Java 获取 Arraylist 中某些元素的平均值

Question

我是 Java 新手，我正在编写一个程序来管理学生、他们的科目和该科目的成绩。

代码询问学生的详细信息，然后将所有信息保存到 Arraylist。我想求一门学科的平均成绩，但没有成功，我已经计算了某个学生的平均成绩，但是得到某个学科的所有成绩，然后计算平均成绩超出了我的理解范围。

这是我用于创建 Arraylist 的代码：

public class options {

public String name;

public ArrayList<students> studentlist = new ArrayList<students>(10);  

    public String createstudent(String name, String subject, Float rade1, Float grade2, Float grade3) {
        studentlist.add(new students(name, subject, grade1, grade2, grade3);
        return "The student has been added to the database. ";
    }
}

这是学生类的代码：

public class students{
    
    public String name;
    public String subject;
    public Float grade1;  
    public Float grade2;  
    public Float grade3;
    
   public students(String name, String subject, Float grade1, Float grade2, Float grade3) {
        
        this.name= name;
        this.subject= subject;
        this.grade1= grade1;
        this.grade2= grade2;
        this.grade3= grade3;
    }
}   

我尝试过类似的东西

public double getAverageValue(){
  double averageValue = 0;
  double sum = 0;

  if(studentlist.size() > 0){
    for ( int i=0; i < studentlist.size() ; i++) {
      sum += studentlist.get(i).getname().getgrade1().getgrade2().getgrade3();
    }
    averageValue = (sum / (double)studentlist.size())
  }

  return averageValue;
}

但是我不知道如何只选择某个学科的成绩，而不是所有学生的所有成绩，所以我有点卡住了。

编辑：

根据这里的答案和我已经完成的一些工作，这是最终代码：

public String showAverageGrade(String clase){
        int countstudents =0;
        double studentaverage =0;

        for(int i=0 ; i < studentlist.size();i++){
            if(studentlist.get(i).asksubject().equals(subject)){
                studentaverage +=studentlist.get(i).studentaverage ();
                countstudents++;

            }
        }
        return "The average grade of " + subject + " is " + (studentaverage / countstudents) + ".";

    }  

英语不是我的母语，所以任何错误都是因为我翻译了它。

感谢大家的反馈。

Answer 1

构建这个的正确方法是首先，您需要将 Student 抽象为一个类。 简单的方法是在主类中创建一个数组属性students，在Student类中它会有一个名为subjects的属性Map来放置一个键（Subject name）和值（Grade）。

如果我理解正确，您需要将一些学生放在主数组中，即他们的成绩，然后计算每个科目所有这些学生的平均值，要计算这一点，您需要创建另一个地图并通过键（主题）总和进行交互所有出现，然后除以计数。

以简单的方式你已经知道所有的主题类型，如果你不知道，它会更复杂，在计算之前你需要另一个没有重复类型的列表来过滤和交互，也许创建这个没有重复的新列表的方法是在这里解释：https://www.baeldung.com/java-remove-duplicates-from-list。

Answer 2

你的方式

使用您的代码，您按职位划分的三个等级中的每一个都可能代表三个不同的课程。我假设您将“法语”课程的所有成绩放在grade1 字段中，将“作文”课程的成绩放在grade2 字段中，将“数学”课程的成绩放在grade3 中。

所以循环学生，分别拉出每个成绩。

float frenchPoints = 0 ;
float compositionPoints = 0 ;
float mathPoints = 0 ;
for( Student student : students )
{
    frenchPoints = ( frenchPoints + student.getGrade1() ) ;
    compositionPoints = ( compositionPoints + student.getGrade2() ) ;
    mathPoints = ( mathPoints + student.getGrade3() ) ;
}

用分数除以分数来取平均值。

如果您想要float 结果，请不要将float（小数）除以int（整数）。因此，在进行除法之前，我们必须将 int 转换为 float。

float frenchAverage = ( frenchPoints / (float) students.size() ) ;
…

我的方式

正如Answer by Bruno Souza Picinini 中所建议的那样，在更现实的情况下，您可以将数据分解为单独的类。您可能会在一个类中表示Student，而该类又可能包含CourseGrade 类的对象集合。

在 Java 16 中，我们可以使用新的 records 特性来简单地编写一个类，其主要目的是透明且不可变地传递数据。编译器隐式创建构造函数、getter、equals & hashCode 和 toString。

public record CourseGrade(String courseName , YearMonth term , Float grade) {}

public record Student(UUID id , String name , Set < CourseGrade > courseGrades) {}

让我们获取一些示例数据。

private List < Student > fetchStudents ( )
{
    List < Student > students =
            List.of(
                    new Student( UUID.fromString( "57ce6790-5094-480c-88f5-79ee9e59b9bb" ) , "Alice" ,
                            Set.of(
                                    new CourseGrade( "French" , YearMonth.of( 2021 , Month.JANUARY ) , 3.2F ) ,
                                    new CourseGrade( "French" , YearMonth.of( 2021 , Month.APRIL ) , 3.6F ) ,
                                    new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.JANUARY ) , 3.3F ) ,
                                    new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.APRIL ) , 3.7F )
                            )
                    ) ,
                    new Student( UUID.fromString( "3ed40f58-fb82-44f0-9ae8-7cc1b317085e" ) , "Bob" ,
                            Set.of(
                                    new CourseGrade( "French" , YearMonth.of( 2021 , Month.JANUARY ) , 2.7F ) ,
                                    new CourseGrade( "French" , YearMonth.of( 2021 , Month.APRIL ) , 2.6F ) ,
                                    new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.JANUARY ) , 2.9F ) ,
                                    new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.APRIL ) , 2.7F )
                            )
                    ) ,
                    new Student( UUID.fromString( "9058567e-8a5e-4606-81b0-1def2bb8ceb5" ) , "Carol" ,
                            Set.of(
                                    new CourseGrade( "French" , YearMonth.of( 2021 , Month.JANUARY ) , 3.0F ) ,
                                    new CourseGrade( "French" , YearMonth.of( 2021 , Month.APRIL ) , 3.1F ) ,
                                    new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.JANUARY ) , 3.2F ) ,
                                    new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.APRIL ) , 3.4F )
                            )
                    )
            );
    return students;
}

编写一个方法来报告特定学期特定课程名称的平均成绩。

我们报告Optional，因为我们可能会在没有提供此类课程的学期中要求课程。因此，如果没有上课，则没有学生获得任何成绩。因此，我们不想报告一个数字，而是要报告一个空的Optional 对象。如果该学期提供的课程有学生注册，那么我们返回一个 Optional，其中包含一个 Float 对象，用于表示平均成绩。

对了，注意浮点类型如float/Floattrade away inaccuracy for speed的执行。如果您关心准确性，请改用BigDecimal 类。

private Optional < Float > reportAverageGradePerCourseInTerm ( String courseName , YearMonth term )
{
    Objects.requireNonNull( courseName );
    Objects.requireNonNull( term );
    List < CourseGrade > courseGrades = new ArrayList <>( this.students.size() );
    for ( Student student : this.students )
    {
        for ( CourseGrade courseGrade : student.courseGrades() )
        {
            if ( courseGrade.courseName().equals( courseName ) && courseGrade.term().equals( term ) )
            {
                courseGrades.add( courseGrade ); // If a match on name and term, remember this `CourseGrade` object. Else, forget/skip.
            }
        }
    }
    if ( courseGrades.size() == 0 ) { return Optional.empty(); }
    float totalPoints = 0F;
    for ( CourseGrade courseGrade : courseGrades )
    {
        totalPoints = totalPoints + courseGrade.grade();
    }
    Float average = ( totalPoints / ( float ) courseGrades.size() );
    return Optional.of( average );
}

使用main 方法将所有代码拉入类App 以驱动演示。

package work.basil.example.reportcard;

import java.time.Month;
import java.time.YearMonth;
import java.util.*;

public class App
{
    public static void main ( String[] args )
    {
        App app = new App();
        app.students = app.fetchStudents();
        System.out.println( "app.students = " + app.students );
        Optional < Float > averageForFrenchIn2021_01 = app.reportAverageGradePerCourseInTerm( "French" , YearMonth.of( 2021 , Month.JANUARY ) );
        System.out.println( "averageForFrenchIn2021_01 = " + averageForFrenchIn2021_01 );
    }

    // Member fields
    List < Student > students;

    // Logic

    private List < Student > fetchStudents ( )
    {
        List < Student > students =
                List.of(
                        new Student( UUID.fromString( "57ce6790-5094-480c-88f5-79ee9e59b9bb" ) , "Alice" ,
                                Set.of(
                                        new CourseGrade( "French" , YearMonth.of( 2021 , Month.JANUARY ) , 3.2F ) ,
                                        new CourseGrade( "French" , YearMonth.of( 2021 , Month.APRIL ) , 3.6F ) ,
                                        new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.JANUARY ) , 3.3F ) ,
                                        new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.APRIL ) , 3.7F )
                                )
                        ) ,
                        new Student( UUID.fromString( "57ce6790-5094-480c-88f5-79ee9e59b9bb" ) , "Bob" ,
                                Set.of(
                                        new CourseGrade( "French" , YearMonth.of( 2021 , Month.JANUARY ) , 2.7F ) ,
                                        new CourseGrade( "French" , YearMonth.of( 2021 , Month.APRIL ) , 2.6F ) ,
                                        new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.JANUARY ) , 2.9F ) ,
                                        new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.APRIL ) , 2.7F )
                                )
                        ) ,
                        new Student( UUID.fromString( "57ce6790-5094-480c-88f5-79ee9e59b9bb" ) , "Carol" ,
                                Set.of(
                                        new CourseGrade( "French" , YearMonth.of( 2021 , Month.JANUARY ) , 3.0F ) ,
                                        new CourseGrade( "French" , YearMonth.of( 2021 , Month.APRIL ) , 3.1F ) ,
                                        new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.JANUARY ) , 3.2F ) ,
                                        new CourseGrade( "Composition" , YearMonth.of( 2021 , Month.APRIL ) , 3.4F )
                                )
                        )
                );
        return students;
    }

    private Optional < Float > reportAverageGradePerCourseInTerm ( String courseName , YearMonth term )
    {
        Objects.requireNonNull( courseName );
        Objects.requireNonNull( term );
        List < CourseGrade > courseGrades = new ArrayList <>( this.students.size() );
        for ( Student student : this.students )
        {
            for ( CourseGrade courseGrade : student.courseGrades() )
            {
                if ( courseGrade.courseName().equals( courseName ) && courseGrade.term().equals( term ) )
                {
                    courseGrades.add( courseGrade ); // If a match on name and term, remember this `CourseGrade` object. Else, forget/skip.
                }
            }
        }
        if ( courseGrades.size() == 0 ) { return Optional.empty(); }
        float totalPoints = 0F;
        for ( CourseGrade courseGrade : courseGrades )
        {
            totalPoints = totalPoints + courseGrade.grade();
        }
        Float average = ( totalPoints / ( float ) courseGrades.size() );
        return Optional.of( average );
    }
}

当main 运行时。

app.students = [Student[id=57ce6790-5094-480c-88f5-79ee9e59b9bb, name=Alice, courseGrades=[CourseGrade[courseName=French, term=2021-04, grade=3.6], CourseGrade[courseName=French, term=2021-01, grade=3.2], CourseGrade[courseName=Composition, term=2021-01, grade=3.3], CourseGrade[courseName=Composition, term=2021-04, grade=3.7]]], Student[id=57ce6790-5094-480c-88f5-79ee9e59b9bb, name=Bob, courseGrades=[CourseGrade[courseName=French, term=2021-04, grade=2.6], CourseGrade[courseName=French, term=2021-01, grade=2.7], CourseGrade[courseName=Composition, term=2021-01, grade=2.9], CourseGrade[courseName=Composition, term=2021-04, grade=2.7]]], Student[id=57ce6790-5094-480c-88f5-79ee9e59b9bb, name=Carol, courseGrades=[CourseGrade[courseName=French, term=2021-04, grade=3.1], CourseGrade[courseName=Composition, term=2021-01, grade=3.2], CourseGrade[courseName=Composition, term=2021-04, grade=3.4], CourseGrade[courseName=French, term=2021-01, grade=3.0]]]]
averageForFrenchIn2021_01 = Optional[2.9666665]

看起来是对的，( 3.2 , 2.7 , 3.0 ) 的平均值是 2.96...。

Answer 3

这些解决方案使您能够为每个学生在每个科目中输入成绩，然后输出所有科目评分的平均值。

将每个类作为自己的文件添加到您的文件夹中。

我们用 SchoolSubject.java 表示一个主题：

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

//abstract class. is init from sub classes
public abstract class SchoolSubjects {

//grades in a subject
protected List<Grade> studentGrades;
Scanner myInput = new Scanner(System.in);

abstract double getAverageValue();

abstract void gradeStudents(List<Student> students) throws IOException;

public SchoolSubjects() {
    this.studentGrades = new ArrayList<>();
}
}

然后我们为每个主题创建一个类：

import java.io.IOException;
import java.util.List;

public class EnglishSubject extends SchoolSubjects {

//constructor
public EnglishSubject() {
    super();
}

//Overrides method in super class
@Override
double getAverageValue() {
    double sum = 0;
    if (!studentGrades.isEmpty()) {
        for (Grade grade : studentGrades) {

            sum += grade.getGrade();
        }
        return sum / studentGrades.size();
    }
    return sum;
}


//Overrides method in super class
//parameter: List of students to be graded.
//Takes console input for grade.
//generates random name to represent student
@Override
void gradeStudents(List<Student> students) throws IOException {
    System.out.println("Grade for english subject");
    for (Student student : students) {
        System.out.print("Student:" + " " + RandomNames.randomIdentifier() + " " + 
"Enter grade: ");
        double grade = myInput.nextDouble();
        studentGrades.add(new Grade(grade, student));
    }
}
}

另一个使用相同方法的主题。请记住，由于继承，每个主题中的方法都可以更改：

import java.io.IOException;
import java.util.List;

public class MathSubject extends SchoolSubjects {

//constructor
public MathSubject() {
    super();
}

//Overrides method in super class
@Override
double getAverageValue() {
    double sum = 0;
    if (!studentGrades.isEmpty()) {
        for (Grade grade : studentGrades) {

            sum += grade.getGrade();
        }
        return sum / studentGrades.size();
    }
    return sum;
}

//Overrides method in super class
//parameter: List of students to be graded.
//Takes console input for grade.
//generates random name to represent student
@Override
void gradeStudents(List<Student> students) throws IOException {
    System.out.println("Grade for math subject");
    for (Student student : students) {
        System.out.print("Student:" + " " + RandomNames.randomIdentifier() + " " + 
"Enter grade: ");
        double grade = myInput.nextDouble();
        studentGrades.add(new Grade(grade, student));
    }
}
}

用于学科课程的年级。

public class Grade {
//has a grade, and a student object
private double grade;
public Student student;

public double getGrade() {
    return grade;
}

public Grade(double grade, Student student) {
    this.grade = grade;
    this.student = student;
}
}

学生班：

public class Student {

}

您的应用程序。你运行这个文件。

public class App {
public static void main(String[] args) throws Exception {
 
    //School
    List<Student> studentList = new ArrayList<>();
    studentList.add(new Student());
    studentList.add(new Student());
    studentList.add(new Student());


    SchoolSubjects mathSubject = new MathSubject();
    SchoolSubjects englishSubject = new EnglishSubject();

    mathSubject.gradeStudents(studentList);
    englishSubject.gradeStudents(studentList);

    System.out.println("Average grade math:" + " " + mathSubject.getAverageValue());
    System.out.println("Average grade english:" + " " + 
englishSubject.getAverageValue());
}
}

生成随机名称的类：

import java.util.HashSet;
import java.util.Set;

public class RandomNames {
// class variable
final static String lexicon = "ABCDEFGHIJKLMNOPQRSTUVWXYZ12345674890";

//final java.util.Random rand = new java.util.Random();

// consider using a Map<String,Boolean> to say whether the identifier is being used 
or not
final static Set<String> identifiers = new HashSet<String>();

public static String randomIdentifier() {
    StringBuilder builder = new StringBuilder();
    while (builder.toString().length() == 0) {
        int length = new java.util.Random().nextInt(5) + 5;
        for (int i = 0; i < length; i++) {
            builder.append(lexicon.charAt(new 
java.util.Random().nextInt(lexicon.length())));
        }
        if (identifiers.contains(builder.toString())) {
            builder = new StringBuilder();
        }
    }
    return builder.toString();
}
}