Accept both 1.x and x for 1.1 to 1.8

main/src/main/scala/sbt/Defaults.scala

@@ -160,9 +160,9 @@ object Defaults extends BuildCommon {
       scalaHome :== None,
       apiURL := None,
       javaHome :== None,
-      discoveredJavaHomes := sbt.internal.CrossJava.discoverJavaHomes,
+      discoveredJavaHomes := CrossJava.discoverJavaHomes,
       javaHomes :== ListMap.empty,
-      fullJavaHomes := discoveredJavaHomes.value ++ javaHomes.value,
+      fullJavaHomes := CrossJava.expandJavaHomes(discoveredJavaHomes.value ++ javaHomes.value),
       testForkedParallel :== false,
       javaOptions :== Nil,
       sbtPlugin :== false,

main/src/main/scala/sbt/internal/CrossJava.scala

@@ -10,7 +10,6 @@ package internal
 
 import java.io.File
 import scala.collection.immutable.ListMap
-import sbt.io.IO
 import sbt.io.syntax._
 
 private[sbt] object CrossJava {
@@ -44,6 +43,17 @@ private[sbt] object CrossJava {
     }
   }
 
+  // expand Java versions to 1-8 to 1.x, and vice versa to accept both "1.8" and "8"
+  private val oneDot = Map((1 to 8).toVector flatMap { i =>
+    Vector(s"$i" -> s"1.$i", s"1.$i" -> s"$i")
+  }: _*)
+  def expandJavaHomes(hs: Map[JavaVersion, File]): Map[JavaVersion, File] =
+    hs flatMap {
+      case (k, v) =>
+        if (oneDot.contains(k.version)) Vector(k -> v, k.withVersion(oneDot(k.version)) -> v)
+        else Vector(k -> v)
+    }
+
   def wrapNull(a: Array[String]): Vector[String] =
     if (a eq null) Vector()
     else a.toVector

sbt/src/sbt-test/java/home-discovery/build.sbt

@@ -1,5 +1,5 @@
 Global / javaHomes += JavaVersion("6") -> file("/good/old/times/java-6")
 
 TaskKey[Unit]("check") := {
-  assert(fullJavaHomes.value(JavaVersion("6")).getAbsolutePath.contains("java-6"))
+  assert(fullJavaHomes.value(JavaVersion("1.6")).getAbsolutePath.contains("java-6"))
 }