集合排序和二进制搜索和 lambda 比较器

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?

Answer 1

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());

Answer 2

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(); }
    }