Java 电话及；在不同类中搜索Arraylist

我正在构建一个程序来读取一个.txt文件，提取学生数据并将其存储在一个集合中。然后，用户应该能够选择几个不同的查询。我请求帮助的问题是选择所有毕业的学生，例如2014年，然后将这些结果打印到屏幕上

简言之，我如何为2014年毕业的学生搜索存储在ProcessRecords类中的Arralist？我只是不明白如何从另一个类中调用它

下面是我的代码：

第一类：使用主方法

import java.util.*;
import java.io.*;
import java.lang.*;

public class ProcessRecords {

 public static void AskUser() 
 throws Exception {
     Scanner preference = new Scanner(System.in);
    //Creating a new scanner will allow us to gather user input

     boolean flag=true; 
    //I will use this for my while loop
    while (flag) {
        System.out.println("What type of Search would you like to run?\n 1)Search for all students\n 2) Search for students graduating in a specific year\n 3)Search for students whose last name begins with a certain string\n");
        Query query = new Query(studentRecords);
        int searchType=preference.nextInt();
        //How would I throw an exception here if the user doesn't enter a number or enters a number less than 1 or great than 4
        //This variable will store what type of query the user would like to run

        switch(searchType) {
            case 1:
            System.out.println("Gathering Records for all students\n");
            //Call Query Method in the Query Class to return all students in the colletion
            case 2:
            System.out.println("What graduation year would you like to search for? \n");
            String yearsearch=preference.next();
            //Call Query Method to return students who are graduating in the specified year
            //Pass the "yearsearch" variable to the Query class
            case 3:
            System.out.println("What string would you like to search for? \n");
            String lstsearch=preference.next();
            //Call Query Method in the Query Class to return students who have the string in their last name
            //Also I need to pass the "lstsearch" variable to the Query class to search through last names                

        }
    }
 }

 public static void main(String[] args)
 throws Exception
 {
    Scanner input = new Scanner(new File("students.txt"));
    //This will import the file
    input.nextLine();
    //This will skip the headers in the file
    System.out.println("Processing file now...");
    //Let the user know that the file is being processed



    int id;
    String last;
    String first;
    int year;
    int i=1;
    // Declare variables that we will extract from the file

    //Now we will being processing the file with a while loop

    List<StudentRecord> studentRecords = new ArrayList<StudentRecord>();
    while(input.hasNext())
    {
        id=input.nextInt();
        last=input.next();
        first=input.next();
        year=input.nextInt();
        StudentRecord record = new StudentRecord(id, last, first, year);
        studentRecords.add(record);
        System.out.println(id + "  " + last + "  " + first + "  " + year + "\n");

    }
    System.out.println(" You have successfully read and printed from the file!");
    for (StudentRecord s : studentRecords)
        System.out.println(s.toString());
}
}

---

---

三等舱（我请求帮助的那一个）

import java.util.*；
导入java.io.*；
公共类查询
{
//public static ProcessRecords studentrecord=new ProcessRecords（）；
私人名单记录；
公开查询（列表记录）{
这个.记录=记录；
}
公共int getYear（int yearSearch）{
整数计数=0；
用于（StudentRecord记录：记录）{
if（record.getYear（）==yearSearch）{
计数++；
}
}
返回计数；
}
}

---

添加了一个新帖子

必须使studentRecords成为静态变量或实例变量

这样说吧，在主体之上：

public static List<StudentRecord> studentRecords ;
public static void main(String[] args)
throws Exception{

studentRecords = new ArrayList<StudentRecord>();

---

您的代码有多个问题。

不鼓励使用公共变量，应使用getter/setter访问对象成员。Eclipse

alt+s，r

将为您生成它们

变量的名称具有误导性。您在查询中访问的列表是什么

您的类的名称也（至少！）具有误导性

ProcessRecords

作为一种行为，似乎特别糟糕。在这种情况下，类不应该是一个名词吗，比如

RecordsProcessor

？它不应该暗示它实际上在做什么吗<代码>学生年搜索器

您不会抛出列表，而是将它（它的引用）作为参数传递，或者以某种方式访问它。你抛出异常

回答您的问题

有多种方法可以做到这一点。一种是使用单例模式并使列表静态可访问。像这样：

class StudentRecord {
  static List<StudentRecord> studentRecords;

  List<StudentRecord> getStudentRecords() {
    if (studentRecords == null) studentRecords= new ArrayList<StudentRecord>();
    return studentRecords;
  }

  //the reest of the class
}

class StudentRecord{
静态列表记录；
列出getStudentRecords（）{
如果（studentRecords==null）studentRecords=new ArrayList（）；
归还录音带；
}
//这个班的学生
}

最简单的解决方案是将整个列表传递给

getYear

方法：

public static int getYear(List<StudentRecord> studentRecords, int yearsearch) {
      // ProcessRecords processRecords = new ProcessRecords();  <- don't need it
      int getYear= yearsearch;
      Iterator itr = studentRecords.iterator();

      // ...

publicstaticintgetyear（列表studentRecords，intyearsearch）{
//ProcessRecords=new ProcessRecords（）；使您的查询类如下所示：

ProcessRecords.studentRecords

public class Query {

    private List<StudentRecord> records;

    public Query(List<StudentRecord> records) {
        this.records = records;
    }

    public int getYear(int yearSearch) {
       int count = 0;

       for(StudentRecord record : records) {
           if(record.getYear() == yearSearch) {
               count++;
           }
       }

       return count;
    }

    public int otherQuery() {
        // code for another query
    }
}

公共类查询{
私人名单记录；
公开查询（列表记录）{
这个.记录=记录；
}
公共int getYear（int yearSearch）{
整数计数=0；
用于（StudentRecord记录：记录）{
if（record.getYear（）==yearSearch）{
计数++；
}
}
返回计数；
}
public int otherQuery（）{
//另一个查询的代码
}
}

然后在你的主课上：

import java.util.*;
import java.io.*;
import java.lang.*;

public class ProcessRecords {

    public static void AskUser(Query query) throws Exception {
        // all the code you have right now except the line where you
        // create a new Query object   
    }

    public static void main(String[] args) throws Exception {
        Scanner input = new Scanner(new File("students.txt"));
        //This will import the file
        input.nextLine();
        //This will skip the headers in the file
        System.out.println("Processing file now...");
        //Let the user know that the file is being processed

        int id;
        String last;
        String first;
        int year;
        int i=1;
        // Declare variables that we will extract from the file

        //Now we will being processing the file with a while loop

        List<StudentRecord> studentRecords = new ArrayList<StudentRecord>();

        while(input.hasNext()) {
            id=input.nextInt();
            last=input.next();
            first=input.next();
            year=input.nextInt();
            StudentRecord record = new StudentRecord(id, last, first, year);
            studentRecords.add(record);
            System.out.println(id + "  " + last + "  " + first + "  " + year + "\n");
        }

        System.out.println(" You have successfully read and printed from the file!");

        for (StudentRecord s : studentRecords) {
            System.out.println(s.toString());
        }

        Query query = new Query(studentRecords); // we've moved this out of AskUser method to here

        // now we call the AskUser method and pass it this query object we just
        // created so it can have access to it, meaning inside the AskUser method we can
        // say things like 'query.getYear(2014);'

        AskUser(query);
    }
}

import java.util.*；
导入java.io.*；
导入java.lang.*；
公共类处理记录{
公共静态void AskUser（查询）引发异常{
//你现在拥有的所有代码，除了你
//创建一个新的查询对象
}
公共静态void main（字符串[]args）引发异常{
扫描仪输入=新扫描仪（新文件（“students.txt”）；
//这将导入该文件
input.nextLine（）；
//这将跳过文件中的标题
System.out.println（“正在处理文件…”）；
//让用户知道文件正在处理中
int-id；
最后一串；
先串；
国际年；
int i=1；
//声明我们将从文件中提取的变量
//现在我们将使用while循环处理该文件
List studentRecords=new ArrayList（）；
while（input.hasNext（））{
id=input.nextInt（）；
last=input.next（）；
first=input.next（）；
年份=input.nextInt（）；
StudentRecord=新的StudentRecord（id、上一个、第一个、年份）；
添加（记录）；
System.out.println（id+“”+最后一个+“”+第一个+“”+年份+“\n”）；
}
System.out.println（“您已成功读取并打印了该文件！”）；
用于（studentRecords:studentRecords）{
System.out.println（s.toString（））；
}
Query Query=new Query（studentRecords）；//我们已经将它从AskUser方法移到这里
//现在我们调用AskUser方法，并将我们刚刚创建的查询对象传递给它
//创建，以便它可以访问它，这意味着在AskUser方法中，我们可以
//比如说“query.getYear（2014）；”
AskUser（查询）；
}
}
您忘记了
中断
您的
案例
s。如果
searchType
为1，则所有三个案例中的代码都将被执行。谢谢！您是否有机会帮我收集特定年份毕业学生列表中的所有记录？为什么会这样说“编译ProcessRecords类时找不到符号-变量StudentRecords？我在上一次编辑中更新了代码。StudentRecords变量在AskUser（）方法中不存在。您需要在访问StudentRecords列表时创建查询对象。您可以在创建列表后（在整个循环之后）立即创建该对象。”或者你可以让studentRecords成为一个实例变量，并以这种方式访问它。或者你可以让Query对象成为一个实例变量，你也可以以这种方式在AskUser（）中访问它。再次查看你的代码，最简单的方法可能是通过
public class Query {

    private List<StudentRecord> records;

    public Query(List<StudentRecord> records) {
        this.records = records;
    }

    public int getYear(int yearSearch) {
       int count = 0;

       for(StudentRecord record : records) {
           if(record.getYear() == yearSearch) {
               count++;
           }
       }

       return count;
    }

    public int otherQuery() {
        // code for another query
    }
}

import java.util.*;
import java.io.*;
import java.lang.*;

public class ProcessRecords {

    public static void AskUser(Query query) throws Exception {
        // all the code you have right now except the line where you
        // create a new Query object   
    }

    public static void main(String[] args) throws Exception {
        Scanner input = new Scanner(new File("students.txt"));
        //This will import the file
        input.nextLine();
        //This will skip the headers in the file
        System.out.println("Processing file now...");
        //Let the user know that the file is being processed

        int id;
        String last;
        String first;
        int year;
        int i=1;
        // Declare variables that we will extract from the file

        //Now we will being processing the file with a while loop

        List<StudentRecord> studentRecords = new ArrayList<StudentRecord>();

        while(input.hasNext()) {
            id=input.nextInt();
            last=input.next();
            first=input.next();
            year=input.nextInt();
            StudentRecord record = new StudentRecord(id, last, first, year);
            studentRecords.add(record);
            System.out.println(id + "  " + last + "  " + first + "  " + year + "\n");
        }

        System.out.println(" You have successfully read and printed from the file!");

        for (StudentRecord s : studentRecords) {
            System.out.println(s.toString());
        }

        Query query = new Query(studentRecords); // we've moved this out of AskUser method to here

        // now we call the AskUser method and pass it this query object we just
        // created so it can have access to it, meaning inside the AskUser method we can
        // say things like 'query.getYear(2014);'

        AskUser(query);
    }
}