Question

I am currently stuck on a particular part of my code. For my class, we are to create a train containing boxcars containing people or cargo. We use generics to define whether a boxcar can hold people or cargo. Then we load individual people/cargo onto the boxcar and if it has the same String “ID” as someone already on the boxcar then we log an error and do not load that person/cargo. This is where I’m having trouble. I cannot for the life of me figure out how to compare their “ID’s” to see if they are equal. The following is the code that I have so far,

package proj5;

public class Person implements Comparable<Person> {

private String id;
private String name;
private int age;

public Person(String id, String name, int age){
    this.id = id;
    this.id = id;
    this.name = name;
    this.age = age;
}   

public Person(String id){
    this.id = id;
}

public String getId(){
    return id;
}

public int getAge(){
    return age;
}

public String getName(){
    return name;
}

public String toString(){
    String str = "        " + "ID: " + id + "  " + " Name: " + name + "  " + " Age: " + age;
    return str;
}

public int compareTo(Person p) {
    int result = this.id.compareTo(p.getId());
    return result;
}

}

package proj5;

import java.util.ArrayList;
import java.util.List;
import java.util.Collections;

public class Boxcar<T extends Comparable<T>> {

private ArrayList<T> boxcar;
private int maxItems;
private int boxcarID;

public Boxcar(){
    boxcar = new ArrayList<T>();
}

public void load(T thing){
    for(int i = 0; i < boxcar.size(); i++){
    if(boxcar.size() < maxItems && !boxcar.get(i).equals(thing)){
        boxcar.add(thing);
        System.out.println(boxcar.get(i));
    }
    else{
        boxcar.remove(thing);
    }
    }
    Collections.sort(boxcar);
}

public int getBoxcarId(){
    return boxcarID;
}

public int getMaxItems(){
    return maxItems;
}

public void setMaxItems(int i){
    maxItems = i;
}

public void unload(T thing){
    for(T item : boxcar){
        if(item.equals(thing)){
            boxcar.remove(item);
        }
    }
}

public List<T> getBoxcar(){
    return boxcar;
}

public String toString(){
    String str = "";
    for(T item : boxcar){
        str += item + "\n";
    }
    return str;
}

}

The problem is with my load function. I do not know how to compare their ID’s. For clarification, the objects ID’s are Strings. I have other classes but I have only included the ones I thought were necessary. If you need more files I will be happy to provide them. I have been stuck on this for hours and would appreciate any help! Thank you very much in advance!

EDIT: I have tried using the contains() method from the Collections API but why does that not work? It sounds like it would work perfectly.

Answer 1

You have to implement the equals and hashCode for your class Person.

The problem is that boxcar.get(i).equals(thing) is calling the generic equals and its only comparing the references.

so a generic equals will look like.

public boolean equals(Object obj){
  if (obj == null) return false;
  if (obj == this) return true;
  if (obj instanceof Person){
    Person p = (Person) obj;
    return p.getId().equals(this.getId());
  }
  return false;
}

and hashCode can be like this

public int hashCode(){
  return 37*this.getId().hashCode();
}