Jave Map wrapper respects put/remove contract

If map contains key, always return nonEmpty.
If binding to null, return Some(null) instead of None.

src/library/scala/collection/convert/JavaCollectionWrappers.scala

@@ -332,11 +332,19 @@ private[collection] object JavaCollectionWrappers extends Serializable {
     def addOne(kv: (K, V)): this.type = { underlying.put(kv._1, kv._2); this }
     def subtractOne(key: K): this.type = { underlying remove key; this }
 
-    override def put(k: K, v: V): Option[V] = Option(underlying.put(k, v))
+    override def put(k: K, v: V): Option[V] = {
+      val present = underlying.containsKey(k)
+      val result  = underlying.put(k, v)
+      if (present) Some(result) else None
+    }
 
     override def update(k: K, v: V): Unit = { underlying.put(k, v) }
 
-    override def remove(k: K): Option[V] = Option(underlying remove k)
+    override def remove(k: K): Option[V] = {
+      val present = underlying.containsKey(k)
+      val result  = underlying.remove(k)
+      if (present) Some(result) else None
+    }
 
     def iterator: Iterator[(K, V)] = new AbstractIterator[(K, V)] {
       val ui = underlying.entrySet.iterator

test/junit/scala/jdk/CollectionConvertersTest.scala

@@ -0,0 +1,55 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala.jdk
+
+import java.util.{
+  Dictionary,
+  HashMap => JMap,
+  Hashtable => JTable,
+  Properties => JProperties,
+}
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+import scala.tools.testkit.AssertUtil.assertThrows
+
+import CollectionConverters._
+
+class CollectionConvertersTest {
+  @Test def `t11894 Map wrapper respects put contract`(): Unit = {
+    val sut = new JMap[String, String].asScala
+    assertEquals(None, sut.put("one", null))
+    assertEquals(Some(null), sut.put("one", "eins"))
+    assertEquals(Some("eins"), sut.put("one", "uno"))
+    assertEquals(Some("uno"), sut.remove("one"))
+    assertEquals(None, sut.remove("one"))
+    assertEquals(None, sut.put("one", null))
+    assertEquals(Some(null), sut.remove("one"))
+    assertEquals(None, sut.remove("one"))
+  }
+  @Test def `t11894 Dictionary wrapper disallows nulls`(): Unit = {
+    val tbl = new JTable[String, String]
+    val sut = tbl.asInstanceOf[Dictionary[String, String]].asScala
+    assertThrows[NullPointerException](sut.put("any", null))
+  }
+  @Test def `t11894 Properties wrapper enforces Strings`(): Unit = {
+    val ps  = new JProperties()
+    val sut = ps.asScala
+    assertThrows[NullPointerException](sut.put("any", null))
+    assertEquals(None, sut.put("one", "eins"))
+    assertEquals("eins", sut("one"))
+    ps.put("one", new Object())
+    assertThrows[ClassCastException](sut.put("one", "uno"))
+  }
+}