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Java 8 新特性:4-断言(Predicate)接口
(原)
这个接口主要用于判断,先看看它的实现,说明,再给个例子。
/* * 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; /** * Represents a predicate (boolean-valued function) of one argument. * 根据一个参数代表了一个基于boolean类型的断言 * <p>This is a <a href="http://www.mamicode.com/package-summary.html">functional interface</a> * whose functional method is {@link #test(Object)}. *这是一个函数式接口,它的函数方法是test * @param <T> the type of the input to the predicate *根据输入类型得到一个断言 * @since 1.8 */ @FunctionalInterface public interface Predicate<T> { /** * Evaluates this predicate on the given argument. *根据给定的参数获得判断的结果 * @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. * 通过这个predicate和它的参数predicate 返回一个逻辑与的判断结果, *当去计算这个复合的predicate时,如果当前的predicate 结果是false,那么就不会计算它的参数other的值。 * <p>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. *如果这二个其中任何一个抛出异常,具体的处理交给调用的人,如果抛出了异常,它将不会被执行。 * @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) { Objects.requireNonNull(other); return (t) -> test(t) && other.test(t); } /** * Returns a predicate that represents the logical negation of this * predicate. * 返回一个predicate 代表了这个predicate的逻辑非 * @return a predicate that represents the logical negation of this * predicate */ default Predicate<T> 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. *通过这个predicate和它的参数predicate 返回一个逻辑或的判断结果, 当计算这个组合的predicate,如果这个predicate是true ,那么它的参数other将不会计算 * <p>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. *如果这二个其中任何一个抛出异常,具体的处理交给调用的人,如果抛出了异常,它将不会被执行。 * @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) { 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)}. *如果二个参数机等的话,根据Objects#equals(Object, Object)返回一个断言的结果 * @param <T> 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) { return (null == targetRef) ? Objects::isNull : object -> targetRef.equals(object); } }
这里其实慢慢看它的doc文档,还真没有直接看它的实现来的快。无非就是一个判断的函数式接口,主要做逻辑与或非的判断,其中还有一个静态方法,其实现是这样的:
return (null == targetRef) ? Objects::isNull : object -> targetRef.equals(object);
null == targetRef这个就不说了,因为它的返回结果是predicate,所以Objects::isNull必需是predicate的实例,它代表了一个方法的引用,为什么它符合这个函数式接口的唯一抽象方法boolean test(T t);这个呢?我们进去看下它的实现。
public static boolean isNull(Object obj) { return obj == null; }
这是一个静态的方法引用,接收一个Object类型的参数,返回一个boolean类型,这完全附合这个函数式接口的boolean test(T t);抽象方法,那么编译器就会认为它是predicate这个函数式接口的一个实现。
下面给出一个例子,看下怎么使用的,结果我就不分析了。
package com.demo.jdk8; import java.util.Arrays; import java.util.List; import java.util.function.Predicate; public class Test4 { public static void main(String[] args) { Predicate<String> p = s -> s.length() > 3; System.out.println(p.test("hello")); List<Integer> list = Arrays.asList(1,2,3,4,5,6,7,8); System.out.println("part1------------------"); findOdd(list); System.out.println("part2------------------"); conditionFilter(list, ppp -> ppp % 2 == 1); System.out.println("part3------------------"); and(list, p1 -> p1 > 3, p2 -> p2 < 7); System.out.println("part4------------------"); or(list, p1 -> p1 > 3, p2 -> p2 % 2 == 1); System.out.println("part5------------------"); negate(list, p1 -> p1 > 3); System.out.println("part6------------------"); System.out.println(isEqual("abc").test("abcd")); } //找到集合中的奇数 public static void findOdd(List<Integer> list){ for (int i = 0; i < list.size(); i++) { if(list.get(i) % 2 == 1){ System.out.println(list.get(i)); } } } public static void conditionFilter(List<Integer> list,Predicate<Integer> p){ for (int i = 0; i < list.size(); i++) { if(p.test(list.get(i))){ System.out.println(list.get(i)); } } } public static void and(List<Integer> list,Predicate<Integer> p1,Predicate<Integer> p2){ for (int i = 0; i < list.size(); i++) { if(p1.and(p2).test(list.get(i))){ System.out.println(list.get(i)); } } } public static void or(List<Integer> list,Predicate<Integer> p1,Predicate<Integer> p2){ for (int i = 0; i < list.size(); i++) { if(p1.or(p2).test(list.get(i))){ System.out.println(list.get(i)); } } } public static void negate(List<Integer> list,Predicate<Integer> p1){ for (int i = 0; i < list.size(); i++) { if(p1.negate().test(list.get(i))){ System.out.println(list.get(i)); } } } public static Predicate isEqual(Object obj){ return Predicate.isEqual(obj); } }
Java 8 新特性:4-断言(Predicate)接口
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