带有链接列表的Java泛型问题

Question

Hello guys i'm trying to search for a particular country in my list that i created. First of all i was successfully able to create the list and able to print out to the console. but when i search for a country that was printed out on a console i tried entering one of the countries displayed on the screen to verify it and it displayed it wasn't found even when the displayed country is on the screen. Please this is my first programming experience with generics in java> i have tried debugging it but to no avail. Please could some one help. Node class holds a generic parameter and class LinkedList creates the list for node objects. Node class holds generic parameter that takes class country. The overridden method is written in class country that stores each country names. I have put below the class LinkedList, class Node and the overridden method in class country as well as the search function in the driver class and the output received from the console.

 public class LinkedList<T>{

private Node<T> first;

public LinkedList()
{
    this.first = null;
}
public boolean isEmpty()
{
    return (first==null);
}

 public T contains(T obj)
{
    if(this.isEmpty())
    {
        System.out.println("Sorry list is empty");
        return null;
    }
    else
    {
        Node<T> current = first;
        while(current != null)
        {
            if(current.getT().equals(obj))
            {
                return current.getT();
            }
            current = current.getNext();
        }
        return null;
    }
}

Node class that has generic detail:

public class Node<T> {

private T data;
private Node<T> next;

public Node(T data){
    this.data = data;
    this.next = null;
}
public Node(T objectT, Node<T> next)
{
    this.data = objectT;
    this.next = next;
}
public void setNode(Node<T> next)
{
    this.next = next;
}
public Node<T> getNext()
{
    return this.next;
}
 public T getT()
{
    return data;
}
}

overriden equals method in a different class.

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

my search function in the main class.

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 = Integer.parseInt(keyboard.nextLine());


    // Build the list out of a random selection of countries.
    Random random = new Random();
    LinkedList<Country> selectedCountries = new LinkedList<Country>();
    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.nextLine();
    //Country obj = new Country(countryToFind);
    Country foundCountry = selectedCountries.contains(new Country(countryToFind));
    if (foundCountry != null)
    {
        System.out.println("Country " + countryToFind + " found with details:\n" + foundCountry);
    }
    else
        System.out.println("Country " + countryToFind + " not found.");

}

Output below:

How many countries do you want to add to the list?
 4
 Turkmenistan   0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.057656946 0.068254835 0.089899216 0.166614128 0.17955684  0.177667308 0.197653332 1.06666644  2.211532447 4.516582511 7.85614367  23.08368224 42.83804536 63.41981696 68.75324576 76.41702547 
 South Asia 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
 Burkina Faso   0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00506143  0.014092851 0.024890963 0.044637705 0.217480374 0.637748952 0.902809026 1.880815092 3.037679629 4.720290206 7.354840819 13.05253397 20.62907931 25.3304628  36.72938024 48.02719397 60.60765215 
 Latin America and the Caribbean    0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
 List of countries: 

What country do you want to search for?
Turkmenistan
Country Turkmenistan not found.

Answer 1

Java Generics remove type information during compilation, and add casts for runtime typing. What this means is that your overriden equals() method is actually never called because it will call Object.equals() instead. You can verify this by adding a print statement to the method or a breakpoint and observe that it never executes.

The way to solve this is with another class called a Comparable (or Comparator). https://docs.oracle.com/javase/7/docs/api/java/lang/Comparable.html

Implement the Comparable interface on your Country class, and change your conditional in your LinkedList to use the compareTo() method. Your generic type declaration will also need to be changed to public class LinkedList<T extends Comparable> .