Can I create a Comparator object from a lambda expression without explicit variable?

Question

I am not yet familiar with lambda expression in Java.

Can

//create a comparator object using a Lambda expression
Comparator<Double> compareDouble = (d1, d2) -> d1.compareTo(d2);

//Sort the Collection in this case 'testList' in reverse order
Collections.sort(testList, Collections.reverseOrder(compareDouble));

be written without explicitly create a variable compareDouble?

I tried the following, but why does it not work?

//Sort the Collection in this case 'testList' in reverse order
Collections.sort(testList, Collections.reverseOrder(new Comparator<Double> ((d1, d2) -> d1.compareTo(d2))));

Thanks.

Does this help? mkyong.com/java8/java-8-lambda-comparator-example — Ray Toal
– Ray Toal, Commented Nov 24, 2017 at 18:24
This can be simplified into testList.sort(Collections.reverseOrder(Double::compareTo)); — ronhash
– ronhash, Commented Nov 24, 2017 at 18:28

rgettman · Accepted Answer · 2017-11-24 18:31:40Z

2

First, your immediate error: You forgot to place parentheses around your casting type. Try:

Collections.sort(testList, Collections.reverseOrder( (Comparator<Double>) ((d1, d2) -> d1.compareTo(d2))));

_{Edit: The above error was when the question didn't have new so it looked like a cast.}

Also, Java's type inference will work without explicitly casting your lambda expression to the necessary functional type. Try:

Collections.sort(testList, Collections.reverseOrder( (d1, d2) -> d1.compareTo(d2) ));

When the comparison operation already exists as in this case, you can make it even simpler with a method reference:

Collections.sort(testList, Collections.reverseOrder(Double::compare));

edited Nov 24, 2017 at 18:31

answered Nov 24, 2017 at 18:25

rgettman

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3 Comments

user1224441 Over a year ago

Thanks. I forgot to type in new to invoke constructor, and didn't intend to use casting. Why doesn't new work?

rgettman Over a year ago

When you're using new, you're attempting to create a new instance of something directly, but Comparator is an interface. The lambda expression itself is sufficient code.

Eugene Over a year ago

@rgettman Double already implements Comparable, a simple Collections.sort(testList, Collections.reverseOrder()) will do

shmosel · Accepted Answer · 2017-11-24 20:52:42Z

2

Double already implements Comparable, so you can use the zero-arg reverseOrder() overload:

testList.sort(Collections.reverseOrder());

Or you can reverse your custom comparator:

testList.sort((d1, d2) -> d2.compareTo(d1));

edited Nov 24, 2017 at 20:52

answered Nov 24, 2017 at 18:50

shmosel

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Comments

Ousmane D. · Accepted Answer · 2017-11-24 20:46:38Z

1

I’d go with something like:

Collections.sort(testList, Comparator.comparingDouble(Type::getDoubleValue).reversed());

Where Type is the name of your class and getDoubleValue is the double value being used as the sort key.

Another shorter alternative.

testList.sort(Comparator.comparingDouble(Type::getDoubleValue).reversed());

edit

I think I’ve misinterpreted your current problem. Nevertheless, altering your current solution to:

Collections.sort(testList, Collections.reverseOrder((e, a) -> e.compareTo(a)));

Would suffice.

It’s just a matter of taking the behavior you’ve assigned to the variable compareDouble and directly pass it into the reverseOrder method.

edited Nov 24, 2017 at 20:46

answered Nov 24, 2017 at 18:27

Ousmane D.

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