generics help needed

[Rusty Wright]

Can someone explain how to get rid of the following warnings? Here is
a simple contrived example. The warnings are when I call sort().

Here is the sample code:

import java.util.*;

class Xyz {
private List<Xyz> xlist = new ArrayList<Xyz>();

public void abc() {
xlist.add(new Xyz());

Collections.sort((List) xlist);
}
}

Here are the warnings I get about sort():

Xyz.java:9: warning: [unchecked] unchecked conversion
found : java.util.List
required: java.util.List<T>
Collections.sort((List) xlist);
^
Xyz.java:9: warning: [unchecked] unchecked method invocation: <T>sort(java.util.List<T>) in java.util.Collections is applied to (java.util.List)
Collections.sort((List) xlist);
^
2 warnings

Thanks.

 

[Chris Uppal]

class Xyz {
private List<Xyz> xlist = new ArrayList<Xyz>();

public void abc() {
xlist.add(new Xyz());

Collections.sort((List) xlist);

Why the cast to (List) ? Collections.sort is a generic operation nowadays.

I think that either you have to make class Xyz implement Comparable<Xyz> (or
Comparable<something that includes Xyz-s>, and then just say:

Collections.sort(xlist);

Or else use the other flavour of Collection.sort() that takes a list of
<anything> plus a custom Comparator<>.

-- chris

 

[Rusty Wright]

Bingo, that's the concept I was missing. Many thanks.