我该如何修复这些错误？ 程序

Question

I have been working on a program.

I keep getting these errors:

StationInformation.java:65: error: non-static variable this cannot be referenced from a static context
      Station mile_end = new Station();
                         ^
StationInformation.java:66: error: non-static variable this cannot be referenced from a static context
      Station oxford_circus  = new Station();
                               ^
StationInformation.java:67: error: non-static variable this cannot be referenced from a static context
      Station kings_cross = new Station();
                            ^
StationInformation.java:68: error: non-static variable this cannot be referenced from a static context
      Station stepney_green = new Station();
                              ^
4 errors

I want to fix the program.

Answer 1

Edit: This answer might seem outdated since the OP decided to edit the question, removing his code in the process.

There are 2 errors in your code:

1. Your inner class Station is not static, meaning you cannot instantiate it in a static context. This is producing the error messages you see.
2. You are thinking that Java is pass-by-reference, and are trying to override the value the variable(s) are pointing to (which you cannot do in Java).

You can fix your mistakes by making your Station -class static ( static class Station ), and by making the stations you use class-variables, and using their fields to create a String.
You could also implement a getInfo() -method for the Station -class that prepares its info on its own. With this, you can just call System.out.println(STATION_YOU_WANT.getInfo()) .

I have taken a bit of my time to write a commented solution to the question.
The most confusing part of it is probably the use of varargs ( String... in the code below). They basically allow you to pass any number of arguments to a method, which will inherently be converted to an array by Java.

import java.util.HashMap;
import java.util.Locale;
import java.util.Scanner;

public class StationInformation {
  private static class Station {
    private String name;
    private int distanceToPlatform;
    private boolean stepFree;
    private String[] alternateNames;
    
    Station(String name, int distanceToPlatform, boolean stepFree, String...alternateNames) {
      this.name = name;
      this.distanceToPlatform = distanceToPlatform;
      this.stepFree = stepFree;
      this.alternateNames = alternateNames; // 'String...' makes that parameter optional, resulting in 'null' if no value is passed
    }
    
    String getInfo() {
      return name + " does" + (stepFree ? " " : " not ")
        + "have step free access.\nIt is " + distanceToPlatform + "m from entrance to platform.";
    }
  }
  
  private static HashMap<String, Station> stations = new HashMap<String, Station>();
  
  public static void main(String[] args) {
    createStations();
    
    // The Program
    Scanner scanner = new Scanner(System.in);
    
    // Keep requesting input until receiving a valid number
    int num;
    for (;;) { // 'for (;;) ...' is effectively the same as 'while (true) ...'
      System.out.print("How many stations do you need to know about? ");
      String input = scanner.nextLine();
      
      try {
        num = Integer.parseInt(input);
        break;
      } catch (Exception exc) {
        // Do nothing
      }
      
      System.out.println("Please enter a correct number.");
    }
    
    for (int i = 0; i < num; ++i) {
      System.out.print("\nWhat station do you need to know about? ");
      String input = scanner.nextLine();
      
      // If available, show Station-information
      if (stations.containsKey(input.toLowerCase(Locale.ROOT))) {
        System.out.println(stations.get(input.toLowerCase(Locale.ROOT)).getInfo());
      } else {
        System.out.println("\"" + input + "\" is not a London Underground Station.");
      }
    }
    
    scanner.close(); // Close Scanner; Here actually not needed because program will be closed right after, freeing its resources anyway
  }
  
  private static void createStations() {
    // Add new Stations here to automatically add them to the HashMap
    Station[] stations = new Station[] {
      new Station("Stepney Green", 100, false),
      new Station("King's Cross", 700, true, "Kings Cross"),
      new Station("Oxford Circus", 200, true)
    };
    
    for (Station station : stations) {
      StationInformation.stations.put(station.name.toLowerCase(Locale.ROOT), station);
      
      // Alternative names will be mapped to the same Station
      if (station.alternateNames == null) continue;
      for (String altName : station.alternateNames)
        StationInformation.stations.put(altName.toLowerCase(Locale.ROOT), station);
    }
  }
}

Answer 2

I have made Station class as static and returning a list from create_messages().

//this program tells the user whether a station is a step free access station or not and how far is it from the platform

import java.util.Scanner; // imports the scanner function to input data from the user
import java.util.ArrayList;
import java.util.List;

class StationInformation {
    public static void main(String[] args) // main method where methods are sequenced
    {
        int numberOfStations = inputint("how many stations do you want to know about?");
        String station;

        for (int i = 1; i <= numberOfStations; i++) {
            station = inputstring("what station do you want to know about?");
            search_station(station);

        }

        System.exit(0);
    }

// A method to input integers
    public static int inputint(String message) {
        Scanner scanner = new Scanner(System.in);
        int answer;

        System.out.println(message);
        answer = Integer.parseInt(scanner.nextLine());

        return answer;
    } // END inputInt

    public static String inputstring(String message) {
        Scanner scanner = new Scanner(System.in);
        String answer;

        System.out.println(message);
        answer = scanner.nextLine();

        return answer;
    }

    public static String create_message(Station station) {
        String message;
        if (station.step_free_access == true) {
            message = (station.name + "does have step free access. " + "it is " + station.distance_from_platform
                    + "m away from the entrance");
        } else {
            message = (station.name + "does not have step free access. " + "it is " + station.distance_from_platform
                    + "m away from the entrance");
        }
        return message;
    }

    public static List<String> create_messages() {
        Station mile_end = new StationInformation.Station();
        Station oxford_circus = new Station();
        Station kings_cross = new Station();
        Station stepney_green = new Station();

        mile_end.distance_from_platform = 50;
        mile_end.name = "Mile End ";
        mile_end.step_free_access = false;
        String message1 = create_message(mile_end);

        oxford_circus.distance_from_platform = 200;
        oxford_circus.name = " Oxford Circus ";
        oxford_circus.step_free_access = true;
        String message2 = create_message(oxford_circus);

        kings_cross.distance_from_platform = 700;
        kings_cross.name = " kings cross ";
        kings_cross.step_free_access = true;
        String message3 = create_message(kings_cross);

        stepney_green.distance_from_platform = 300;
        stepney_green.name = " Stepney Green ";
        stepney_green.step_free_access = false;
        String message4 = create_message(stepney_green);

        List<String> list = new ArrayList<>();
        list.add(message1);
        list.add(message2);
        list.add(message3);
        list.add(message4);
        return list;
    }

    public static void search_station(String station) {

        List<String> list = create_messages();
        String mileEndMessage = list.get(0);
        String oxfordCircusMessage = list.get(1);
        String kingsCrossMessage = list.get(2);
        String stepneyGreenMessage = list.get(3);

        if (station.equals("Mile End")) {
            System.out.println(mileEndMessage);
        } else if (station.equals("kings cross")) {
            System.out.println(kingsCrossMessage);
        } else if (station.equals("oxford circus")) {
            System.out.println(oxfordCircusMessage);
        } else if (station.equals("stepney green")) {
            System.out.println(stepneyGreenMessage);
        } else {
            System.out.println(station + " is not a London underground station ");
        }

    }

    static class Station // a record to store information about stations
    {
        int distance_from_platform;
        String name;
        boolean step_free_access;
    }

}