如果我有 ArrayList，如何比较两个 HashMap？

Question

我不能继续我的HashMap作业了。你能帮助我吗？我有 2 HashMap:mtutors和mstudents.mtutors关键：他们的名字。mtutors价值：一个主题。mstudents关键：他们的名字。mstudents值：主题（ArrayList）我必须找到匹配的导师-学生。例如： mtutors 键之一：Peter Parker。mtutors 的价值之一：数学。学生关键之一：比尔·盖茨。学生的价值之一：数学，IT。配对：彼得帕克和比尔盖茨 - 数学。HashMap<String,String> mtutors = new HashMap<>();HashMap<String,ArrayList> mstudents = new HashMap<>();for(int i=0;i<tutors.size();i++)    mtutors.put(tutors.get(i).firstName + " "           + tutors.get(i).lastName, tutors.get(i).subject);for(int i=0;i<students.size();i++)    mstudents.put(students.get(i).firstName + " "           + students.get(i).lastName, students.get(i).students);

Answer 1

如果您使用的是 Java8 或更高版本，请尝试以下操作：

mtutors.forEach((tutorName, subject) -> {

        mstudents.forEach((studentName, subjects) -> {

            if (subjects.contains(subject)) {

                System.out.println(tutorName + ":" + studentName + ":" + subject);

            }

        });

    });

我假设您将科目（不是学生）放到学生地图上。

仅获得第一次拟合的数据流示例。

mtutors.forEach((tutorName, subject) -> {

        mstudents.entrySet().stream()

                .filter(entry -> entry.getValue().contains(subject))

                .findFirst()

                .ifPresent(entry -> System.out.println(tutorName + ":" + entry.getKey() + ":" + subject));

    });

Answer 2

我的回答类似于@ŁukaszKucik 的回答，只是使用 Java 流。您可能不应该将我的解决方案用于您的作业，因为我想这不是您应该解决的方法。不过，只是为了多样化：

List<String> matches = mtutors.entrySet()
                .stream()
                .flatMap(tutor -> mstudents.entrySet()
                        .stream()
                        .filter(student -> student.getValue()
                                .contains(tutor.getValue()))
                        .map(student -> tutor.getKey() + " and " + student.getKey() + " - " + tutor.getValue()))
                .collect(Collectors.toList());

如果缺少的部分是list.contains()：https ://www.tutorialspoint.com/java/util/arraylist_contains.htm

如果您缺少的部分是如何解决这种组合，那么仅仅在这里寻求解决方案并不是学习和家庭作业的工作方式。

Answer 3

两个解决方案示例：

1. forEach Lambda - Java 8 或更高版本

2. 经典for循环

对未来的建议：

一个。不要直接使用类变量——而是根据 Java Bean 规范创建 getter/setter 对。然后使用它们：例如，改为tutors.get(i).lastName使用tutors.get(i).getLastName()

湾。for即使只有一行，在编写循环时也要使用大括号 - 它可以防止循环体出现意外的拼写错误。例如

for (int i = 0; i < students.size(); i++) {

        mstudents.put(students.get(i).getFirstName() + " " + students.get(i).getLastName(), students.get(i).getSubjects());

    }

代替

for (int i = 0; i < students.size(); i++)

        mstudents.put(students.get(i).firstName + " " + students.get(i).lastName, students.get(i).subjects);

======================

package org.tempuri.tests;

import java.util.Arrays;

import java.util.HashMap;

import java.util.LinkedList;

import java.util.List;

import java.util.Map.Entry;

public class SimpleTest {

  public static void main(String[] args) {

    // Just for test - inbound data

    Tutor mathTutor = new Tutor("Peter", "Parker", "MATH");

    Tutor physicsTutor = new Tutor("Tonny", "Stark", "PHYSICS");

    Tutor computersTutor = new Tutor("Steve", "Rogers", "COMPUTERS");

    Student freshmen = new Student("Bill", "Gates", Arrays.asList("MATH", "COMPUTERS"));

    Student sophomore = new Student("Peter", "Quill", Arrays.asList("MATH", "COMPUTERS", "PHYSICS"));

    Student senior = new Student("Peggy", "Carter", Arrays.asList("MATH", "PHYSICS"));

    List<Tutor> tutors = Arrays.asList(mathTutor, physicsTutor, computersTutor);

    List<Student> students = Arrays.asList(freshmen, sophomore, senior);

    // Untouched question code (almost ...)

    HashMap<String, String> mtutors = new HashMap<>();

    HashMap<String, List<String>> mstudents = new HashMap<>();

    for (int i = 0; i < tutors.size(); i++)

        mtutors.put(tutors.get(i).firstName + " " + tutors.get(i).lastName, tutors.get(i).subject);

    for (int i = 0; i < students.size(); i++)

        mstudents.put(students.get(i).firstName + " " + students.get(i).lastName, students.get(i).subjects);

    // variation 1. using forEach Lambda (> Java 8)

    List<MatchingPair> matchingPairs1 = new LinkedList<>();

    mtutors.entrySet().forEach(tutorEntry -> {

        mstudents.entrySet().forEach(studentEntry -> {

            if (studentEntry.getValue().contains(tutorEntry.getValue()))

                matchingPairs1

                        .add(new MatchingPair(tutorEntry.getKey(), studentEntry.getKey(), tutorEntry.getValue()));

        });

    });

    System.out.println("result variation1: ");

    System.out.println("------------------ ");

    matchingPairs1.forEach(onePair -> System.out.println("Tutor: " + onePair.getTutorName() + " -> Student: "

            + onePair.getStudentName() + " -> Subject: " + onePair.getSubject()));

    // variation 2. using classical for loops (< Java 7)

    final List<MatchingPair> matchingPairs2 = new LinkedList<>();

    for (Entry<String, String> oneTutor : mtutors.entrySet()) {

        for (Entry<String, List<String>> oneStudent : mstudents.entrySet()) {

            if (oneStudent.getValue().contains(oneTutor.getValue())) {

                matchingPairs2.add(new MatchingPair(oneTutor.getKey(), oneStudent.getKey(), oneTutor.getValue()));

            }

        }

    }

    System.out.println("------------------ ");

    System.out.println();

    System.out.println("result variation2: ");

    System.out.println("------------------ ");

    for (MatchingPair onePair : matchingPairs2) {

        System.out.println("Tutor: " + onePair.getTutorName() + " -> Student: " + onePair.getStudentName() + " -> Subject: "

                + onePair.getSubject());

    }

    System.out.println("------------------ ");

}

    // supporting classes for test

private static class MatchingPair {

    private String tutorName;

    private String studentName;

    private String subject;

    MatchingPair(String tutorName, String studentName, String subject) {

        this.tutorName = tutorName;

        this.studentName = studentName;

        this.subject = subject;

    }

    public String getTutorName() {

        return tutorName;

    }

    public void setTutorName(String tutorName) {

        this.tutorName = tutorName;

    }

}

private static class Tutor {

    String firstName;

    String lastName;

    String subject;

    Tutor(String firstName, String lastName, String subject) {

        this.firstName = firstName;

        this.lastName = lastName;

        this.subject = subject;

    }

    public String getFirstName() {

        return firstName;

    }

    public void setFirstName(String firstName) {

        this.firstName = firstName;

    }

    public String getLastName() {

        return lastName;

    }

    public void setLastName(String lastName) {

        this.lastName = lastName;

    }

    public String getSubject() {

        return subject;

    }

    public void setSubject(String subject) {

        this.subject = subject;

    }

}

private static class Student {

    String firstName;

    String lastName;

    List<String> subjects;

    Student(String firstName, String lastName, List<String> subjects) {

        this.firstName = firstName;

        this.lastName = lastName;

        this.subjects = subjects;

    }

    public String getFirstName() {

        return firstName;

    }

    public void setFirstName(String firstName) {

        this.firstName = firstName;

    }

    public String getLastName() {

        return lastName;

    }

    public void setLastName(String lastName) {

        this.lastName = lastName;

    }

    public List<String> getSubjects() {

        return subjects;

    }

    public void setSubjects(List<String> subjects) {

        this.subjects = subjects;

    }

}

}