blocks|key|199917|text|Java文档说：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|199918|199919|请注意，不覆盖Object.equals(Object)始终是安全的。然而，在某些情况下，通过允许程序确定两个不同的比较器施加相同的顺序，覆盖此方法可能会提高性能。|blockquote|199920|199921|199922|也许比较器是特别的？也许，尽管它是一个接口，但不知何故有一个调用compare()的equals()默认实现？在算法上，这是微不足道的。|offset|length|style|CODE|199923|我认为在接口中声明的所有方法都是抽象的(即没有默认实现)。但也许我漏掉了什么。|199924|entityMap^0|0|0|0|0|0|W|9|16|8|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|S|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|T|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|U|8|@]|9|@]|A|$]]|$1|G|3|-4|5|6|7|V|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|W|8|@$J|X|K|Y|L|M]|$J|Z|K|10|L|M]]|9|@]|A|$]]|$1|N|3|O|5|6|7|11|8|@]|9|@]|A|$]]|$1|P|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|Q|$]]

The Java docs say:

<blockquote>
 Note that it is always safe not to override Object.equals(Object).
 However, overriding this method may, in some cases, improve
 performance by allowing programs to determine that two distinct
 comparators impose the same order.
</blockquote>

Maybe Comparator is special? Maybe, even though it's an interface, there is somehow a default implementation of <code>equals()</code> that calls <code>compare()</code>? Algorithmically, it's trivial.

I thought all methods that were declared in interfaces were abstract (i. e. no default implementation). But maybe I'm missing something.

blocks|key|199349|text|从same+page+you+linked+to|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|199350|199351|接口比较器是功能性的，因为尽管它声明了两个抽象方法，但其中一个-equals-在Object中有一个对应于公共方法的签名。接口总是声明与Object的公共方法相对应的抽象方法，但它们通常隐式地这样做。无论是隐式声明还是显式声明，这些方法都会被排除在计数之外。|blockquote|199352|199353|199354|我真的不能说得更好了。|199355|entityMap|0|LINK|mutability|MUTABLE|url|http://www.lambdafaq.org/what-is-a-functional-interface/^0|1|N|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@$A|U|B|V|1|W]]|C|$]]|$1|D|3|-4|5|6|7|X|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|Y|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|Z|8|@]|9|@]|C|$]]|$1|I|3|-4|5|6|7|10|8|@]|9|@]|C|$]]|$1|J|3|K|5|6|7|11|8|@]|9|@]|C|$]]|$1|L|3|-4|5|6|7|12|8|@]|9|@]|C|$]]]|M|$N|$5|O|P|Q|C|$R|S]]]]

From the <a href="http://www.lambdafaq.org/what-is-a-functional-interface/" rel="noreferrer">same page you linked to</a>:
<blockquote>
The interface Comparator is functional because although it declares two abstract methods, one of these—equals— has a signature corresponding to a public method in Object. Interfaces always declare abstract methods corresponding to the public methods of Object, but they usually do so implicitly. Whether implicitly or explicitly declared, such methods are excluded from the count.
</blockquote>
I can't really say it better.

blocks|key|201653|text|但是在比较器接口中，compare()和equals()方法都是抽象的，这意味着它有两个抽象的方法。那么，如果定义要求一个接口恰好有一个抽象方法，那么这怎么可能呢？这里我漏掉了什么？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|201654|一个。|201655|201656|函数接口可以指定Object定义的任何公共方法，例如equals(+)，而不会影响其“函数接口”状态。公共对象方法被视为函数接口的隐式成员，因为它们是由函数接口的实例自动实现的。|blockquote|201657|201658|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|M|8|@]|9|@]|A|$]]|$1|E|3|F|5|G|7|N|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|P|8|@]|9|@]|A|$]]]|J|$]]

Q. But in the Comparator interface both compare() and equals() methods are abstract, which means it has two abstract methods. So how can this be working, if the definition requires an interface to have exactly one abstract method? What am I missing here?

A.

<blockquote>
 A functional interface may specify any public method defined by Object, such as equals( ),
 without affecting its “functional interface” status. The public Object methods are considered implicit
 members of a functional interface because they are automatically implemented by an instance of a
 functional interface.
</blockquote>

blocks|key|199952|text|@FunctionalInterface+page中给出了另一种解释|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|199953|199954|从概念上讲，函数接口只有一个抽象方法。因为默认方法有一个实现，所以它们不是抽象的。如果接口声明了一个抽象方法，该抽象方法重写了java.lang.Object**，的公共方法之一，该方法也不计入接口的抽象方法计数，因为该接口的任何实现都将具有来自**+java.lang.Object、或其他地方的实现。|blockquote|style|BOLD|CODE|199955|199956|199957|您可以使用@FunctionalInterface.测试哪个接口是正确的功能接口|199958|例如：|199959|199960|这是可行的|unordered-list-item|199961|199962|@FunctionalInterface公共接口FunctionalInterf+{+void+m()；boolean+equals(Object+o)；}|199963|这会产生错误：|199964|199965|@函数接口公共接口FunctionalInterf+{+void+m()；boolean+equals()；}|199966|在接口FunctionalInterf中找到多个非重写抽象方法|199967|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html^0|0|P|0|0|0|1R|I|3I|G|3Z|9|1R|G|3I|G|0|0|0|5|K|10|4|5|K|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1C|8|@]|9|@$A|1D|B|1E|1|1F]]|C|$]]|$1|D|3|-4|5|6|7|1G|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|1H|8|@$A|1I|B|1J|H|I]|$A|1K|B|1L|H|I]|$A|1M|B|1N|H|I]|$A|1O|B|1P|H|J]|$A|1Q|B|1R|H|J]]|9|@]|C|$]]|$1|K|3|-4|5|6|7|1S|8|@]|9|@]|C|$]]|$1|L|3|-4|5|6|7|1T|8|@]|9|@]|C|$]]|$1|M|3|N|5|6|7|1U|8|@$A|1V|B|1W|H|I]|$A|1X|B|1Y|H|I]|$A|1Z|B|20|H|J]]|9|@]|C|$]]|$1|O|3|P|5|6|7|21|8|@]|9|@]|C|$]]|$1|Q|3|-4|5|6|7|22|8|@]|9|@]|C|$]]|$1|R|3|S|5|T|7|23|8|@]|9|@]|C|$]]|$1|U|3|-4|5|6|7|24|8|@]|9|@]|C|$]]|$1|V|3|W|5|6|7|25|8|@]|9|@]|C|$]]|$1|X|3|Y|5|T|7|26|8|@]|9|@]|C|$]]|$1|Z|3|-4|5|6|7|27|8|@]|9|@]|C|$]]|$1|10|3|11|5|6|7|28|8|@]|9|@]|C|$]]|$1|12|3|13|5|6|7|29|8|@]|9|@]|C|$]]|$1|14|3|-4|5|6|7|2A|8|@]|9|@]|C|$]]]|15|$16|$5|17|18|19|C|$1A|1B]]]]

Another explanation is given in the <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html" rel="noreferrer">@FunctionalInterface page</a>:

<blockquote>
 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 <code>java.lang.Object</code>, that also does not count toward the interface's abstract method count since any implementation of the interface will have an implementation from <code>java.lang.Object</code> or elsewhere.
</blockquote>

You can test which interface is a correct functional interface using <code>@FunctionalInterface</code>.

E.g.:

<ul>
<li>this works

<pre><code>@FunctionalInterface
public interface FunctionalInterf {

 void m();

 boolean equals(Object o);

}
</code></pre></li>
<li>this generates an error:

<pre><code>@FunctionalInterface
public interface FunctionalInterf {

 void m();

 boolean equals();

}
</code></pre>

<blockquote>
 Multiple non-overriding abstract methods found in interface FunctionalInterf
</blockquote></li>
</ul>

blocks|key|199397|text|定义：|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|199398|如果一个接口只包含一个抽象方法，那么这种类型的接口称为函数式接口。|199399|使用：|199400|199401|一旦我们编写了Lambda表达式"->“来调用它的功能，那么在这个上下文中我们需要函数接口。|ordered-list-item|199402|我们可以使用函数接口引用来引用Lambda函数接口，我们可以有一个抽象方法和n个默认/静态方法。|199403|199404|与继承相关的函数接口：|199405|如果接口扩展了函数接口，并且子接口不包含任何抽象方法，那么该子接口也被认为是函数接口。|199406|函数接口对于java来说并不是新事物，它已经在以下接口API中使用过：|199407|call|199408|Runnable:包含()方法only.|unordered-list-item|199409|Callable+:包含call()方法only.|199410|Comparable+:仅包含compareTo()方法。|199411|199412|199413|entityMap^0|0|3|0|0|0|3|0|0|0|0|0|6|5|0|0|0|Z|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|19|8|@$9|1A|A|1B|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1C|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|1D|8|@$9|1E|A|1F|B|C]]|D|@]|E|$]]|$1|J|3|-4|5|6|7|1G|8|@]|D|@]|E|$]]|$1|K|3|L|5|M|7|1H|8|@]|D|@]|E|$]]|$1|N|3|O|5|M|7|1I|8|@]|D|@]|E|$]]|$1|P|3|-4|5|6|7|1J|8|@]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1K|8|@$9|1L|A|1M|B|C]]|D|@]|E|$]]|$1|S|3|T|5|6|7|1N|8|@]|D|@]|E|$]]|$1|U|3|V|5|6|7|1O|8|@$9|1P|A|1Q|B|C]]|D|@]|E|$]]|$1|W|3|X|5|6|7|1R|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|10|7|1S|8|@]|D|@]|E|$]]|$1|11|3|12|5|M|7|1T|8|@]|D|@]|E|$]]|$1|13|3|14|5|M|7|1U|8|@]|D|@]|E|$]]|$1|15|3|-4|5|6|7|1V|8|@]|D|@]|E|$]]|$1|16|3|-4|5|6|7|1W|8|@]|D|@]|E|$]]|$1|17|3|-4|5|6|7|1X|8|@]|D|@]|E|$]]]|18|$]]

Definition:

If an interface contains only one abstract method, then such type of interface is called functional interface.

Usage:

<ol>
<li>Once we write Lambda expression "->" to invoke its functionality ,
then in this context we require Functional Interface.</li>
<li>We can use the Functional Interface reference to refer Lambda
expression.</li>
<li>Inside functional interface we can have one abstract method and n
number of default/static methods.</li>
</ol>

Functional interface with respect to inheritance:

If an interface extends Functional interface and the child interface does not contain any abstract method , then the child interface is also considered to be Functional Interface.

Functional interface is not new to java, its already used in following interface API's:

<ol>
<li>Runnable : contains run() method only.</li>
<li>Callable : contains call() method only.</li>
<li>Comparable : contains compareTo() method only.</li>
</ol>

blocks|key|2829357|text|在Java+8之前，接口只能声明一个或多个方法，也称为抽象方法(没有实现的方法，只有签名)。从Java+8开始，接口还可以实现一个或多个方法(称为接口默认方法)和静态方法以及抽象方法。接口默认方法被标记为Default关键字。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2829358|所以问题是，什么是函数接口？具有单一抽象方法(SAM)的接口称为函数接口。|2829359|这意味着-|2829360|2829361|具有单个抽象方法的接口是函数接口|ordered-list-item|2829362|具有单个抽象方法和零个或多个默认方法以及零个或多个静态方法的接口也是有效的函数接口。|2829363|2829364|有关示例代码https://readtorakesh.com/functional-interface-java8/的更多细节|offset|length|2829365|entityMap|0|LINK|mutability|MUTABLE|url|https://readtorakesh.com/functional-interface-java8/^0|0|0|0|0|0|0|0|6|1G|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|10|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|11|8|@]|9|@]|A|$]]|$1|G|3|H|5|I|7|12|8|@]|9|@]|A|$]]|$1|J|3|K|5|I|7|13|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|14|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|15|8|@]|9|@$O|16|P|17|1|18]]|A|$]]|$1|Q|3|-4|5|6|7|19|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

Before Java 8, an interface could only declare one or more methods also known as Abstract Method (method with no implementation, just the signature). Starting with Java 8 an interface can also have implementation of one or more methods (knows as Interface Default Method) and static methods along with abstract methods. Interface Default Methods are marked default keyword.

So the question is, what is Functional Interface?
An interface with Single Abstract Method (SAM) is called Functional Interface.

Which means - 

<ol>
<li>An interface with Single Abstract Method is a Functional Interface</li>
<li>An interface with Single Abstract Method and zero or more default
methods and zero or more static method is also a valid Functional
Interface.</li>
</ol>

More detail with example code <a href="https://readtorakesh.com/functional-interface-java8/" rel="nofollow noreferrer">https://readtorakesh.com/functional-interface-java8/</a>

blocks|key|201740|text|函数接口只有一个抽象方法，但它可以有多个默认方法和静态方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|201741|由于默认方法不是抽象的，因此您可以随意向函数接口添加任意数量的默认方法。|201742|@FunctionalInterface
public+interface+MyFuctionalInterface+
{
public+void+perform();

default+void+perform1(){
//Method+body
}

default+void+perform2(){
//Method+body
}
}|code-block|syntax|javascript|201743|如果接口声明了一个抽象方法覆盖了java.lang.Object的公共方法之一，这也不会计入该接口的抽象方法计数，因为该接口的任何实现都将具有来自java.lang.Object或其他地方的实现。|CODE|201744|比较器是一个函数接口，尽管它声明了两个抽象方法。因为其中一个抽象方法“equals()”具有与对象类中的公共方法相同的签名。例如，下面的接口是一个有效的功能接口。|201745|@FunctionalInterface
++++public+interface+MyFuctionalInterface+
++++{
++++public+void+perform();
+
++++@Override
++++public+String+toString();++++++++++++++++//Overridden+from+Object+class
+
++++@Override
++++public+boolean+equals(Object+obj);++++++++//Overridden+from+Object+class
++++}|201746|entityMap^0|0|U|0|0|0|G|G|21|G|0|0|3|Z|8|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Y|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|Z|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|10|8|@$9|11|A|12|B|O]|$9|13|A|14|B|O]]|D|@]|E|$]]|$1|P|3|Q|5|6|7|15|8|@$9|16|A|17|B|C]|$9|18|A|19|B|O]]|D|@]|E|$]]|$1|R|3|S|5|J|7|1A|8|@]|D|@]|E|$K|L]]|$1|T|3|-4|5|6|7|1B|8|@]|D|@]|E|$]]]|U|$]]

A functional interface has only one abstract method but it can have multiple default and static methods.
Since default methods are not abstract you’re free to add default methods to your functional interface as many as you like.
<pre><code>@FunctionalInterface
public interface MyFuctionalInterface 
{
public void perform();

default void perform1(){
//Method body
}

default void perform2(){
//Method body
}
}
</code></pre>
If an interface declares an abstract method overriding one of the public methods of <code>java.lang.Object</code>, that also does not count toward the interface’s abstract method count since any implementation of the interface will have an implementation from <code>java.lang.Object</code> or elsewhere.
Comparator is a functional interface even though it declared two abstract methods. Because one of these abstract methods “<code>equals()</code>” which has signature equal to public method in Object class.
e.g. Below interface is a valid functional interface.
<pre><code>@FunctionalInterface
 public interface MyFuctionalInterface 
 {
 public void perform();
 
 @Override
 public String toString(); //Overridden from Object class
 
 @Override
 public boolean equals(Object obj); //Overridden from Object class
 }
</code></pre>

blocks|key|200026|text|接口不能扩展Object类，因为接口必须具有公共方法和抽象方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|200027|对于Object类中的每个公共方法，接口中都有一个隐式的公共和抽象方法。|200028|这是标准的Java语言规范，它是这样规定的，|200029|200030|“如果接口没有直接的超级接口，则该接口隐式声明一个带有签名s、返回类型r和抛出子句t的公共抽象成员方法m，该方法对应于每个公共实例方法m，带有对象中声明的签名s、返回类型r和抛出子句t，除非接口显式声明了具有相同签名、相同返回类型和兼容的throws子句的方法。”|blockquote|200031|200032|200033|这就是Object类的方法在接口中声明的方式。根据JLS，这不能算作接口的抽象方法。因此，比较器接口是一个函数接口。|200034|entityMap^0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]|$1|G|3|H|5|I|7|T|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|U|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|V|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|W|8|@]|9|@]|A|$]]|$1|N|3|-4|5|6|7|X|8|@]|9|@]|A|$]]]|O|$]]

An interface cannot extend Object class, because Interface has to have public and abstract methods.
For every public method in the Object class, there is an implicit public and abstract method in an interface.
This is the standard Java Language Specification which states like this,
<blockquote>
“If an interface has no direct super interfaces, then the interface implicitly declares a public abstract member method m with signature s, return type r, and throws clause t corresponding to each public instance method m with signature s, return type r, and throws clause t declared in Object, unless a method with the same signature, same return type, and a compatible throws clause is explicitly declared by the interface.”
</blockquote>
That's how Object class' methods are declared in an interface. And according to JLS, this does not count as interface' abstract method. Hence, Comparator interface is a functional interface.

Recently I started exploring Java 8 and I can't quite understand the concept of "functional interface" that is essential to Java's implementation of lambda expressions. There is a <a href="http://www.lambdafaq.org/" rel="noreferrer">pretty comprehensive guide</a> to lambda functions in Java, but I got stuck on <a href="http://www.lambdafaq.org/what-is-a-functional-interface/" rel="noreferrer">the chapter that gives definition to the concept of functional interfaces</a>. The definition reads:

<blockquote>
 More precisely, a functional interface is defined as any interface that has exactly one abstract method.
</blockquote>

An then he proceeds to examples, one of which is <code>Comparator</code> interface:

<blockquote>
<pre><code>public interface Comparator&lt;T&gt; {
 int compare(T o1, T o2);
 boolean equals(Object obj);
} 
</code></pre>
</blockquote>

I was able to test that I can use a lambda function in place of Comparator argument and it works(i.e. <code>Collections.sort(list, (a, b) -&gt; a-b)</code>). 

But in the Comparator interface both <code>compare</code> and <code>equals</code> methods are abstract, which means it has two abstract methods. So how can this be working, if the definition requires an interface to have exactly one abstract method? What am I missing here?

Precise definition of "functional interface" in Java 8

Java

最近，我开始探索Java 8，我不太理解“函数接口”的概念，它是Java实现lambda表达式所必需的。在Java中有一个指向lambda函数的，但是我被卡住了。定义是这样的：更准确地说，函数接口被定义为只有一个抽象方法的任何接口。然后他继续举例，其中之一是Comparator接口：公共接口比较器{ int compa...

问Java 8中“函数接口”的精确定义
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