Fix #3409: Remove defects from Collectors#joining method

javalib/src/main/scala/java/util/stream/Collectors.scala

@@ -579,59 +579,33 @@ object Collectors {
       prefix: CharSequence,
       suffix: CharSequence
   ): Collector[CharSequence, AnyRef, String] = {
-    val delimiterLength = delimiter.length()
-
-    val supplier = new Supplier[StringBuilder] {
-      def get(): StringBuilder = {
-        val sb = new StringBuilder()
-        if (prefix != "")
-          sb.append(prefix)
-        sb
+
+    val supplier = new Supplier[StringJoiner] {
+      def get(): StringJoiner = {
+        new StringJoiner(delimiter, prefix, suffix)
       }
     }
 
-    val accumulator = new BiConsumer[StringBuilder, CharSequence] {
-      def accept(accum: StringBuilder, element: CharSequence): Unit = {
-        val acc = accum.append(element)
-        if (delimiter != "")
-          accum.append(delimiter)
+    val accumulator = new BiConsumer[StringJoiner, CharSequence] {
+      def accept(accum: StringJoiner, element: CharSequence): Unit = {
+        accum.add(element)
       }
     }
 
-    val combiner = new BinaryOperator[StringBuilder] {
-      def apply(
-          sb1: StringBuilder,
-          sb2: StringBuilder
-      ): StringBuilder = {
-        sb1.append(sb2)
+    val combiner = new BinaryOperator[StringJoiner] {
+      def apply(sj1: StringJoiner, sj2: StringJoiner): StringJoiner = {
+        sj1.merge(sj2)
       }
     }
 
-    val finisher =
-      new Function[StringBuilder, String] {
-        def apply(accum: StringBuilder): String = {
-
-          if ((accum.length() > prefix.length()) && (delimiterLength > 0)) {
-            /* This branch means accum has contents beyond a possible prefix.
-             * If a delimiter arg was is specified, accumlator() will have
-             * appended that delimiter. A delimiter is unwanted after what is
-             * now known to be the last item, so trim it off before possibly
-             * adding a suffix.
-             */
-            val lastIndex = accum.length() - delimiterLength
-            accum.setLength(lastIndex) // trim off last delimiter sequence.
-          }
-          // Else empty stream; no token accepted, hence no delimiter to trim.
-
-          if (suffix != "")
-            accum.append(suffix)
-
-          accum.toString()
-        }
+    val finisher = new Function[StringJoiner, String] {
+      def apply(accum: StringJoiner): String = {
+        accum.toString()
       }
+    }
 
     Collector
-      .of[CharSequence, StringBuilder, String](
+      .of[CharSequence, StringJoiner, String](
         supplier,
         accumulator,
         combiner,

unit-tests/shared/src/test/scala-3/scala/org/scalanative/testsuite/javalib/util/stream/CollectorsTest.scala

@@ -1,5 +1,6 @@
 package org.scalanative.testsuite.javalib.util.stream
 
+import java.{lang => jl}
 import java.{util => ju}
 import java.util._
 
@@ -719,6 +720,52 @@ class CollectorsTest {
     assertEquals("unexpected joined", expected, joined)
   }
 
+  // Issue #3409
+  @Test def collectorsJoining_Merge(): Unit = {
+    /* The idea is to test that a delimiter is added between the
+     * two arguments when Collectors.joining() merge() method is called.
+     *
+     * One would not normally call merge() directory, but writers of
+     * parallel library methods might. So the method should match its
+     * JVM description.
+     *
+     * The complexity comes from not wanting to know the actual implementation
+     * type of the accumulator A in <T, ?, R>. combiner() takes two arguments
+     * of that exact type. To get the unknown type right, each of the
+     * arguments passed to combiner() should come from the same supplier of
+     * the same Collector.joining().
+     *
+     * So far, this type fun & games is true with both JVM and Scala Native.
+     *
+     * This gets the interior implementation type correct, but also means
+     * that both arguments use the same prefix, suffix, & delimiter.
+     * Experience with parallel Collectors may show a way around this
+     * restriction/feature.
+     */
+
+    val left = "Left"
+    val right = "Right"
+    val delim = "|"
+
+    val expected = s"${left}${delim}${right}"
+
+    val collector = Collectors.joining(delim)
+
+    val supplier = collector.supplier
+    val accumulator = collector.accumulator
+    val combiner = collector.combiner
+
+    val accLeft = supplier.get()
+    accumulator.accept(accLeft, left)
+
+    val accRight = supplier.get()
+    accumulator.accept(accRight, right)
+
+    val combined = combiner.apply(accLeft, accRight).toString()
+
+    assertEquals("unexpected combined", expected, combined)
+  }
+
   @Test def collectorsMapping(): Unit = {
     /* This implements one of the examples in the Java 19 description of the
      * java.util.Collectors class: