java8之默认接口实现

简介

定义：申明方法的同时提供实现

1. 允许在接口内声明静态方法
2. 指定接口方法的默认实现

Predicate源码

/*
 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
package java.util.function;

import java.util.Objects;

/**
 1. Represents a predicate (boolean-valued function) of one argument.
 2.  3. 
This is a functional interface
 4. whose functional method is {@link #test(Object)}.
 5.  6. @param  the type of the input to the predicate
 7.  8. @since 1.8
 */
@FunctionalInterface
public interface Predicate<T> {

    /**
     * Evaluates this predicate on the given argument.
     *
     * @param t the input argument
     * @return {@code true} if the input argument matches the predicate,
     * otherwise {@code false}
     */
    boolean test(T t);

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * AND of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code false}, then the {@code other}
     * predicate is not evaluated.
     *
     * 
Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ANDed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * AND of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> and(Predicate<? super T> other) { //接口方法的默认实现
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }

    /**
     * Returns a predicate that represents the logical negation of this
     * predicate.
     *
     * @return a predicate that represents the logical negation of this
     * predicate
     */
    default Predicate<T> negate() { //接口方法的默认实现
        return (t) -> !test(t);
    }

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * OR of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code true}, then the {@code other}
     * predicate is not evaluated.
     *
     * 
Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ORed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * OR of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> or(Predicate<? super T> other) { //接口方法的默认实现
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }

    /**
     * Returns a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}.
     *
     * @param  the type of arguments to the predicate
     * @param targetRef the object reference with which to compare for equality,
     *               which may be {@code null}
     * @return a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}
     */
    static <T> Predicate<T> isEqual(Object targetRef) { //静态方法
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }
}

代码演示

public interface A {
	//默认接口实现
	default void hello(){
		System.out.println("Hello World");
	}
	//静态方法
	static void say(){
		System.out.println("Hello World");
	}
}

冲突解决

1. 类中的方法优先级最高(类或父类中声明的方法优先于任何声明为默认方法的优先级)
2. 如果无法依据第一条，那么子接口优先级更高：函数签名相同时，优先选择拥有最具体实现的默认方法的接口，如果B继承了A，那么B就比A更加具体
3. 继承了多个接口的类必须通过显示覆盖和调用期望的方法

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