Void-type not allowed here 错误

Question

我正在尝试将从文件中读取的这些数据添加到我的地图中。我的地图是一个树形图 TreeMap<String, Student>，其中 Student 在另一个 class 中。我正在尝试使用代码 map.put(formatSNumber, student.setCourses(courses)); 将读取的文件元素添加到我的地图中，但我一直遇到 void type not allowed here 错误。

sNumber = Integer.parseInt(Breader.readLine());
formatSNumber = String.format("%03d", sNumber);
hours = Integer.parseInt(Breader.readLine());
grade = Double.parseDouble(Breader.readLine());
Student student = map.get(formatSNumber);
Course course = new Course(hours, grade);
List<Course> courses = student.getCourses();
courses.add(course);
map.put(formatSNumber, student.setCourses(courses));
end = Breader.ready();

这是我的完整代码：

import java.io.*;
import java.util.Scanner;
import java.util.TreeMap;
import java.util.Iterator;
import java.util.List;
public class FinalProgram {
    public static void main(String[] args) throws IOException {
        String nameFile = " ";
        String classFile = " ";
        TreeMap<String, Student> map = new TreeMap<>();
        Scanner input = new Scanner(System.in);

        try {
            System.out.print("Enter the Name file(c:filename.txt): ");
            nameFile = input.nextLine();
        } catch(IllegalArgumentException e) {
            System.out.printf("Invalid input. Please enter"
                    + " filename in the form of "
                    + "c:filename.txt\n", e.getMessage());
        }
        nameReader(nameFile, map);

        try {
            System.out.print("Enter the Class file(c:filename.txt): ");
            classFile = input.nextLine();
        } catch(IllegalArgumentException e) {
            System.out.printf("Invalid input. Please enter"
                    + " filename in the form of "
                    + "c:filename.txt\n", e.getMessage());
        }
        classReader(classFile, map);
    }

    private static void nameReader(String file, TreeMap<String, Student> map)
            throws IOException {

        String nameFile = file;
        int sNumber = 0;
        String formatSNumber = " ";
        String sName = " ";

        //Instantiate FileReader and BufferedReader
        FileReader freader = new FileReader(nameFile);
        BufferedReader Breader = new BufferedReader(freader);
        boolean end = Breader.ready();

        do {
            sNumber = Integer.parseInt(Breader.readLine());
            formatSNumber = String.format("%03d", sNumber);
            sName = Breader.readLine();
            Student student = new Student(sName);
            map.put(formatSNumber, student);
            end = Breader.ready();
        } while(end);
        Iterator<String> keySetIterator = map.keySet().iterator();
        while(keySetIterator.hasNext()) {
            String key = keySetIterator.next();
            System.out.println("key: " + key + " value: " + map.get(key).getName());
        }
    }
    private static void classReader(String file, TreeMap<String, Student> map) 
            throws IOException {
        String classFile = file;
        int sNumber = 0;
        String formatSNumber = " ";
        int hours = 0;
        double grade = 0.0;
        double points = grade * hours;
        double GPA = points / hours;

        //Instantiate FileReader and BufferedReader
        FileReader freader = new FileReader(classFile);
        BufferedReader Breader = new BufferedReader(freader);
        boolean end = Breader.ready();

        do {
            sNumber = Integer.parseInt(Breader.readLine());
            formatSNumber = String.format("%03d", sNumber);
            hours = Integer.parseInt(Breader.readLine());
            grade = Double.parseDouble(Breader.readLine());
            Student student = map.get(formatSNumber);
            Course course = new Course(hours, grade);
            List<Course> courses = student.getCourses();
            courses.add(course);
            map.put(formatSNumber, student.setCourses(courses));
            end = Breader.ready();
        } while(end);

        points = grade * hours;
        GPA = points / hours;
    }
}

学生class：

import java.util.ArrayList;
import java.util.List;
public class Student {
    private String name = " ";
    private List<Course> courses = new ArrayList<>();
    public Student(String name) {
        this.name = name;
    }
    public Student(String name, List courses) {
        this.name = name;
        this.courses = courses;
    }
    public List getCourses() {
        return courses;
    }
    public void setCourses(List courses) {
        this.courses = courses;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }
}

课程class：

public class Course {
    private int hours = 0;
    private double grade = 0.0;
    public Course(int hours, double grade) {
        this.hours = hours;
        this.grade = grade;
    }
    public void setHours(int hours) {
        this.hours = hours;
    }
    public int getHours() {
        return hours;
    }
    public void setGrade(double grade) {
        this.grade = grade;
    }
    public double getGrade() {
        return grade;
    }
}

Answer 1

map.put(formatSNumber, student.setCourses(courses)) 中的第二个参数必须是 Student 类型。 student.setCourses(courses) 是一个 setter 方法，return 类型为 void，即没有 return。这不匹配。

你必须有类似 map.put("someString", new Student("name")) 的东西，或者 map.put("someString", student) 其中 student 是 Student.

类型

Answer 2

put 的想法是 put something into that Map.

更准确地说，您通常提供（非空）键和值对象。

您正在使用 student.setCourses(courses) 作为 put() 期望的 "value" 参数的参数。

该参数是一个表达式。该表达式的结果将是 方法调用 .

的结果

该方法被定义为而不是return任何东西（即无效）。

显然 nothing 与 something 不同。这就是编译器试图告诉你的。

两种解决方案：

传递一个学生对象
更改方法 setCourses()

像这样：

Student setCourses(... {
....
  return this;
}

（你最好选择选项 1；2 更像是一个肮脏的 hack，本质上是糟糕的做法）

Void-type not allowed here 错误

Void-type not allowed here error

java

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treemap