Collections.sort(), gives "unchecked or unsafe operation" Error?

Question

I get the error from Java Version 8: "uses unchecked or unsafe operations".

It seems like the problem is comming from Collections.sort(), but what is the problem? I've checked the Java Doc and everything seems to be fine, except that the argument is a List, but ArrayList is a List as far as I'm concerned?

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

public class Driver
{
    public static void test() 
    {
        ArrayList<Person> persons = new ArrayList<Person>();
        persons.add(new Person("Hans", "Car License"));
        persons.add(new Person("Adam", "Motorcycle License"));
        persons.add(new Person("Tom", "Car License"));
        persons.add(new Person("Kasper", "Car License"));

        System.out.println(persons);        
        Collections.sort(persons);   
        System.out.println(persons);

        System.out.println(Collections.max(persons));
        System.out.println(Collections.min(persons));
    }
}

possible duplicate of [How to sort by two fields in Java?](http://stackoverflow.com/questions/4805606/how-to-sort-by-two-fields-in-java) — Jae Heon Lee, Sep 29 '15 at 06:42
And you forgot the most important part: show us the code of class `Person` — Nir Alfasi, Sep 29 '15 at 06:44
@JaeHeonLee, disagreed. The question is about compiler warning, not about how to sort. — Tagir Valeev, Sep 29 '15 at 06:44
Have a look at [Generics Trail](https://docs.oracle.com/javase/tutorial/java/generics/) more of an idea of what's going on — MadProgrammer, Sep 29 '15 at 06:45

score 11 · Answer 1 · edited May 23 '17 at 12:06

11

I suspect your class Person is declared this way:

class Person implements Comparable {
    ...

    @Override
    public int compareTo(Object o) {
        ...
    }
}

Change it to

class Person implements Comparable<Person> {
    ...

    @Override
    public int compareTo(Person o) {
        ...
    }
}

Do not use rawtypes.

edited May 23 '17 at 12:06

Community

1
1

answered Sep 29 '15 at 06:43

Tagir Valeev

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3

Good answer. Please also explain shortly/link why using rawtypes should be avoided? – Rob Audenaerde Sep 29 '15 at 06:44
1

@RobAu, added link to the canonical do-not-use-rawtypes SO question. – Tagir Valeev Sep 29 '15 at 06:46

score 0 · Answer 2 · answered Sep 29 '15 at 06:46

This error comes when the compiler is not able to check that the collection is used in a type-safe way, using Generics. So you need to specify the type of objects you are storing in your collection.

As Tagir has pointed out that you are probably not providing the type when you are declaring your class Person, so the compiler is giving you the error. If you want a more detailed information you can recompile with the "-Xlint:unchecked" switch.

Collections.sort(), gives "unchecked or unsafe operation" Error?

2 Answers2