如何将compareTo方法强制转换为实现Comparable的泛型类？

Question

I have an Arraylist with type Person. Name is a class that I have created. In my mainapp I'm creating some Name objects, setting each name-object to a person object and then adding the person objects to an ArrayList. When I try to sort the list using Collections.sort(), I get a ClassCastException.

I can not see what the problem is.

Here is my code so far:

public class Person<T> implements Comparable<Person<T>> {

private T name;

public T getName() {
    return name;
}

public void setName(T name) {
    this.name = name;
}

@Override
public int compareTo(Person<T> o) {
    return ((Person<T>) this.getName()).compareTo((Person<T>) o.getName());
}

}

Answer 1

The return value from getName() has type T , but you are trying to cast it to Person<T> . To fix the problem, you need to compare the names directly without casting:

public int compareTo(Person<T> o) {
    return this.getName().compareTo(o.getName());
}

Now this will cause another error because the compiler doesn't know if T has the compareTo() method. To fix that, you need to add a bound to the type T :

public class Person<T implements Comparable<T>> implements Comparable<Person<T>>

Note:

To me, this seems like overuse of generics. Why should name be allowed a generic type when String is the only natural fit? And why do you need a Name class? It is likely only a wrapper around a String so you should just use String directly.

Answer 2

You are casting this.getName() , which is of type T , to type Person<T> . Ie you are casting a Name to a Person, hence the ClassCastException ("name is not a person").

To fix this, please remove the casts and compare the names directly. Ignoring null-safety, it would be:

return getName().compareTo(o.getName());

Note that class Name also needs to implements Comparable<Name> .

Answer 3

Why is your name a type? This looks a bit weird. Could it be you meant to do something like this:

public class Person implements Comparable<Person> {

    private String name;

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public int compareTo(Person other) {
        return this.name.compareTo(other.getName());
    }
}