Question

How do I create a list of generic list? I have a Boxcar class that takes a generic argument and a Train class that is supposed to create a list of Boxcars. We are supposed to specify the type that will be in Boxcar in a separate main class, so until then boxcar has to stay generic. The following is the code that I have written. It compiles but in a separate driver class when calling the load method I get the error The method load(capture#1-of ?) in the type Boxcar<capture#1-of ?> is not applicable for the arguments (Person)

package proj5;

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

public class Train {

private List<Boxcar<?>> train;
private int maxSpeed;
private int minSpeed;
private String position;
private int numBoxcars;
private int maxNumBoxcars;
private int speed;
private String destination;
private boolean stopped = true;

public Train(int maxSpeed, int minSpeed, int maxNumBoxcars, String position){
    train = new ArrayList<Boxcar<?>>();
    this.maxSpeed = maxSpeed;
    this.minSpeed = minSpeed;
    this.maxNumBoxcars = maxNumBoxcars;
    this.position = position;
}

public int getMaxNumBoxcars(){
    return maxNumBoxcars;
}

public int getSpeed(){
    return speed;
}

public String getPosition(){
    return position;
}

public int getMaxSpeed(){
    return maxSpeed;
}

public int getNumBoxcars(){
    return numBoxcars;
}

public List<Boxcar<?>> getTrain(){
    return train;
}

public void depart(String destination){
    this.destination = destination;
    speed = minSpeed;
    stopped = false;
}

public void arrive(){
    stopped = true;
    position = destination;
}

public void addCar(Boxcar<?> boxcar, int i){
    if(stopped){
        boxcar.setMaxItems(i);
        train.add(boxcar);
    }
}

public void removeCar(int i){
    if(stopped){
        train.remove(i);
    }
}

}

package proj5;

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

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

private List<T> boxcar;
private int maxItems;

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

public void load(T thing){
    if(!boxcar.contains(thing) && boxcar.size() < maxItems){
        boxcar.add(thing);
        Collections.sort(boxcar);
    }else{

    }
}

public int getMaxItems(){
    return maxItems;
}

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

public void unload(T thing){
    boxcar.remove(thing);
}

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

}

I hope this better conveys what I am trying to accomplish

Answer 1

You need to supply a type in place of T and ? when creating a Generic List. For example a List of Boxcars containing Strings would look like this:

List<Boxcar<String>> train = new ArrayList<Boxcar<String>>();

The ? is an example of a wildcard, while a T represents a type that is referenced inside the source of List. That point can be tricky to understand without a deeper understanding of Generics, but I wanted to be sure address it for completeness sake. Take a look at this page for more information about how to use Generics inside your code.

Looking at your revised question, I would direct you to this line of code:

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

and then just below it this line:

private List<T> boxcar;

This means whatever type you pass to new Boxcar<type>() will be carried over to the inner list (and other methods that expect an object of type T).