函数接口 Predicate

Metadata

title: 函数接口 Predicate
date: 2023-02-21 10:47
tags:
  - 行动阶段/完成
  - 主题场景/程序
  - 笔记空间/KnowladgeSpace/ProgramSpace/BasicsSpace
  - 细化主题/Java/函数式编程
categories:
  - Java
keywords:
  - Java
description: 有时候我们需要对某种类型的数据进行判断，从而得到一个boolean值结果。这时可以使用 java.util.function.Predicate  接口。

函数接口 Predicate

有时候我们需要对某种类型的数据进行判断，从而得到一个boolean值结果。这时可以使用java.util.function.Predicate<T> 接口。接口定义如下：

package java.util.function;

import java.util.Objects;

/**
 * Represents a predicate (boolean-valued function) of one argument.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #test(Object)}.
 *
 * @param <T> the type of the input to the predicate
 *
 * @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.
     *
     * <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.
     *
     * @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.
     *
     * @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.
     *
     * <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.
     *
     * @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)}.
     *
     * @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);
    }
}

抽象方法:test

Predicate 接口中包含一个抽象方法: boolean test(T t) 。用于条件判断的场景:

public class Demo {
    private static void method(Predicate<String> predicate) {
        boolean veryLong = predicate.test("HelloWorld");
        System.out.println("字符串很长吗:" + veryLong);
    }

    public static void main(String[] args) {
        method(s -> s.length() > 5);
    }
}

条件判断的标准是传入的Lambda表达式逻辑，只要字符串长度大于5则认为很长。

默认方法:and

既然是条件判断，就会存在与、或、非三种常见的逻辑关系。其中将两个 Predicate 条件使用“与”逻辑连接起来实 现“并且”的效果时，可以使用default方法 and 。

如果要判断一个字符串既要包含大写“H”，又要包含大写“W”，那么:

public class Demo {
    private static void method(Predicate<String> one, Predicate<String> two) {
        boolean isValid = one.and(two).test("Helloworld");
        System.out.println("字符串符合要求吗:" + isValid);
    }

    public static void main(String[] args) {
        method(s -> s.contains("H"), s -> s.contains("W"));
    }
}

默认方法:or

与 and 的“与”类似，默认方法 or 实现逻辑关系中的“或”。

如果希望实现逻辑“字符串包含大写H或者包含大写W”，那么代码只需要将“and”修改为“or”名称即可，其他都不变:

public class Demo {
    private static void method(Predicate<String> one, Predicate<String> two) {
        boolean isValid = one.or(two).test("Helloworld");
        System.out.println("字符串符合要求吗:" + isValid);
    }

    public static void main(String[] args) {
        method(s -> s.contains("H"), s -> s.contains("W"));
    }
}

默认方法:negate

“与”、“或”已经了解了，剩下的“非”(取反)也会简单。

从上面的实现中很容易看出，它是执行了test方法之后，对结果boolean值进行“!”取反而已。一定要在 test 方法调用之前调用 negate 方法，正如 and 和 or 方法一样:

public class Demo {
    private static void method(Predicate<String> predicate) {
        boolean veryLong = predicate.negate().test("HelloWorld");
        System.out.println("字符串很长吗:" + veryLong);
    }

    public static void main(String[] args) {
        method(s -> s.length() < 5);
    }
}

作用

集合信息筛选测试

数组当中有多条“姓名+性别”的信息如下，通过 Predicate 接口的拼装将符合要求的字符串筛选到集合 ArrayList 中，需要同时满足两个条件:

• 性别必须为女
• 姓名为3个字

public class Demo {
    public static void main(String[] args) {
        String[] array = {"张三,女", "李四五,女", "李四六,男", "张二三,女"};
        List<String> list = filter(array,
                s -> "女".equals(s.split(",")[1]),
                s -> s.split(",")[0].length() == 3);
        System.out.println(list);
    }

    private static List<String> filter(String[] array, Predicate<String> one,
                                       Predicate<String> two) {
        List<String> list = new ArrayList<>();
        for (String info : array) {
            if (one.and(two).test(info)) {
                list.add(info);
            }
        }
        return list;
    }
}