Collections Sort and Binarysearch and lambda comparator

Question

I have an object Customer with an attribute id that is an int. I'm trying to use Collections builtin sort and search features with a lambda comparator. My question is, why does this seem to work:

Collections.sort(customers, (a, b) -> a.getID() - b.getID());

But this does not:

foundIndex = Collections.binarySearch(customers, custID, 
                    (a, b) -> a.getID() - b.getID());

When I say 'seem to work' I mean it runs with no errors. Specifically with the second line, Eclipse has a problem with .getID() . It gave me a casting suggestion, which I tried:

foundIndex = Collections.binarySearch(customers, custID, 
                    (a, b) -> ((Customer) a).getID() - ((Customer) b).getID());

But that ran into a runtime error that said "Integer can not be cast to class Customer" When I try to cast inside the (a, b) Eclipse doesn't like that either. Will any lambda do what I want in the binarySearch part?

binarySearch expects a value of the same type as your list elements, so you need to pass it a customer, not a customer's ID. Admittedly, I would be inclined to agree that the API is flawed (accepting a value of the key type makes more sense here), but that's Java for you. — Silvio Mayolo
– Silvio Mayolo, Commented Jul 10, 2021 at 22:36
omg, Thank you so much! That's all it was. Works great now. I found it so weird there is so much help regarding lambdas and sort, but nothing really on search. Again, thank you! — aegarten
– aegarten, Commented Jul 10, 2021 at 22:53

Maurício Avellar · Accepted Answer · 2021-07-10 23:46:43Z

4

Have you tried providing an instance of the Customer class instead of an ID?

foundIndex = Collections.binarySearch(customers, customer, 
                    (a, b) -> a.getID() - b.getID());

answered Jul 10, 2021 at 23:46

Maurício Avellar

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1 Comment

aegarten Over a year ago

Yes, as per Silvio Mayolo's comment and my answer below.

aegarten · Accepted Answer · 2021-07-11 00:18:36Z

0

As commenter Silvio Mayolo stated, the problem isn't really the lambda it is what is expected by the key argument in Collections.binarySearch().

This works exactly as I wanted:

//assumes presorted customers ArrayList, thank you commenter gOOse
public Customer findCustomer(int custID) {
        int foundIndex;
        Customer key = new Customer(custID, null);
        
        readLock.lock();
        try {
            foundIndex = Collections.binarySearch(customers, key, 
                (a, b) -> a.getID() - b.getID());       
            return (foundIndex > -1) ? customers.get(foundIndex) : null;
        }
        finally { readLock.unlock(); }
    }

edited Jul 11, 2021 at 0:18

answered Jul 10, 2021 at 22:56

aegarten

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2 Comments

g00se Over a year ago

Don't forget that collections need to be sorted before binary searches work properly

aegarten Over a year ago

Yes, this answer assumes the list is already sorted. This code is directly from what I am using, and the list is already sorted.

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