Re: GWT vs js performance: Collections and Strings

GWT already optimizes a HashMap<String, ...> similar to what you have implemented. You can see this in [1] and the specialized methods if the key type is String.class. Also ArrayList is backed by a native JS array. Of course GWT carries some overhead because it emulates the full Java API but because it's already quite optimized you don't see a huge improvement when switching from Java to JavaJsCollection. The largest improvement is on IE but IMHO thats not a surprise.

StringBuilder in GWT is just appending strings and does not use any buffer array like in real Java. It is basically s += toAppend which is pretty fast in browsers.

The biggest difference I see is that the Java / JavaJsCollection version use String.toCharArray() which actually creates a new array, walks the string and fills the array. It is probably faster to do

for (int i = 0; i < string.length(); i++) { 

  char c = string.charAt(i); 

  ....

}

because Java's String.charAt(i) is directly mapped to JavaScript's String.charCodeAt(i) (see [2]) and you avoid the char array creation.

[1] https://gwt.googlesource.com/gwt/+/master/user/super/com/google/gwt/emul/java/util/AbstractHashMap.java

[2] https://gwt.googlesource.com/gwt/+/master/user/super/com/google/gwt/emul/java/lang/String.java#609

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