Extend Iterable instead of implementing it.

sdk/lib/utf/utf16.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 
 /**
  * Decodes the UTF-16 bytes as an iterable. Thus, the consumer can only convert
  * as much of the input as needed. Determines the byte order from the BOM,
  * or uses big-endian as a default. This method always strips a leading BOM.
  * Set the [replacementCodepoint] to null to throw an ArgumentError
@@ skipping 197 matching lines @@
   }
 }
 
 typedef _ListRangeIterator _CodeUnitsProvider();
 
 /**
  * Return type of [decodeUtf16AsIterable] and variants. The Iterable type
  * provides an iterator on demand and the iterator will only translate bytes
  * as requested by the user of the iterator. (Note: results are not cached.)
  */
-class IterableUtf16Decoder implements Iterable<int> {
+class IterableUtf16Decoder extends Iterable<int> {

[Ivan Posva, 2012/11/26 18:13:01]

Please remember by extending rather than implementing you add unnecessary type
arguments on the class extending the parameterized interface.

This ripples all the way down into the implementation causing you to allocate
two objects (object and its type arguments) for each new IterableUtf16Decoder.

[floitsch, 2012/11/28 14:16:01]

Added TODO.

[Lasse Reichstein Nielsen, 2012/11/28 21:29:35]

Is there any way that this could be made cheaper?
E.g., when the class and its superclass always have the same type
parameterization, and since type parameters are final on the object, could it be
stored only once?

   final _CodeUnitsProvider codeunitsProvider;
   final int replacementCodepoint;
 
   IterableUtf16Decoder._(this.codeunitsProvider, this.replacementCodepoint);
 
   Utf16CodeUnitDecoder iterator() =>
       new Utf16CodeUnitDecoder.fromListRangeIterator(codeunitsProvider(),
           replacementCodepoint);
 }
 
@@ skipping 114 matching lines @@
       skip();
     }
   }
 
   int decode() {
     int lo = utf16EncodedBytesIterator.next();
     int hi = utf16EncodedBytesIterator.next();
     return (hi << 8) + lo;
   }
 }