Java .stream() 未在 .toList() 中保存 List<> 排序 - 打印空白

Question

-- 我正在使用 .stream() 按用户最初选择的名称过滤列表并将其打印出来，但我无法正确打印排序后的列表；它只打印白色 space。我认为它没有在列表中正确保存排序。我怎样才能解决这个问题？

StudentFinder.java:

    import java.util.*;
    import java.util.stream.*; //throws error w/o it :(

    public class StudentFinder {

        //Print method
            public static void contactPrint(List<Student> arr) {
                for(int x = 0; x < arr.size(); ++x) {
                    System.out.println("[" + x + "]" + " Name: " + arr.get(x).name + " - ID: " + arr.get(x).id + " - GPA: " + arr.get(x).gpa);
                }// end for loop
            }// end contactPrint()

        public static void main(String[] args) {                

            // "File" to hold Student's field 'name' info
            String[] names = {
                    "Albert Einstein",
                    "Ada Lovelace",
                    "Blaise Pascal",
                    "Bruna Louise",
                    "Caroline Herschel",
                    "Cecilia Payne-Gaposchkin",
                    "Dorothy Hodgkin",
                    "Douglas Junior",
                    "Edwin Powell Hubble",
                    "Elizabeth Blackburn",
                    "Flossie Wong-Staal",
                    "Frieda Robscheit-Robbins",
                    "Geraldine Seydoux",
                    "Gertrude B. Elion",
                    "Ingrid Daubechies",
                    "Irma Sanchez",
                    "Jacqueline K. Barton",
                    "Johannes Kepler",
                    "Lene Vestergaard Hau",
                    "Lord Kelvin",
                    "Maria Mitchell",
                    "Max Planck",
                    "Nicolaus Copernicus",
                    "Niels Bohr",
                    "Patricia S. Goldman-Rakic",
                    "Patty Jo Watson",
                    "Richard Phillips Feynman",
                    "Rita Levi-Montalcini",
                    "Sarah Boysen",
                    "Stephen Hawking",
                    "Werner Karl Heisenberg",
                    "Wilhelm Conrad Roentgen"
            };

            int numStudents = names.length;
            String holdUserInput;

            //Create new Student object students
            List<Student> students = new ArrayList<Student>(); 

            // Initialize List of objects Student students with **CONSTRUCTOR**
            for(int x = 0; x < numStudents; ++x) {
                students.add(new Student(names[x], (100 + x), (double)((2.2 + (x + 2))))); // constructor
            }

            //Prints original object students
            contactPrint(students); 

            //prompt user for input
            System.out.println("Enter the first letter of the student's name:");
            Scanner userInput = new Scanner(System.in);
            holdUserInput = userInput.nextLine();//<<<******INT??? String???********

            //handle user's input
            List<String> studentsNames = 
                    students.stream()
                                   .filter(n -> n.getName().startsWith(holdUserInput.toUpperCase())) 
                                   .sorted(Comparator.comparing(Student::getName))
                                   .map(Student::getName)
                                   .collect(Collectors.toList());

            //print sorted list
            System.out.println(" \n");
            for(int i = 0; i < studentsNames.size(); i++) {
                System.out.println(studentsNames.get(i));
            }

            //end program message to user
            System.out.println("Program Ended.");

        }// end main()
}// end class StudentFinder

Student.java:

public class Student {

    String name = "";
    int id;
    double gpa;

    // constructor
    public Student(String sName, int sId, double sGpa) {
        name = sName;
        id = sId;
        gpa = sGpa;

    }

    //get name value to main()
    public String getName() {
        return name;

    }

    //get ID value to main()
    public int getId() {
        return id;

    }

    //get GPA value to main()
        public double getGpa() {
            return gpa;

        }
}

Answer 1

我不认为你的问题来自于排序...下面的代码可能与你没有内部 class 的代码非常相似似乎有效。向我们提供有关其余代码的更多详细信息，我们可能会提供帮助。

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Collectors;

public class SortTest
{
    public static class Student
    {
        String string;
        public Student(String s) { this.string = s; }
        public String getName() { return this.string; }
    }

    public static void main(String[] args)
    {
        List<Student> students = new ArrayList<Student>();
        students.add(new Student("AA"));
        students.add(new Student("AC"));
        students.add(new Student("AB"));
        students.add(new Student("BA"));
    
        System.out.println("Initial Order of all students: ");
        for(Student student : students) { System.out.print(student.getName() + " "); }

        System.out.print("\nPick your letter: ");
        Scanner scanner = new Scanner(System.in);
        String letter = scanner.next();
        scanner.close();

        List<String> studentNames = students.stream()
        .filter(n -> n.getName().toUpperCase().startsWith(letter.toUpperCase()))   
        .sorted(Comparator.comparing(Student::getName))
        .map(Student::getName)
        .collect(Collectors.toList());
    
        System.out.println("\nNew order of names starting by " + letter + ":");
        for(String name : studentNames) { System.out.print(name + " "); }
    }
}

输出为：

Initial order of all students: AA AC AB BA

Pick your letter: a

New order of names starting by A: AA AC AB

Java .stream() 未在 .toList() 中保存 List<> 排序 - 打印空白

Java .stream() not saving List<> sorting in .toList() - prints blank

java

sorting

list

tolist

java-stream