JDK8新特性：函数式接口@FunctionalInterfac

我们常用的一些接口Callable、Runnable、Comparator等在JDK8中都添加了@FunctionalInterface注解

Callable Runnable

/**
 * An informative annotation type used to indicate that an interface
 * type declaration is intended to be a <i>functional interface</i> as
 * defined by the Java Language Specification.
 *
 * Conceptually, a functional interface has exactly one abstract
 * method.  Since {@linkplain java.lang.reflect.Method#isDefault()
 * default methods} have an implementation, they are not abstract.  If
 * an interface declares an abstract method overriding one of the
 * public methods of {@code java.lang.Object}, that also does
 * <em>not</em> count toward the interface's abstract method count
 * since any implementation of the interface will have an
 * implementation from {@code java.lang.Object} or elsewhere.
 *
 * <p>Note that instances of functional interfaces can be created with
 * lambda expressions, method references, or constructor references.
 *
 * <p>If a type is annotated with this annotation type, compilers are
 * required to generate an error message unless:
 *
 * <ul>
 * <li> The type is an interface type and not an annotation type, enum, or class.
 * <li> The annotated type satisfies the requirements of a functional interface.
 * </ul>
 *
 * <p>However, the compiler will treat any interface meeting the
 * definition of a functional interface as a functional interface
 * regardless of whether or not a {@code FunctionalInterface}
 * annotation is present on the interface declaration.
 *
 * @jls 4.3.2. The Class Object
 * @jls 9.8 Functional Interfaces
 * @jls 9.4.3 Interface Method Body
 * @since 1.8
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface FunctionalInterface {}

通过JDK8源码javadoc，可以知道这个注解有以下特点：

1、该注解只能标记在"**有且仅有一个抽象方法"的接口上。

2、JDK8接口中的静态方法和默认方法，都不算是抽象方法。

3、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。

4、该注解不是必须的，如果一个接口符合"函数式接口"定义，那么加不加该注解都没有影响。加上该注解能够更好地让编译器进行检查。如果编写的不是函数式接口，但是加上了@FunctionInterface，那么编译器会报错。

@FunctionalInterface标记在接口上，“函数式接口”是指仅仅只包含一个抽象方法的接口 。

package io.fredia;

@FunctionalInterface
public interface MyFunctionalInterface {

    public void test();
}

如果一个接口中包含不止一个抽象方法，那么不能使用@FunctionalInterface，编译会报错。

package io.fredia;

@FunctionalInterface
public interface MyFunctionalInterface {

    public void test();

    public void test2();
}

如下面这个接口就是一个正确的函数式接口：

package io.fredia;

@FunctionalInterface
public interface MyFunctionalInterface {

    public void test();

    // java.lang.Object中的方法不是抽象方法  
    public boolean equals(Object var1);  
  
    // default不是抽象方法  
    public default void defaultMethod(){ }  
  
    // static不是抽象方法  
    public static void staticMethod(){  }
}