Java Language
- Getting started with Java Language
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- Java Pitfalls - Performance Issues
- Pitfall - Calling 'new String(String)' is inefficient
- Pitfall - Calling System.gc() is inefficient
- Pitfall - Efficiency concerns with regular expressions
- Pitfall - Interning strings so that you can use == is a bad idea
- Pitfall - Iterating a Map's keys can be inefficient
- Pitfall - Over-use of primitive wrapper types is inefficient
- Pitfall - Small reads / writes on unbuffered streams are inefficient
- Pitfall - String concatenation in a loop does not scale
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- Pitfall - Using 'new' to create primitive wrapper instances is inefficient
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Java Language Java Pitfalls - Performance Issues Pitfall - Iterating a Map's keys can be inefficient
Example
The following example code is slower than it needs to be :
Map<String, String> map = new HashMap<>();
for (String key : map.keySet()) {
String value = map.get(key);
// Do something with key and value
}
That is because it requires a map lookup (the get() method) for each key in the map. This lookup may not be efficient (in a HashMap, it entails calling hashCode on the key, then looking up the correct bucket in internal data structures, and sometimes even calling equals). On a large map, this may not be a trivial overhead.
The correct way of avoiding this is to iterate on the map's entries, which is detailed in the Collections topic
