功能接口FunctionalInterface中抽象方法的理解

一直记得:如果接口中只有一个抽象方法,那么它就是一个功能接口FunctionalInterface。

最近看到一个功能接口:

@FunctionalInterface
public interface Comparator<T> {
    int compare(T o1, T o2);   
    boolean equals(Object obj);
......


发现定义了两个方法,但仍然是一个FunctionalInterface。


具体解释:

package java.lang;

import java.lang.annotation.*;

/**
 * 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 {}


两个要点:

1.明确只有一个抽象类

2.有具体实现的不计算在内。所有类都有Object的实现,所有上面的equals方法不会计算在内。


  

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