如何判断列表是否已经加载？

【问题标题】：How to tell if a list has already been loaded?如何判断列表是否已经加载？
【发布时间】：2021-11-12 21:45:12
【问题描述】：

我有一个 Java 程序，允许教师加载并列出 GPA 高于用户输入的学生。该程序具有 3 个类：一个为每个学生提供 get 和 set 方法 (Student)，一个提供四种不同的选项供教师选择 (Communicator)，以及一个调用 Communicator 的类类启动（HW4）。 Communicator 类中的四个选项包括 1) 使用文本文件加载学生列表，2) 显示 GPA 高于用户输入的 GPA 的学生，3) 打印已加载到程序，以及 4) 退出程序。

每个程序的所有基本部分都可以正常工作。但是，我在识别文件是否已加载时遇到困难，它应该回复“文件已加载！”加载到程序中的每个文件都被添加到名为“logHistory”的 TreeSet 中，我尝试使用 if 语句，如果日志历史记录包含输入，则基本上将打印出文件已经加载。但是，它只是打印出“[文本文件]已成功加载！文件已加载！”即使我已经输入过一次。有关如何解决此问题的任何想法？

这是我每个类的代码：

HW4.java：

public class HW4 {

    public static void main(String[] args) throws Exception {
        //Start communicator
        Communicator.start();

    }

}

学生.java：

public class Student {

        private String firstName;
        private String lastName;
        private float gpa;
        
        public Student(String firstName, String lastName, float gpa){
            this.firstName = firstName;
            this.lastName = lastName;
            this.gpa = gpa;         
        }
        
        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 float getGPA() {
            return gpa;
        }
        
        public void setGPA(float gpa) {
            this.gpa = gpa;
        }
        
        @Override
        public String toString() {
            return "Student{" + "First Name = " + firstName + ", Last Name = " + lastName + ", GPA = " + gpa + '}';
        }
}

Communicator.java：

import java.util.*;
import java.io.*;
public class Communicator {

    static Set<String> logHistory;
    
    static LinkedList<Student> studentList;
    
    public static void start() throws Exception{
        logHistory = new TreeSet<String>();
        studentList = new LinkedList<Student>();
        System.out.println("Hello professor. How can I help you?");
        displayMainMenu();
    }
    
    public static void displayMainMenu() throws Exception {
        
        String input;
        Scanner in = new Scanner(System.in);
        do {
            System.out.printf("1: Load student list.\n");
            System.out.printf("2: Display students by gpa.\n");
            System.out.printf("3: Print load log.\n");
            System.out.printf("4: Exit.\n");
            System.out.printf(">> ");
            input = in.next();
            if(invalidInput(input)) {
                continue;
            }
        
        switch(input) {
        case "1":
            OptionOne();
            break;
        case "2":
            OptionTwo();
            break;
        case "3":
            printLog();
            break;
        case "4":
            System.out.println("Good bye! :)");
            return;
            }
        } while (true);
    }
    
    public static boolean invalidInput(String input) {
        if(!(input.charAt(0) == '1' || input.charAt(0) == '2' || input.charAt(0) == '3' || input.charAt(0) == '4')) {
            System.out.println("Error! Not an option!");
            return true;
        }
        return false;
    }
    
    public static void OptionOne() throws FileNotFoundException {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter the name of the class list or press 'B' to go back to the main menu.\n>>");
        String input = in.next();
        if (!input.equals("B")){
            File File1 = new File("C:\\Users\\myName\\eclipse-workspace\\HW4\\" + input);
            if(File1.exists()) {
                Scanner inFile = new Scanner(File1);
                while(inFile.hasNext()) {
                    String inFirstName = inFile.next();
                    String inLastName = inFile.next();
                    float inGPA= Float.parseFloat(inFile.next());
                    Student stu = new Student(inFirstName, inLastName, inGPA);
                    studentList.add(stu);
                }
                System.out.println(input + " loaded successfully!");
                logHistory.add(input);
            }
            else {
                System.out.println("File does not exist!");
            }
        }
    }
    
    public static void OptionTwo() {
        Scanner in = new Scanner(System.in);
        System.out.print("Type in a GPA for which you would like to display students\n>>");
        float user_input = in.nextFloat();
        System.out.println("Hey");
        for(Student myStudent : studentList) {
            if(user_input <= myStudent.getGPA()) {
                System.out.println(myStudent);
            }
        }
    }
    
    public static void printLog() {
        for(int i=0; i<logHistory.size();i++) {
            String log = logHistory.toString().replace("[", "").replace("]", "");
            System.out.println(log);
        }
        return;
    }
    
    public static void main(String[] args) throws Exception {
        start();
    }
}

还有一个我正在使用的文本文件的示例：

Madeline    Holmes  0.3966
Cassandra   Wilkerson   1.2734
Sherman Tucker  0.8281
Ray Owen    1.6671
Irving  Klein   1.3626
Chris   Young   0.801
Ora Reynolds    0.6448
Charlie Collins 0.4683
Wayne   White   0.0056
Bobby   Price   2.9535
Troy    Keller  1.9381
Herbert Lloyd   3.1048
Anita   Robinson    2.6867
Lorene  Brady   2.8133
Wendy   Coleman 0.9067
Rochelle    Fernandez   2.5683
Wanda   Clayton 1.0686
Jamie   Simon   0.6709
Maggie  Gonzalez    3.9742
Felicia Powers  1.7226
Allison Summers 3.4843
Kristopher  Maxwell 1.4895
Donna   Rose    1.519
Sue Alvarado    0.0465
Pearl   Carlson 3.035
Abraham Rodgers 2.3123
Billie  Doyle   0.9774
Edna    Fitzgerald  3.8697
Christie    Day 0.8813
Terri   Pena    3.9045
Maria   Carpenter   2.2858
Jamie   Manning 0.9225
Rose    Bridges 1.5402
Zachary Parker  3.723
Julie   Rowe    0.9275
Javier  Warren  1.1316
Cora    Townsend    0.6736
Yvette  Rodriguez   0.2977
Gloria  Henderson   0.4383
Rafael  Weaver  1.9113
Kim Barrett 0.1046
Orlando Dunn    3.0004
Stuart  Terry   0.4609
Elaine  Davis   2.3482
Simon   Jones   3.5721
Faith   Rivera  1.1192
Jodi    Kelly   1.9566
Loren   Spencer 0.7206
Ignacio Hicks   3.3818
Johanna James   0.3527
Arturo  Huff    2.3178
Arnold  Barker  0.7946
Marc    Weber   1.1336
Joy Bush    0.6794
Brenda  Hubbard 3.0953
Carroll Romero  2.5379
Ann Moore   2.1482
Archie  Duncan  2.897
Eddie   Fields  3.1792
Jan Andrews 3.4771
Della   Waters  2.0199
Nadine  Pierce  1.3615
Carlos  Ortega  1.4948
Toni    Ross    0.1255
Israel  Carson  0.1968
Essie   Morton  3.561
Wesley  Poole   1.3881
Philip  Benson  2.071
Jonathon    Morgan  3.4305
Theresa Hunt    1.7487
Jaime   Ruiz    2.5617
Maxine  Evans   1.8658
Meghan  Hopkins 0.4686
Darnell Hines   2.8667
Rhonda  Garner  2.9759
Blake   Mills   2.0325
Cedric  Jennings    2.2025
Lisa    Miles   3.9877
Steve   Ball    1.1752
Sherri  Gutierrez   0.3944
Rudolph Cross   0.3921
Mario   Cunningham  1.5849
Eileen  Gonzales    1.2047
Tami    Malone  0.1306
Nathan  Becker  1.1747
Brett   Alexander   1.5998
Colleen Cole    2.7064
Ivan    Harris  1.539
Tiffany Gibbs   0.7235
Judith  Kim 2.8998
Christy Blake   0.7685
Gerardo Hammond 0.88
Dwight  Bailey  0.8722
Brian   Santiago    2.634
Andrew  Watkins 3.8533
Garrett Fox 1.1582
Claire  Graham  0.5412
Hubert  Mitchell    1.0085
Grant   Bowman  2.8718
Yolanda Gomez   1.2913

【问题讨论】：

  • 简单的方法是保存一个加载路径列表。在类中有像 Collection loaded = new HashSet() 这样的东西来跟踪这些东西，并在加载完成后将文件路径添加到列表中。然后只需检查是否加载了.contains(path) 来检测加载的数据。
  • 你能给我举个例子吗？

标签： java linked-list treeset


【解决方案1】：

添加检查输入文件名是否已存在于您的日志历史记录中的条件。您可以根据需要设置条件。

public static void OptionOne() throws FileNotFoundException {
            Scanner in = new Scanner(System.in);
            System.out.print("Enter the name of the class list or press 'B' to go back to the main menu.\n>>");
            String input = in.next();
            if (!input.equals("B")){
                if(!logHistory.contains(input)) {//check if the input file does not exist
                    File File1 = new File("C:\\Users\\myName\\eclipse-workspace\\HW4\\" + input);
                    if(File1.exists()) {
                        Scanner inFile = new Scanner(File1);
                        while(inFile.hasNext()) {
                            String inFirstName = inFile.next();
                            String inLastName = inFile.next();
                            float inGPA= Float.parseFloat(inFile.next());
                            Student stu = new Student(inFirstName, inLastName, inGPA);
                            studentList.add(stu);
                        }
                        System.out.println(input + " loaded successfully!");
                        logHistory.add(input);
                    }
                    else {
                        System.out.println("File does not exist!");
                    }
                }
                else {//the input file exist in logHistory
                    System.out.println("File already loaded!");
                }
            }
        }

【讨论】：

    【解决方案2】：

    您需要检查input 文件之前是否已经加载。为此，您需要检查input 是否已经在您的加载文件列表中。以下内容应该可行：

    import java.util.*;
import java.io.*;
public class Communicator {

    static List<String> loadedStudentLists;

    static LinkedList<Student> studentList;

    public static void start() throws Exception{
        loadedStudentLists = new ArrayList<>();
        studentList = new LinkedList<Student>();
        System.out.println("Hello professor. How can I help you?");
        displayMainMenu();
    }

    public static void displayMainMenu() throws Exception {

        String input;
        Scanner in = new Scanner(System.in);
        do {
            System.out.printf("1: Load student list.\n");
            System.out.printf("2: Display students by gpa.\n");
            System.out.printf("3: Print load log.\n");
            System.out.printf("4: Exit.\n");
            System.out.printf(">> ");
            input = in.next();
            if(invalidInput(input)) {
                continue;
            }

            switch(input) {
                case "1":
                    OptionOne();
                    break;
                case "2":
                    OptionTwo();
                    break;
                case "3":
                    printLog();
                    break;
                case "4":
                    System.out.println("Good bye! :)");
                    return;
            }
        } while (true);
    }

    public static boolean invalidInput(String input) {
        if(!(input.charAt(0) == '1' || input.charAt(0) == '2' || input.charAt(0) == '3' || input.charAt(0) == '4')) {
            System.out.println("Error! Not an option!");
            return true;
        }
        return false;
    }

    public static void OptionOne() throws FileNotFoundException {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter the name of the class list or press 'B' to go back to the main menu.\n>>");
        String input = in.next();
        if (!input.equals("B")){
            if (loadedStudentLists.contains(input)) {
                System.out.println("File already loaded!");
            } else {
                File File1 = new File("C:\\Users\\myName\\eclipse-workspace\\HW4\\" + input);
                if (File1.exists()) {
                    Scanner inFile = new Scanner(File1);
                    while (inFile.hasNext()) {
                        String inFirstName = inFile.next();
                        String inLastName = inFile.next();
                        float inGPA = Float.parseFloat(inFile.next());
                        Student stu = new Student(inFirstName, inLastName, inGPA);
                        studentList.add(stu);
                    }
                    System.out.println(input + " loaded successfully!");
                    loadedStudentLists.add(input);
                } else {
                    System.out.println("File does not exist!");
                }
            }
        }
    }

    public static void OptionTwo() {
        Scanner in = new Scanner(System.in);
        System.out.print("Type in a GPA for which you would like to display students\n>>");
        float user_input = in.nextFloat();
        System.out.println("Hey");
        for(Student myStudent : studentList) {
            if(user_input <= myStudent.getGPA()) {
                System.out.println(myStudent);
            }
        }
    }

    public static void printLog() {
        for(int i=0; i<loadedStudentLists.size();i++) {
            String log = loadedStudentLists.toString().replace("[", "").replace("]", "");
            System.out.println(log);
        }
        return;
    }

    public static void main(String[] args) throws Exception {
        start();
    }
}

    【讨论】：

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