Implementing Comparable With a Generic Type

[Kaydell Leavitt]

I'm trying to learn how to use the Comparable interface for Java 5.0+. In the Java 5.0 API, when I implement the Comparable interface in the code below, I get a warning that says that Comparable is "unchecked".

Will somebody explain to me why Comparable has a generic type and how to implement the Comparable interface in Java 5.0 and up.

package comparable;
 
public class ComparableTest {
 
public static void main(String[] args) {
Person robert = new Person(48);
Person charles = new Person(49);
System.out.format("robert.compareTo(charles) = %d \n",robert.compareTo(charles));
}
 
}
 
class Person implements Comparable {
 
private Integer age;
 
Person(Integer age) {
this.age = age;
}
 
private Integer getAge() {
return age;
}
 
@Override
public int compareTo(Object o) {
 
// return zero if we're comparing an object to itself
if (o == this) { return 0; }
 
// a person object that is null is not allowed
if (o == null) { 
throw new NullPointerException("null objects can't be compared to each other.");
}
 
// For the Person class, the types must both be Person to be comparable
if (o.getClass() != this.getClass()) { 
throw new IllegalArgumentException("in the Person class, objects must be the same type to be compared to each other.");
}
 
// everything else is OK, so compare the ages of the Person objects to each other.
Integer age2 = ((Person)o).getAge();
return this.age.compareTo(age2);
 
}
 

}

[victor kamat]

The person class should be written so:

class Person implements Comparable<Person> {
 
private Integer age;
Person(Integer age) { this.age = age;  }
 
private Integer getAge() { return age;  }

public int compareTo(Person o) {
 
// return zero if we're comparing an object to itself
if (o == this) { return 0; }
 
// a person object that is null is not allowed
if (o == null)
throw new NullPointerException("null objects can't be compared to each other.");

 
// everything else is OK, so compare the ages of the Person objects to each other.
Integer age2 = o.getAge();
return this.age.compareTo(age2);
}
}

[Kaydell Leavitt]

Thank you Victor. I appreciate the help.

I think that my next task would be to implement the following methods:

toString()
equals()
hashCode()

And, now I've got that done.
[ December 27, 2008: Message edited by: Kaydell Leavitt ]