warning: [unchecked] unchecked method invocation

[Mike Mitchell]

Hi guys. Apologies if this should be obvious. When I compile the following -Xlint I'm warned:

CompareMe.java:27: warning: [unchecked] unchecked method invocation: <T>sort(java.util.List<T> ;) in java.util.Collections is applied to (java.util.List<scratch.Item> ;)
Collections.sort(list);
^
1 warning

list is declared to hold only <Items>, and I assume Collections.sort is a generic method. What am I missing?

Thanks much.

  
package scratch;
 
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
 
/**
 *
 * @author mike
 */
public class CompareMe {
 
    public static void main(String[] args) {
 
        List<Item> list = new ArrayList<Item>();
        list.add(new LittleItem(-1, 'a'));
        list.add(new LittleItem(0, 'd'));
        list.add(new Item(0, '?'));
        list.add(new Item(17, '_'));
        list.add(new LittleItem(-20, 'p'));
 
        System.out.println(list);
        Collections.sort(list);
        System.out.println(list);
 
        System.out.println(Item.sum(list));
    }
}
 
class LittleItem extends Item {
 
    LittleItem(int i, char c) {
        super(i, c);
    }
}
 
class Item implements Comparable {
 
    int number;
    int letter;
 
    public Item() {
        this(0, '.');
    }
 
    public Item(int i, char c) {
        number = i;
        letter = c;
    }
 
    public int compareTo(Object o) {
 
        Item i = (Item) o;
        if (this.number < i.number) {
            return -1;
        } else if (this.number > i.number) {
            return 1;
        } else {
            return 0;
        }
    }
 
    static <T extends Item> int sum(List<T> list) {
        int sum = 0;
        for (T i : list) {
            sum += i.number;
        }
        return sum;
    }
 
    @Override
    public String toString() {
 
        return String.format("[%d, %c]", number, letter);
    }
}

[ June 18, 2008: Message edited by: Mike Mitchell ]

[Matt Russell]

You need to make Item implement Comparable<Item>, not just Comparable (which is a raw type). That is:

...
class Item implements Comparable<Item> {
...
    public int compareTo(Item i) {
       ...
    }
}

[Krishnamoorthy Vuyala Muralidharan]

public int compareTo(Object o) { Item i = (Item) o; if (this.number < i.number) { return -1; } else if (this.number > i.number) { return 1; } else { return 0; } }

Hi

compareTo() method's signature must be the same, as you are overriding it and not overloading. That means, you are implementing the method from the interface Comparable.

public int compareTo(Object o) {
Item i = (Item) o;
if (this.number < i.number) {
return -1;
} else if (this.number > i.number) {
return 1;
} else {
return 0;
} }

and not

public int compareTo(Item o) {
Item i = (Item) o;
if (this.number < i.number) {
return -1;
} else if (this.number > i.number) {
return 1;
} else {
return 0;
} }

Kind Regards
Kris

[Mike Mitchell]

Thanks much Matt and Kris -- that got me straightened out.