws Exception if unable to compute a result
     */
    V call() throws Exception;
}

　　因此能夠咱們經過lambda表達式省略了接口類型、方法名和方法參數（由於這裏call方法根本就不須要參數），->符號的左邊是參數列表（若是有參數的話），右邊是方法體。lambda實際上是經過編譯器的強大而賦予的能力，編譯器經過自動匹配接口類型、智能識別參數列表，能夠像考古學家根據蛛絲馬跡恢復物品的原貌的同樣，還原出匿名內部類的原本面目。