需要帮助搜索链接列表

Question

大家好，我在搜索链表时遇到问题。基本上，我是从csv文件中读取并将其存储在链接列表中。我能够在最后添加列表。但当我搜索列表时，它一直说没有找到。方法函数被调用为contains。方法"contains"将Country对象作为参数，并检查是否可以在列表中找到国家的名称。要检查Country类型的对象foo是否等于Country类型对象bar，必须重写Country类中的"equals方法"。当我运行代码时，它返回not found，并且我发现countryNode类中包含的方法返回null，这就是它返回not find的原因。谢谢你的帮助。除了contains方法之外，其他一切都可以工作。以下是我的代码：

public Country contains(Country obj)
{
   if(this.isEmpty())
   {
       System.out.println("Sorry this is an Empty list");
       return null;
   }
   else{
       CountryNode current = first;
        while(current!=null)
       {
           if(current.getCountry().equals(obj))
           {
                return current.getCountry();
               // break;
           }
           current = current.getNext();
       }
      return null;
   }     
}

类Country和overrides方法等于：

public class Country {
private String countryNames;
private SubscriptionYear[] subscriptions;
private int size;
private int location;
public Country(String country)
{
    this.countryNames = country;
}
public Country(String country, int arraylength)
{
    this.countryNames = country;
    this.size = arraylength;
    subscriptions = new SubscriptionYear[size];
    location = 0;
}
public void addSubscriptionYear(int year, double subscription)
{
        subscriptions[location]= new SubscriptionYear(year, subscription);
        ++location;
}

public String toString()
{
    System.out.print(countryNames+"t");
    for(SubscriptionYear s: subscriptions)
    {
        //System.out.print(countryNames+"t");
        System.out.print(s.getSubscription()+"t");
    }
    System.out.println();
    return  "";
}
 public String getName()
 {
    return this.countryNames;
 }
public boolean equals(Country obj)
{
    return (this.countryNames==obj.countryNames);
}
}

这是我的测试主文件：

import java.util.Random;
import java.util.Scanner;
 public class TestCountryList 
 {
  /**
  * Builds a list of countries to debug.
  */
  private void debugListOfCountries(Country [] allCountries)
  {
    // TO COMPLETE
  }
  /**
  * Builds a random list of countries.
  */
private void testRandomListOfCountries(Country [] allCountries)
{   
    Scanner keyboard = new Scanner(System.in);
    System.out.println("How many countries do you want to add to the list?");
    int requestedSize = keyboard.nextInt();
    // Build the list out of a random selection of countries.
    Random random = new Random();
    CountryList selectedCountries = new CountryList();
    for (int i = 0; i < requestedSize; i++)
    {
        int selectedIndex = random.nextInt(allCountries.length);
        selectedCountries.add(allCountries[selectedIndex]);
    }

    // Note: To debug your list, comment this line in
    System.out.println("List of countries: " + selectedCountries);

    // Check if the name of a country is in the list.
    // If the country is found, print the details.
    // Otherwise output not found.
    System.out.println("nWhat country do you want to search for?");
    String countryToFind = keyboard.next();
    Country obj = new Country(countryToFind);
    Country foundCountry = selectedCountries.contains(obj);
    if (foundCountry != null)
    {
    System.out.println("Country " + countryToFind + " found with details:" + foundCountry);
    }
    else
        System.out.println("Country " + countryToFind + " not found.");
}
/**
 * Includes test examples for class GraphView.
 */
public static void main(String[] args) 
{
    // Create and set objects of type Country 
    //
    final String FILENAME = "data/cellular.csv";    // Directory path for Mac OS X
    //final String FILENAME = "datacellular.csv";  // Directory path for Windows OS (i.e. Operating System)
    final int NUM_COUNTRIES_TO_TEST = 3;            // Note: Include test cases in addition to 3

    // Parse the CSV data file
    //
    CSVReader parser = new CSVReader(FILENAME);
    String [] countryNames = parser.getCountryNames();
    int [] yearLabels = parser.getYearLabels();
    double [][] parsedTable = parser.getParsedTable();      

    // Create and set objects of type Country 
    //
    Country [] countries;
    countries = new Country[NUM_COUNTRIES_TO_TEST];
    Country current;
    countries = new Country[countryNames.length];
    for (int countryIndex = 0; countryIndex < countries.length; countryIndex++)
    {
        int numberOfYears = yearLabels.length;   // OR numberOfYears = dataTable[countryIndex].length;
        current = new Country(countryNames[countryIndex], numberOfYears);
        for (int yearIndex = 0; yearIndex < numberOfYears; yearIndex++)
        {
            double [] allSubscriptions = parsedTable[countryIndex];
            double countryData = allSubscriptions[yearIndex];
            current.addSubscriptionYear(yearLabels[yearIndex], countryData);
        }
        countries[countryIndex] = current;
    }
    TestCountryList application = new TestCountryList();
    // Note: Initially, to test your output you may hard code the number of 
    //       countries added, and the array positions selected.
    //       However, make sure to comment this out before submitting your work.
    //application.debugListOfCountries(countries);
    application.testRandomListOfCountries(countries);
}
}

Answer 1

尝试覆盖Object的equals方法，如下所示：

public boolean equals(Object obj)
{
   if (this == obj) {
       return true;
   }
   if (obj == null) {
     return false;
   }
   if (getClass() != obj.getClass()) {
      return false;
   }
   return (this.countryNames.equals(((Country)obj).countryNames));
}

内部包含callcountryList.equals方法，而equals方法的签名是

 public boolean equals(Object obj) {}

与相反

 public boolean equals(Country obj) {}

此外，您只是在比较字符串的两个引用，而您需要比较字符串的内容。所以不是

 this.countryNames==obj.countryNames

你应该说：

 this.countryNames.equals(obj.countryNames);

Answer 2

您需要使用equals或equalsIgnoreCase来比较字符串

public boolean equals(Country obj)
{
    return this.countryNames.equals(obj.countryNames);
}