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JDefaultDict

A Java-8 version of the Python defaultdict pattern that lazily creates entries in a map when they are needed.

JDefaultDict redirects the ConcurrentMap.get(key) method to the ConcurrentMap.computeIfAbsent(key, mappingFunction) method, using the mapping function provided to the JDefaultDict constructor.

Build Status Coverage Status

Requirements

  • Java-8
  • Maven, or you can download the jar files from Maven Central manually

Usage

From Maven

<dependency>
    <groupId>com.github.ansell.jdefaultdict</groupId>
    <artifactId>jdefaultdict</artifactId>
    <version>0.1</version>
</dependency>

Code example

Create a multi-map using JDefaultDict

ConcurrentMap<String, List<String>> defaultDict 
		= new JDefaultDict<>(k -> new ArrayList<>());
defaultDict.get("myKey1").add("myValue1");
defaultDict.get("myKey2").add("myValue2");

Wrap an existing map using JDefaultDict

ConcurrentMap<String, List<String>> unwrapped = new ConcurrentMap<>();
ConcurrentMap<String, List<String>> defaultDict 
		= new JDefaultDict<>(k -> new ArrayList<>(), unwrapped);
defaultDict.get("myKey1").add("myValue1");
defaultDict.get("myKey2").add("myValue2");

Wrap multiple layers of JDefaultDict

ConcurrentMap<String, ConcurrentMap<String, List<String>>> defaultDict 
		= new JDefaultDict<>(k1 -> new JDefaultDict(k2 -> new ArrayList<>()));
defaultDict.get("myKey1").get("yourKey1").add("myValue1");
defaultDict.get("myKey2").get("yourKey2").add("myValue2");

Lazily create the internal JDefaultDict as necessary, to avoid creating a map if it is never actually used

ConcurrentMap<String, List<String>> defaultDict 
		= new JDefaultDict<>(k -> new ArrayList<>(), k -> new ConcurrentHashMap<>());
if(testValue) {
	// Only if the code reaches here will the internal default dict map be created
	defaultDict.get("test").add(testValue);
}

Avoid using the given default and substitute another default temporarily

ConcurrentMap<String, List<String>> defaultDict 
		= new JDefaultDict<>(k -> new ArrayList<>(), k -> new ConcurrentHashMap<>());
defaultDict.computeIfAbsent("test", new CopyOnWriteArrayList<>()).add("testValue");

Thread-safe named counters

ConcurrentMap<String, AtomicInteger> defaultDict 
		= new JDefaultDict<>(k -> new AtomicInteger(0));
defaultDict.get("counter").incrementAndGet();

Related

License

Public Domain

For more information, please refer to the LICENSE file and http://unlicense.org/