Fix ClassCastException when using StringBuilder/Buffer #496

src/org/mozilla/javascript/ConsString.java

@@ -32,6 +32,12 @@ public class ConsString implements CharSequence, Serializable {
     private boolean isFlat;
 
     public ConsString(CharSequence str1, CharSequence str2) {
+        if (!(str1 instanceof String) && !(str1 instanceof ConsString)) {
+            str1 = str1.toString();
+        }
+        if (!(str2 instanceof String) && !(str2 instanceof ConsString)) {
+            str2 = str2.toString();
+        }
         left = str1;
         right = str2;
         length = left.length() + right.length();

testsrc/org/mozilla/javascript/tests/Issue1206Test.java

@@ -0,0 +1,38 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.javascript.tests;
+
+import org.junit.Test;
+import org.mozilla.javascript.Context;
+import org.mozilla.javascript.Scriptable;
+
+/**
+ * Tests the ConsString class to ensure it properly supports String, StringBuffer and StringBuilder.
+ */
+public class Issue1206Test {
+    @Test
+    public void testConsStringUsingString() {
+        Context cx = Context.enter();
+        Scriptable scope = cx.initStandardObjects(null);
+        scope.put("var1", scope, "hello");
+        cx.evaluateString(scope, "var1 = var1 + ' world'", "test", 1, null);
+    }
+
+    @Test
+    public void testConsStringUsingStringBuffer() {
+        Context cx = Context.enter();
+        Scriptable scope = cx.initStandardObjects(null);
+        scope.put("var1", scope, new StringBuffer("hello"));
+        cx.evaluateString(scope, "var1 = var1 + ' world'", "test", 1, null);
+    }
+
+    @Test
+    public void testConsStringUsingStringBuilder() {
+        Context cx = Context.enter();
+        Scriptable scope = cx.initStandardObjects(null);
+        scope.put("var1", scope, new StringBuilder("hello"));
+        cx.evaluateString(scope, "var1 = var1 + ' world'", "test", 1, null);
+    }
+}