Quick Answer

Passing functions as parameters and returning functions

Understanding the Issue

Kotlin treats functions as first-class citizens. Higher-order functions (HOFs) are functions that take other functions as parameters or return them. Lambdas provide concise syntax for function literals.

The Problem

This code demonstrates the issue:

Kotlin Error 
// Problem: Verbose anonymous class
view.setOnClickListener(object : View.OnClickListener {
    override fun onClick(v: View) {
        doSomething()
    }
})

The Solution

Here's the corrected code:

Kotlin Fixed 
// Solution: Lambda syntax
view.setOnClickListener { doSomething() }

// Custom HOF example:
fun measureTime(block: () -> Unit): Long {
    val start = System.currentTimeMillis()
    block()
    return System.currentTimeMillis() - start
}

Key Takeaways

Lambdas and HOFs enable concise functional-style programming.