Java经典类库-Guava中的函数式编程讲解
lylove1220
8年前
<p>如果我要新建一个java的项目,那么有两个类库是必备的,一个是junit,另一个是 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 。选择junit,因为我喜欢TDD,喜欢自动化测试。而是用 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> ,是因为我喜欢简洁的API。 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 提供了很多的实用工具函数来弥补java标准库的不足,另外 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 还引入了函数式编程的概念,在一定程度上缓解了java在JDK1.8之前没有lambda的缺陷,使使用java书写简洁易读的函数式风格的代码成为可能。</p> <p>下面就简单的介绍下 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 中的一些体现了函数式编程的API。</p> <h2>Filter</h2> <p>我们先创建一个简单的Person类。</p> <p>Person.java</p> <pre> <code class="language-java">public class Person { public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } private String name; private int age; public Person(String name, int age) { this.name = name; this.age = age; } }</code></pre> <p>如果要产生一个Person类的List,通常的写法可能是这样子。</p> <pre> <code class="language-java">List<Person> people = new ArrayList<Person>(); people.add(new Person("bowen",27)); people.add(new Person("bob", 20)); people.add(new Person("Katy", 18)); people.add(new Person("Logon", 24));</code></pre> <p>而 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 提供了一个newArrayList的方法,其自带类型推演,并可以方便的生成一个List,并且通过参数传递初始化值。</p> <pre> <code class="language-java">List<Person> people = newArrayList(new Person("bowen", 27), new Person("bob", 20), new Person("Katy", 18), new Person("Logon", 24));</code></pre> <p>当然,这不算函数式编程的范畴,这是 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 给我们提供的一个实用的函数。</p> <p>如果我们选取其中年龄大于20的人,通常的写法可能是这样子。</p> <pre> <code class="language-java">List<Person> oldPeople = new ArrayList<Person>(); for (Person person : people) { if (person.getAge() >= 20) { oldPeople.add(person); } }</code></pre> <p>这就是典型的filter模式。filter即从一个集合中根据一个条件筛选元素。其中person.getAge() >=20就是这个条件。 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 为这种模式提供了一个filter的方法。所以我们可以这样写。</p> <pre> <code class="language-java">List<Person> oldPeople = newArrayList(filter(people, new Predicate<Person>() { public boolean apply(Person person) { return person.getAge() >= 20; } }));</code></pre> <p>这里的Predicate是 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 中的一个接口,我们来看看它的定义。</p> <p>Predicate.java</p> <pre> <code class="language-java">@GwtCompatible public interface Predicate<T> { /** * Returns the result of applying this predicate to {@code input}. This method is <i>generally * expected</i>, but not absolutely required, to have the following properties: * * <ul> * <li>Its execution does not cause any observable side effects. * <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal * Objects.equal}{@code (a, b)} implies that {@code predicate.apply(a) == * predicate.apply(b))}. * </ul> * * @throws NullPointerException if {@code input} is null and this predicate does not accept null * arguments */ boolean apply(@Nullable T input); /** * Indicates whether another object is equal to this predicate. * * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}. * However, an implementation may also choose to return {@code true} whenever {@code object} is a * {@link Predicate} that it considers <i>interchangeable</i> with this one. "Interchangeable" * <i>typically</i> means that {@code this.apply(t) == that.apply(t)} for all {@code t} of type * {@code T}). Note that a {@code false} result from this method does not imply that the * predicates are known <i>not</i> to be interchangeable. */ @Override boolean equals(@Nullable Object object); }</code></pre> <p>里面只有一个apply方法,接收一个泛型的实参,返回一个boolean值。由于java世界中函数并不是一等公民,所以我们无法直接传递一个条件函数,只能通过Predicate这个类包装一下。</p> <h2>And Predicate</h2> <p>如果要再实现一个方法来查找People列表中所有名字中包含b字母的列表,我们可以用 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 简单的实现。</p> <pre> <code class="language-java">List<Person> namedPeople = newArrayList(filter(people, new Predicate<Person>() { public boolean apply(Person person) { return person.getName().contains("b"); } }));</code></pre> <p>一切是这么的简单。 那么新需求来了,如果现在需要找年龄>=20并且名称包含b的人,该如何实现那? 可能你会这样写。</p> <pre> <code class="language-java">List<Person> filteredPeople = newArrayList(filter(people, new Predicate<Person>() { public boolean apply(Person person) { return person.getName().contains("b") && person.getAge() >= 20; } }));</code></pre> <p>这样写的话就有一定的代码重复,因为之前我们已经写了两个Predicate来分别实现这两个条件判断,能不能重用之前的Predicate那?答案是能。 我们首先将之前生成年龄判断和名称判断的两个Predicate抽成方法。</p> <pre> <code class="language-java">private Predicate<Person> ageBiggerThan(final int age) { return new Predicate<Person>() { public boolean apply(Person person) { return person.getAge() >= age; } }; } private Predicate<Person> nameContains(final String str) { return new Predicate<Person>() { public boolean apply(Person person) { return person.getName().contains(str); } }; }</code></pre> <p>而我们的结果其实就是这两个Predicate相与。 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 给我们提供了and方法,用于对一组Predicate求与。</p> <pre> <code class="language-java">List<Person> filteredPeople = newArrayList(filter(people, and(ageBiggerThan(20), nameContains("b"))));</code></pre> <p>由于and接收一组Predicate,返回也是一个Predicate,所以可以直接作为filter的第二个参数。如果不熟悉函数式编程的人可能感觉有点怪异,但是习惯了就会觉得它的强大与简洁。 当然除了and, <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 还为我们提供了or,用于对一组Predicate求或。这里就不多讲了,大家可以自己练习下。</p> <h2>Map(transform)</h2> <p>列表操作还有另一个常见的模式,就是将数组中的所有元素映射为另一种元素的列表,这就是map pattern。举个例子,求People列表中的所有人名。程序员十有八九都会这样写。</p> <pre> <code class="language-java">List<String> names = new ArrayList<String>(); for (Person person : people) { names.add(person.getName()); }</code></pre> <p><a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 已经给我们提供了这种Pattern的结果办法,那就是使用transform方法。</p> <pre> <code class="language-java">List<String> names = newArrayList(transform(people, new Function<Person, String>() { public String apply( Person person) { return person.getName(); } }));</code></pre> <p>Function是另外一种用于封装函数的接口对象。它的定义如下:</p> <p>Function.java</p> <pre> <code class="language-java">@GwtCompatible public interface Function<F, T> { /** * Returns the result of applying this function to {@code input}. This method is <i>generally * expected</i>, but not absolutely required, to have the following properties: * * <ul> * <li>Its execution does not cause any observable side effects. * <li>The computation is <i>consistent with equals</i>; that is, {@link Objects#equal * Objects.equal}{@code (a, b)} implies that {@code Objects.equal(function.apply(a), * function.apply(b))}. * </ul> * * @throws NullPointerException if {@code input} is null and this function does not accept null * arguments */ @Nullable T apply(@Nullable F input); /** * Indicates whether another object is equal to this function. * * <p>Most implementations will have no reason to override the behavior of {@link Object#equals}. * However, an implementation may also choose to return {@code true} whenever {@code object} is a * {@link Function} that it considers <i>interchangeable</i> with this one. "Interchangeable" * <i>typically</i> means that {@code Objects.equal(this.apply(f), that.apply(f))} is true for all * {@code f} of type {@code F}. Note that a {@code false} result from this method does not imply * that the functions are known <i>not</i> to be interchangeable. */ @Override boolean equals(@Nullable Object object); }</code></pre> <p>它与Predicate非常相似,但不同的是它接收两个泛型,apply方法接收一种泛型实参,返回值是另一种泛型值。正是这个apply方法定义了数组间元素一对一的map规则。</p> <h2>reduce</h2> <p>除了filter与map模式外,列表操作还有一种reduce操作。比如求people列表中所有人年龄的和。 <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 并未提供reduce方法。具体原因我们并不清楚。但是我们可以自己简单的实现一个reduce pattern。 先定义一个Func的接口。</p> <p>Func.java</p> <pre> <code class="language-java">public interface Func<F,T> { T apply(F currentElement, T origin); }</code></pre> <p>apply方法的第一个参数为列表中的当前元素,第二个参数为默认值,返回值类型为默认值类型。 然后我们定义个reduce的静态方法。</p> <p>Reduce.java</p> <pre> <code class="language-java">public class Reduce { private Reduce() { } public static <F,T> T reduce(final Iterable<F> iterable, final Func<F, T> func, T origin) { for (Iterator iterator = iterable.iterator(); iterator.hasNext(); ) { origin = func.apply((F)(iterator.next()), origin); } return origin; } }</code></pre> <p>reduce方法接收三个参数,第一个是需要进行reduce操作的列表,第二个是封装reduce操作的Func,第三个参数是初始值。</p> <p>我们可以使用这个reduce来实现求people列表中所有人的年龄之和。</p> <pre> <code class="language-java">Integer ages = Reduce.reduce(people, new Func<Person, Integer>() { public Integer apply(Person person, Integer origin) { return person.getAge() + origin; } }, 0);</code></pre> <p>我们也可以轻松的写一个方法来得到年龄的最大值。</p> <pre> <code class="language-java">Integer maxAge = Reduce.reduce(people, new Func<Person, Integer>() { public Integer apply(Person person, Integer origin) { return person.getAge() > origin ? person.getAge() : origin; } }, 0);</code></pre> <h2>Fluent pattern</h2> <p>现在新需求来了,需要找出年龄>=20岁的人的所有名称。该如何操作那?我们可以使用filter过滤出年龄>=20的人,然后使用transform得到剩下的所有人的人名。</p> <pre> <code class="language-java">private Function<Person, String> getName() { return new Function<Person, String>() { public String apply( Person person) { return person.getName(); } }; } public void getPeopleNamesByAge() { List<String> names = newArrayList(transform(filter(people, ageBiggerThan(20)), getName())); }</code></pre> <p>这样括号套括号的着实不好看。能不能改进一下那? <a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 为我们提供了fluent模式的API,我们可以这样来写。</p> <pre> <code class="language-java">List<String> names = from(people).filter(ageBiggerThan(20)).transform(getName()).toList();</code></pre> <p><a href="/misc/goto?guid=4958835396856351731" rel="nofollow,noindex">Guava</a> 中还有很多好玩的东西,大家时间可以多发掘发掘。这篇文章的源码已经被我放置到 <a href="/misc/goto?guid=4959673435394354324" rel="nofollow,noindex">github</a> 中,感兴趣的可以自行查看。</p> <p> </p> <p>来自: <a href="/misc/goto?guid=4959673435501481068">http://www.cnblogs.com/huang0925/p/3160094.html</a></p> <p> </p>