8267529: StringJoiner can create a String that breaks String::equals

Reviewed-by: naoto

Author: cl4es

src/java.base/share/classes/java/lang/String.java

@@ -3239,8 +3239,12 @@ public static String join(CharSequence delimiter, CharSequence... elements) {
      */
     @ForceInline
     static String join(String prefix, String suffix, String delimiter, String[] elements, int size) {
-        int icoder = prefix.coder() | suffix.coder() | delimiter.coder();
-        long len = (long) prefix.length() + suffix.length() + (long) Math.max(0, size - 1) * delimiter.length();
+        int icoder = prefix.coder() | suffix.coder();
+        long len = (long) prefix.length() + suffix.length();
+        if (size > 1) { // when there are more than one element, size - 1 delimiters will be emitted
+            len += (long) (size - 1) * delimiter.length();
+            icoder |= delimiter.coder();
+        }
         // assert len > 0L; // max: (long) Integer.MAX_VALUE << 32
         // following loop wil add max: (long) Integer.MAX_VALUE * Integer.MAX_VALUE to len
         // so len can overflow at most once

test/jdk/java/lang/String/StringJoinTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2013, 2021, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -21,13 +21,15 @@
  * questions.
  */
 /**
- * @test @bug 5015163
+ * @test
+ * @bug 5015163 8267529
  * @summary test String merge/join that is the inverse of String.split()
  * @run testng StringJoinTest
  * @author Jim Gish
  */
 import java.util.ArrayList;
 import java.util.List;
+import java.util.StringJoiner;
 import org.testng.annotations.Test;
 
 import static org.testng.Assert.*;
@@ -111,4 +113,14 @@ public void testJoinNullStringList() {
     public void testJoinNullDelimiter() {
         String.join(null, JIM, JOHN);
     }
+
+    public void testIgnoreDelimiterCoderJoin() {
+        // 8267529: Ensure that joining zero or one latin-1 Strings with a UTF-16
+        // delimiter produce a String with a latin-1 coder, since the delimiter
+        // is not added.
+        assertEquals("", new StringJoiner("\u2013").toString());
+        assertEquals("foo", new StringJoiner("\u2013").add("foo").toString());
+        assertEquals("", String.join("\u2013"));
+        assertEquals("foo", String.join("\u2013", "foo"));
+    }
 }