JVM原生不支持尾遞歸優化，可是Scala編譯器支持

The JVM doesn’t support TCO natively, so tail recursive methods will need to rely on the Scala compiler performing the optimization.----------"Scala in Depth" 3.5.2

Jvm自己是不支持尾遞歸優化得，須要編譯器支持，而Java編譯器不支持，可是Scala支持。寫一個簡單的計算1到n的和的遞歸算法驗證一下。

public class TestTailRecursion {

    private static long sum(long n, long total) {
        if (n <= 0) {
            return total;
        }
        return sum(n - 1, total + n);
    }

    public static void main(String[] args) {
        long sum = sum(100000, 0);
        System.out.println(sum);
    }
}

10w（可能每一個機器不同）的時候棧溢出。

object TestTailRecursion {

  def sum(n: Long, total: Long): Long = {
    if (n <= 0) total
    else sum(n - 1, total + n)
  }

  def main(args: Array[String]) {
    val total = sum(10000000, 0)
    println(total)
  }
}

Scala表示毫無壓力。

能夠講Scala編譯獲得的bytecode用JavaDecompiler反編譯，看到以下：

import scala.Predef.;
import scala.runtime.BoxesRunTime;

public final class TestTailRecursion$
{
  public static final  MODULE$;
  
  private TestTailRecursion$()
  {
    MODULE$ = this;
  }
  
  public long sum(long n, long total)
  {
    for (;;)
    {
      if (n <= 0L) {
        return 
          total;
      }
      total += n;n -= 1L;
    }
  }
  
  public void main(String[] args)
  {
    long total = sum(10000000L, 0L);
    Predef..MODULE$.println(BoxesRunTime.boxToLong(total));
  }
  
  static
  {
    new ();
  }
}