Comparator - comparing() & reversed()

[henry leu]

On line 21: I try to reverse my comparator.  But I get compile error when calling a chained Comparator reversed() method.

Java needs me to cast it to Test type in the lambda expression as follows line #24:

Comparator<Integer> comparator = Comparator.comparing( (Test) x -> x.getNum() ).reversed();

public class Test{

              int num;

              Test(int num){
                  this.num = num;
              }
 
              public static void main(String args[]){
                     var list = new ArrayList<>();
                     list.add(12);
                     list.add(110);
                     list.add(34);
                     list.add(20);
                     list.add(0);
 
                     // This line works.
                     //Comparator<Integer> comparator = Comparator.comparing( x -> x.getNum() );

                     // Compile error when using the chained Comparator reversed()
                     Comparator<Integer> comparator = Comparator.comparing( x -> x.getNum() ).reversed();

                     // Compiles - Java needs me to cast it to type Test. Why?
                     Comparator<Integer> comparator = Comparator.comparing( (Test) x -> x.getNum() ).reversed();
         
 
                     Collections.sort(list, comparator);
                     
                     System.out.println(list);
 
              }

              public int getNum(){ return num; }
}

Compile error output when I call chained Comparator reversed() on line #21.

Test.java:21: error: cannot find symbol
                Comparator<Test> comparator = Comparator.comparing(x -> x.getNum()).reversed();
                                                                          ^
  symbol:   method getNum()
  location: variable x of type Object
Test.java:21: error: incompatible types: Comparator<Object> cannot be converted to Comparator<Test>
                Comparator<Test> comparator = Comparator.comparing(x -> x.getNum()).reversed();
                                                                                             ^
2 errors

[Paul Clapham]

You need to decide whether you want a Comparator<Integer> or a Comparator<Test>. Your posted code uses the former but your error messages are based on code which uses the latter.

(Hint: to sort a List<Integer> you need a Comparator<Integer>.)

So once you have that straightened out, could you post the corrected code and the error messages which it produces?

[Campbell Ritchie]

If the field is an int, wouldn't you use comparingInt()?

[henry leu]

Paul Clapham wrote:You need to decide whether you want a Comparator<Integer> or a Comparator<Test>. Your posted code uses the former but your error messages are based on code which uses the latter.

(Hint: to sort a List<Integer> you need a Comparator<Integer>.)

So once you have that straightened out, could you post the corrected code and the error messages which it produces?

Hi Paul,

I just updated  my code. It should be Comparator<Test>, not Comparator<Integer>. Sorry, that was a typo.

This is what I thought of last night. On line 21, the chained reversed() is too deep down the chain method call. x-> x.getNum() needs to know the type Test in order to call x.getNum()


                    Comparator<Test> comparator = Comparator.comparing( (Test) x -> x.getNum() ).reversed();

However, without the chained method call reversed(),

                    Comparator<Test> comparator = Comparator.comparing(  x -> x.getNum()); // compile ok

This line alone can inferred the Comparator<Test> from the target assignment.

Let me know whether or not my understanding is valid.

Thanks!

[Campbell Ritchie]

Please don't edit posts after they have been replied to; that makes the replies look incorrect I have reversed the edit.

[Paul Clapham]

henry leu wrote:I just updated  my code. It should be Comparator<Test>, not Comparator<Integer>. Sorry, that was a typo.

I don't understand why it should be that. After all you have a List<Integer> so there's no point in creating a Comparator<Test>.

As for your three lines of code:

Comparator<Test> comparator1 = Comparator.comparing( x -> x.getNum() );
Comparator<Test> comparator2 = Comparator.comparing( x -> x.getNum() ).reversed();
Comparator<Test> comparator3 = Comparator.comparing( (Test) x -> x.getNum() ).reversed();

Only the first of the three lines compiles for me. When I look at the tutorials for how to do reversed Comparators like that, what I see them doing is this:

Comparator<Test> comparator2 = Comparator.comparing( x -> x.getNum() )
comparator2 = comparator2.reversed();

Which compiles successfully.

However as I said, none of those three Comparators can be used in your Collections.sort line of code.