jdk1.8新特性之函数式接口

　　函数式接口就是只有一个抽象方法的接口。如果这个接口里没有或者包含了两个以上的抽象方法，对不起，你不叫函数式接口，只能叫你接口。那这个函数式有啥用呢？如果配合Lambda表达式的话，可以大大的简化代码。这里不说Lambda，只说函数式接口，先看它长啥样：

/*
* Copyright (c) 1994, 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;

/**
* The <code>Runnable</code> interface should be implemented by any
* class whose instances are intended to be executed by a thread. The
* class must define a method of no arguments called <code>run</code>.
* <p>
* This interface is designed to provide a common protocol for objects that
* wish to execute code while they are active. For example,
* <code>Runnable</code> is implemented by class <code>Thread</code>.
* Being active simply means that a thread has been started and has not
* yet been stopped.
* <p>
* In addition, <code>Runnable</code> provides the means for a class to be
* active while not subclassing <code>Thread</code>. A class that implements
* <code>Runnable</code> can run without subclassing <code>Thread</code>
* by instantiating a <code>Thread</code> instance and passing itself in
* as the target.  In most cases, the <code>Runnable</code> interface should
* be used if you are only planning to override the <code>run()</code>
* method and no other <code>Thread</code> methods.
* This is important because classes should not be subclassed
* unless the programmer intends on modifying or enhancing the fundamental
* behavior of the class.
*
* @author  Arthur van Hoff
* @see     java.lang.Thread
* @see     java.util.concurrent.Callable
* @since   JDK1.0
*/
@FunctionalInterface
public interface Runnable {
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* <p>
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see     java.lang.Thread#run()
*/
public abstract void run();
}

　　它最明显的一个标志就是FunctionalInterface注解，如果你使用了FunctionalInterface注解却在接口里写了两个抽象方法，那么编译器是不会放过你的。当然我们也可以不加该注解，只要你满足了一个抽象方法的接口这个标准，那么它仍然是函数式接口：

/*
* 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);
}
}

　　虽然上面接口除了一个抽象方法（接口里方法的默认修饰符是public abstract）外，还有两个默认方法，但它依然满足只有一个抽象方法的标准。除了默认方法，静态方法、Object对象里的方法（接口默认继承java.lang.Object，所以如果接口显示声明覆盖了Object中方法，那么也不算抽象方法），都没关系。