CODEWITHSUNDEEP
scalaint
St.Antario
St.Antario

Reputation: 27395

Why does scalac box Int?

I wrote the following very simple test:

import scala.collection.immutable.HashSet

class Test {
  def m() = {
    var s = new HashSet[Int]
    s = s + 1
  }
}

And figured out that it compiles to this code:

public class Test {
  public void m();
    Code:
       0: new           #12                 // class scala/collection/immutable/HashSet
       3: dup
       4: invokespecial #15                 // Method scala/collection/immutable/HashSet."<init>":()V
       7: astore_1
       8: aload_1
       9: iconst_1
      10: invokestatic  #21                 // Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer;
      13: invokevirtual #25                 // Method scala/collection/immutable/HashSet.$plus:(Ljava/lang/Object;)Lscala/collection/immutable/HashSet;
      16: astore_1
      17: return

  public Test();
    Code:
       0: aload_0
       1: invokespecial #30                 // Method java/lang/Object."<init>":()V
       4: return
}

As you can see, 1 is boxed before creating a new immutable set with 1 element. Why is it boxed. Int extends AnyVal, not AnyRef.

Upvotes: 4

Views: 72

Answers (1)

Andrey Tyukin
Andrey Tyukin

Reputation: 44918

Exactly, Int extends AnyVal, not AnyRef. But the immutable HashSet[A] has A as parameter, which is not specialized for primitive types, that is, it is not @specialized(Int) A or anything like that, so it can deal only with reference types that extend AnyRef, the primitive types must be boxed. This, fortunately, is hidden by the Scala compiler.

Upvotes: 6

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