jdk8函数式编程之Predicate

java.uti.function包下，JDK 8 中提供了一组常用的核心函数接口：

接口	参数	返回类型	描述
Predicate	T	boolean	用于判别一个对象。比如求一个人是否为男性
Consumer	T	void	用于接收一个对象进行处理但没有返回，比如接收一个人并打印他的名字
Function	T	R	转换一个对象为不同类型的对象
Supplier	None	T	提供一个对象
UnaryOperator	T	T	接收对象并返回同类型的对象
BinaryOperator	(T, T)	T	接收两个同类型的对象，并返回一个原类型对象

其中Predicate 用于判断对象是否符合某个条件，经常被用来过滤对象

先看一下Predicate源码

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

import java.util.Objects;

/**
 * Represents a predicate (boolean-valued function) of one argument.
 *
 * 
This is a functional interface
 * whose functional method is {@link #test(Object)}.
 *
 * @param  the type of the input to the predicate
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Predicate {

    /**
     * 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 and(Predicate other) {
        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 negate() {
        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 or(Predicate other) {
        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  Predicate isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }
}

Predicate这个类比较简单，只有5个方法

1.boolean test(T t);

这是最核心的一个方法，验证传入的参数是举个匹配这个函数接口，返回值类型为Boolean

举个栗子：

Predicate p = s->s.equals("a");
System.out.println(p.test("a"));

2.default Predicate and(Predicate other)

值得一提的是jdk1.8接口引入了default关键字，接口中也可以有非抽象的方法了，给了接口一个默认的实现方法

这个方法是把传入的函数和当前函数拼接为一个新的函数，必须同时满足两个函数才会返回True

看一个栗子：

        Predicate predicate1 = s->s.contains("a");
        Predicate predicate2 = s->s.contains("b");
        Predicate predicate3 = predicate1.and(predicate2);
        System.out.println(predicate3.test("a"));
        System.out.println(predicate3.test("abc"));

3.negate和or方法就很好理解了，negate是对test方法取非运算，or是或运算，isEqual是一个静态方法，判断参数是否和给定目标相等。