Why is this an issue?

Functional interfaces can be instantiated from lambda expressions directly. If the only purpose of a class or a singleton object is to implement a functional interface, that class or object is redundant and should be replaced with a lambda expression.

What is the potential impact?

Complexity

When an interface is declared functional, SAM conversion is enabled for that interface. This means that any lambda expression that matches the interface’s single function’s signature can be converted into an instance of the interface without the need for an explicit class or singleton object to implement the interface. This change makes the code more concise and easier to read.

How to fix it

Replace the class or singleton object with a lambda expression that implements the interface’s single function.

Code examples

Noncompliant code example

fun interface ProgressCallback {
    fun progressChanged(percent: Double)
}

fun loadResource(callback: ProgressCallback) {
    // ...
}

val resource = loadResource(object: ProgressCallback { // Noncompliant
    override fun progressChanged(percent: Double) {
        // ...
    }
})

val callback = object: ProgressCallback {
    override fun progressChanged(percent: Double) { // Noncompliant
        // ...
    }
}

Compliant solution

val resource = loadResource() { // Compliant
    // ...
}

val callback = ProgressCallback { // Compliant
    // ...
}