Iterator - Scala

The Iterator pattern is a behavioral design pattern that provides sequential access to a collection’s elements without exposing its underlying representation. This allows traversing complex data structures without needing to know their internal workings. In Scala, the Iterator trait is built-in and extensively used, especially with collections. My example demonstrates a custom iterator for a sequence of Fibonacci numbers. It implements Iterator[Int] by maintaining the current and next values and updating them in the next() method. Using a trait and the next()/hasNext() methods is the standard Scala way to define an Iterator, seamlessly integrating with for comprehensions and other functional constructs.

trait FibonacciIterator extends Iterator[Int] {
  private var current: Int = 0
  private var nextVal: Int = 1

  def hasNext: Boolean = true

  def next(): Int = {
    val result = current
    val temp = current + nextVal
    current = nextVal
    nextVal = temp
    result
  }
}

object FibonacciSequence {
  def iterator: FibonacciIterator = new FibonacciIterator()

  def take(n: Int): Seq[Int] = iterator.take(n).toSeq

  def main(args: Array[String]): Unit = {
    println(take(10)) // Output: Seq(0, 1, 1, 2, 3, 5, 8, 13, 21, 34)
    
    //Using a for comprehension
    println("First 5 Fibonacci numbers:")
    for (fib <- iterator.take(5)) {
      println(fib)
    }
  }
}