Since JsonBuffer is reading character-by-character anyway, there is no need to use String.codePointAt. And doing so is incorrect

unless there is a corresponding call to Character.charCount instead of just incrementing the position by a single character.  The fix
is simply to use use String.charAt instead of String.codePointAt.

 JAVA-1793

Author: jyemin

bson/src/main/org/bson/json/JsonBuffer.java

@@ -34,11 +34,11 @@ public void setPosition(final int position) {
     }
 
     public int read() {
-        return (position >= buffer.length()) ? -1 : buffer.codePointAt(position++);
+        return (position >= buffer.length()) ? -1 : buffer.charAt(position++);
     }
 
     public void unread(final int c) {
-        if (c != -1 && buffer.codePointAt(position - 1) == c) {
+        if (c != -1 && buffer.charAt(position - 1) == c) {
             position--;
         }
     }

bson/src/test/unit/org/bson/json/JsonReaderTest.java

@@ -486,10 +486,32 @@ public void testRegularExpressionStrict() {
 
     @Test
     public void testString() {
-        String json = "\"abc\"";
+        String str = "abc";
+        String json = '"' + str + '"';
         bsonReader = new JsonReader(json);
         assertEquals(BsonType.STRING, bsonReader.readBsonType());
-        assertEquals("abc", bsonReader.readString());
+        assertEquals(str, bsonReader.readString());
+        assertEquals(AbstractBsonReader.State.DONE, bsonReader.getState());
+
+        str = "\ud806\udc5c";
+        json = '"' + str + '"';
+        bsonReader = new JsonReader(json);
+        assertEquals(BsonType.STRING, bsonReader.readBsonType());
+        assertEquals(str, bsonReader.readString());
+        assertEquals(AbstractBsonReader.State.DONE, bsonReader.getState());
+
+        str = "\\ud806\\udc5c";
+        json = '"' + str + '"';
+        bsonReader = new JsonReader(json);
+        assertEquals(BsonType.STRING, bsonReader.readBsonType());
+        assertEquals("\ud806\udc5c", bsonReader.readString());
+        assertEquals(AbstractBsonReader.State.DONE, bsonReader.getState());
+
+        str = "꼢𑡜ᳫ鉠鮻罖᧭䆔瘉";
+        json = '"' + str + '"';
+        bsonReader = new JsonReader(json);
+        assertEquals(BsonType.STRING, bsonReader.readBsonType());
+        assertEquals(str, bsonReader.readString());
         assertEquals(AbstractBsonReader.State.DONE, bsonReader.getState());
     }