Java Predicate

Predicate 接口说明

/*
 * 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.
 *
 * <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);
    }
}

根据接口说明，Predicate 提供的为逻辑判断操作，即断言。

静态方法isEqual：判断是否相等，并返回一个Predicate对象

调用：Predicate.isEqual(Object1).test(Object2)

含义：使用Object1的equals方法判断Object2是否与其相等。

默认方法and，or，negate：分别代表逻辑判断与、或、非并都返回一个Predicate对象

调用：Predicate1.and(Predicate2).test(Object)

含义：判断Object对象是否满足Predicate1 && Predicate2

方法test：按照给定的Predicate条件进行逻辑判断。

package org.htsg;

import java.util.ArrayList;
import java.util.List;
import java.util.Objects;
import java.util.function.Predicate;

/**
 * @author HTSG
 */
public class PredicateTest {
    public static void main(String[] args) {
        // 添加十个学生
        List<Student> studentList = new ArrayList<>(10);
        for (int i = 0; i < 10; i++) {
            studentList.add(new Student("student" + i, 10 + i));
        }
        // 获取年龄大于15的学生
        // [Student{name='student6', age=16}, Student{name='student7', age=17}, Student{name='student8', age=18}, Student{name='student9', age=19}]
        List<Student> filteredStudents = test(studentList, PredicateTest::filterAge1);
        System.out.println(filteredStudents);
        // 获取年龄大于15并且名字叫 "student7" 的学生
        // [Student{name='student7', age=17}]
        filteredStudents = and(studentList, PredicateTest::filterAge1, PredicateTest::filterName);
        System.out.println(filteredStudents);
        // 获取年龄不大于15的学生
        // [Student{name='student0', age=10}, Student{name='student1', age=11}, Student{name='student2', age=12}, Student{name='student3', age=13}, Student{name='student4', age=14}, Student{name='student5', age=15}]
        filteredStudents = negate(studentList, PredicateTest::filterAge1);
        System.out.println(filteredStudents);
        // 获取年龄不大于15或名字叫 "student7" 的学生
        // [Student{name='student0', age=10}, Student{name='student1', age=11}, Student{name='student2', age=12}, Student{name='student3', age=13}, Student{name='student4', age=14}, Student{name='student5', age=15}, Student{name='student7', age=17}]
        filteredStudents = or(studentList, PredicateTest::filterAge2, PredicateTest::filterName);
        System.out.println(filteredStudents);
        // 获取和目标学生属性值相同的学生列表
        // [Student{name='student1', age=11}]
        filteredStudents = isEqual(studentList, new Student("student1", 11));
        System.out.println(filteredStudents);

    }


    public static boolean filterAge1(Student student) {
        return student.getAge() > 15;
    }

    public static boolean filterAge2(Student student) {
        return student.getAge() <= 15;
    }

    public static boolean filterName(Student student) {
        return "student7".equals(student.getName());
    }

    public static List<Student> test(List<Student> students, Predicate<Student> pre) {
        List<Student> result = new ArrayList<>(10);
        for (Student student : students) {
            if (pre.test(student)) {
                result.add(student);
            }
        }
        return result;
    }

    public static List<Student> and(List<Student> students, Predicate<Student> pre1, Predicate<Student> pre2) {
        List<Student> result = new ArrayList<>(10);
        for (Student student : students) {
            if (pre1.and(pre2).test(student)) {
                result.add(student);
            }
        }
        return result;
    }

    public static List<Student> negate(List<Student> students, Predicate<Student> pre) {
        List<Student> result = new ArrayList<>(10);
        for (Student student : students) {
            if (pre.negate().test(student)) {
                result.add(student);
            }
        }
        return result;
    }

    public static List<Student> or(List<Student> students, Predicate<Student> pre1, Predicate<Student> pre2) {
        List<Student> result = new ArrayList<>(10);
        for (Student student : students) {
            if (pre1.or(pre2).test(student)) {
                result.add(student);
            }
        }
        return result;
    }

    public static List<Student> isEqual(List<Student> students, Student student) {
        List<Student> result = new ArrayList<>(10);
        for (Student studentTemp : students) {
            if (Predicate.isEqual(student).test(studentTemp)) {
                result.add(studentTemp);
            }
        }
        return result;
    }

    // 创建静态内部类
    public static class Student {
        private String name;
        private int age;

        public Student() {
        }

        public Student(String name, int age) {
            this.name = name;
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }

        @Override
        public String toString() {
            return "Student{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()) {
                return false;
            }
            Student student = (Student) o;
            return age == student.age &&
                    Objects.equals(name, student.name);
        }
    }
}