Kotlin Koans—Part 16

This portion of the Kotlin Koans tutorial continued with collection operations. Again, many of these operations are available in JDK8, but are missing in Android Java and previous versions of JDK. I got a few different demonstrations about what you can do with Kotlin here.

Operator Overloading

Kotlin supports operator overloading. I think this was a good move. C++ has operator overloading, but it was left out in Java. At the time, some people brought up that operator overloading was a source of bugs and it impacted code readability. I used to be one of those people, but frankly, I have been swayed back towards operator overloading. Operator overloading cuts down on verbosity in your code, and I can’t truthfully say I have seen additional bugs do to operator overloading.

Here is the first portion of Kotlin code

fun Customer.isFrom(city: City): Boolean {
    // Return true if the customer is from the given city
    return this.city == city
}

All

Kotlin collections have an all { } method that accepts a predicate. You can use it check if all items in a collection match the criteria specified in the predicate.

fun Shop.checkAllCustomersAreFrom(city: City): Boolean {
    // Return true if all customers are from the given city
    return customers.all { it.city == city }
}

Any

The any { } method is similar to all, but only one item in the collection has to match the predicate for it to return true.

 fun Shop.hasCustomerFrom(city: City): Boolean {
    // Return true if there is at least one customer from the given city
    return customers.any{ it.city == city }
}

Count

Kotlin collections have a count method also that returns the number of items that match the criteria provided in the predicate.

fun Shop.countCustomersFrom(city: City): Int {
    // Return the number of customers from the given city
    return customers.count { it.city == city }
}

Find Any

We also have a find any method which returns an item that matches the criteria in the predicate.

fun Shop.findAnyCustomerFrom(city: City): Customer? {
    // Return a customer who lives in the given city, or null if there is none
    return customers.find { it.city == city }
}

it

You may have noticed it popping up in all of these lambda expressions. As long as a SAM interface has only one parameter, developers can use a special it variable to refer to that parameter.

You can click here to see Part 15