我正在构建一个程序来读取.txt文件并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。我要求帮助的查询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。
简而言之，如何为2014年毕业的学生搜索存储在ProcessRecords课程中的Arralist？我只是不明白如何从另一个班级打电话。
以下是我的代码：
头等舱：使用主要方法
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());
}
}
二等学生记录
public class StudentRecord
{
private int id;
private String last;
private String first;
private int year;
public StudentRecord(int id, String last, String first, int year)
{
this.id=id;
this.last=last;
this.first=first;
this.year=year;
}
public String toString()
{
return id + " " + last + " " + first + " " + year;
}
public int getYear()
{
return year;
}
}
三等（我要求帮助的那个）
import java.util.*;
import java.io.*;
public class Query
{
//public static ProcessRecords studentrecord = new ProcessRecords();
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;
}
}
添加了新帖子！
答案 0 :(得分：0)
你必须让studentRecords成为静态变量或实例变量，
这样说，主要在：
public static List<StudentRecord> studentRecords ;
public static void main(String[] args)
throws Exception{
studentRecords = new ArrayList<StudentRecord>();
然后就这样称呼它：
ProcessRecords.studentRecords
答案 1 :(得分：0)
您的代码存在多个问题。
不建议使用公共变量，应使用getter / setter来访问对象成员。 Eclipse alt+s, r将为您生成
您的变量名称具有误导性。您在Query中访问的列表是什么？
您的课程名称也（至少！）具有误导性。 ProcessRecords特别是行动似乎很糟糕。在这种情况下，该类不应该是名词，如RecordsProcessor吗？它不应该暗示它实际上在做什么吗？ StudentYearSearcher
您不会抛出列表，将它（它的引用）作为参数传递，或以某种方式访问它。你抛出例外
回答您的问题
有多种方法可以做到。一种方法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
}
答案 2 :(得分：0)
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();
// ...
答案 3 :(得分：0)
让您的Query类看起来像这样：
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
}
}
然后在你的主要课程中：
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);
}
}