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Tip: Avoiding the HashMap Size Trap

See how to avoid having the HashMap resize half way through the insertion.


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Map map = new HashMap(collection.size());
for (Object obj : collection) {
       map.put(obj.key, obj.value);
}

The code above wants to make sure that the HashMap doesn't need to be resized, so its sets the initial size to the number of elements that it will use. But, the HashMap implementation doesn't behave like this, it sets its internal threshold to threshold = (int)(capacity * loadFactor), this means that it will resize after 75% of the collection has been inserted into the map, causing extra garbage. Avoid this using the code below:

Map map = new HashMap(1+ (int) (collection.size() / 0.75));

 

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Octavia Andreea Anghel is a senior PHP developer currently working as a primary trainer for programming teams that participate at national and international software-development contests. She consults on developing educational projects at a national level. She is a coauthor of the book "XML Technologies--XML in Java" (Albastra, ISBN 978-973-650-210-1), for which she wrote the XML portions. In addition to PHP and XML, she's interested in software architecture, web services, UML, and high-performance unit tests.
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