Kotlin Extension Functions further uses

1. Extension for List<T>: List<T>.secondOrNull()

• Purpose: This function is designed to safely retrieve the second element of a list if it exists.
• Extension Function: As an extension function, secondOrNull can be called on any List<T> instance, where T is a generic type representing the type of elements in the list.
• Return Type: The function returns an element of type T if the list has a second element, otherwise null.

Implementation

fun <T> List<T>.secondOrNull(): T? = if (this.size >= 2) this[1] else null

• The function first checks if the size of the list is greater than or equal to 2 (this.size >= 2).
• If the list has two or more elements, it returns the second element (this[1]). In Kotlin, list indices start at 0, so the second element is at index 1.
• If the list has fewer than two elements, it returns null.

Usage

override fun getSecondOrNullCustomer(): Customer? {
        return generateCustomers().secondOrNull()
    }

2. Extension for Int: Int.isEven

fun Int.isEven(): Boolean = this % 2 == 0

• Purpose: This function determines whether an integer (Int) is even.
• Extension Function: Being an extension function, isEven can be called on any instance of Int, like a method of Int itself.
• Return Type: The function returns a Boolean value (true or false).

Implementation

this % 2 == 0

• this refers to the instance of Int on which the function is called.
• The expression performs a modulo operation (%) to find the remainder when the integer is divided by 2.
• If the remainder is 0 (== 0), the number is even, so the function returns true. Otherwise, it returns false.

Usage

override fun getCustomersByAgeIsEven(): List<Customer> {
  return generateCustomers().filter { customer -> customer.age.isEven() }
}

3. Extension for Customer.isEmailValid

fun Customer.isEmailValid(): Boolean {
    return email.contains("@") && email.contains(".")
}

• Purpose: To determine if the email attribute of a Customer object is in a valid format.
• Extension Function: As an extension function, isEmailValid can be invoked on any instance of Customer, treating it like a method of Customer.
• Return Type: It returns a Boolean value, true if the email is valid, and false otherwise.

Implementation

    return email.contains("@") && email.contains(".")

• The function checks two conditions in the email string of the Customer:
  • email.contains("@"): Checks if the email contains the “@” symbol, which is a standard part of an email address.
  • email.contains("."): Checks if the email contains a period “.”, which is typically present in the domain part of an email address.
• Both conditions must be met (&&) for the function to return true.

Usage

fun Customer.isValidCustomer(): Boolean {
    return id != null && isEmailValid() // Add more conditions as needed
}

description:

4. Extension for Customer.isValidCustomer

fun Customer.isValidCustomer(): Boolean {
    return id != null && isEmailValid() // Add more conditions as needed
}

• Purpose: To determine if a Customer object is valid based on specified criteria.
• Extension Function: This function can be called on any instance of Customer as if it were a method inherent to the Customer class.
• Return Type: The function returns a Boolean value: true if the customer is considered valid, false otherwise.

Implementation

    return id != null && isEmailValid() // Add more conditions as needed

• id != null: Checks whether the id property of the Customer is not null, implying that a valid Customer should have an assigned identifier.
• isEmailValid(): Calls another extension function, presumably defined elsewhere, that checks if the customer’s email is in a valid format.
• The function returns true only if both conditions are satisfied (&& operator).
• The comment “// Add more conditions as needed” suggests that the function is designed to be flexible and can be extended to include additional validity checks.

Usage

   override fun saveCustomer(customerDto: CustomerDto): Customer {
        //map to entity
        val customer = customerDto.toEntity()
        if(customer.isValidCustomer()) {
            //for testing purpose log and return
            logger.info("Customer $customer")
            return customer
        }else{
            throw error("Invalid Email")
        }
    }

This text is another installment in the series of posts about Kotlin extensions.

Github: https://github.com/Kotlin-server-squad/articles