源碼 

package java.lang;
import java.lang.annotation.*;
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface FunctionalInterface {}

Conceptually, a functional interface has exactly one abstract method.

Since default methods have an implementation, they are not abstract.

If an interface declares an abstract method overriding one of the public methods of java.lang.Object, that also does not count toward the interface's abstract method count since any implementation of the interface will have an implementation from java.lang.Object or elsewhere.

打撈要點(diǎn)之后有三點(diǎn)

• 要聲明FunctionalInterface的接口只能有1個(gè)抽象方法;

a functional interface has exactly one abstract method.

• 因?yàn)閐efault方法有實(shí)現(xiàn)，所以它也不是抽象方法, 所以不在列;

Since default methods have an implementation, they are not abstract.

• 因?yàn)檫@里限定抽象方法只能有1個(gè)，那么如果我的接口里聲明了一個(gè) 抽象方法"String toString();"或者"boolean equals(Object obj);"這樣的，算不算呢？ 答案是： 不算這個(gè)，因?yàn)槿魏螌?shí)現(xiàn)接口的對(duì)象都會(huì)繼承Object，所以這些public的Object的方法，不算在列的。

If an interface declares an abstract method overriding one of the public methods of java.lang.Object, that also does not count toward the interface's abstract method count since any implementation of the interface will have an implementation from java.lang.Object or elsewhere.

接口聲明

也就是說，你的接口可以聲名如下：

/**
 * Created by niewj on 2017/10/26.
 */
@FunctionalInterface
public interface FunctionalInterfaceSummary {
    int doSum(int x, int y); // 抽象方法
    String toString(); // 不占1的數(shù)額
    boolean equals(Object obj); // 不占1的數(shù)額
    // boolean equals(); // 占1的數(shù)額，這不是Object方法
    default void display() {
        System.out.println("show sth..");
    }
}

小結(jié)

1. 接口只限一個(gè)抽象方法，可注解@FunctionalInterface 限定；

2. 聲明函數(shù)式接口之后，有以下特征：

a. 只有一個(gè)抽象方法；

b. Object類的public的方法也可以在接口里寫出來，因?yàn)檫@不算；

c. default方法不算， 因?yàn)樗鼈兪莍mplementation。不是abstract！

以上就是jdk8 FunctionalInterface注解源碼解讀的詳細(xì)內(nèi)容，更多關(guān)于jdk8 FunctionalInterface注解的資料請(qǐng)關(guān)注腳本之家其它相關(guān)文章！

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