Tired of the following pattern?
value = map.get(key);
if (value == null) {
// Map value lazy initialization in act
value = /* Init value */;
map.put(key, value);
}
// Use value
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Java2html
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Here is the elegant solution from the Java 8:
import java.util.HashMap;
import java.util.Map;
public class MapInit {
public static void main(String[] args) {
Map<String, String> map = new HashMap<>();
String ret;
// Lambda function is called here as there is no value with "theKey" in the map
// The same newly added value returned by the lambda function is returned
ret = map.computeIfAbsent("theKey", key -> "theValue");
System.out.println(ret);
System.out.println(map);
// Lambda function isn't called here as there is the value with "theKey" in the map
// Instead, the value with "theKey" is acquired from the map and returned
ret = map.computeIfAbsent("theKey", key -> "theValue");
System.out.println(ret);
System.out.println(map);
// The "mapping function" is called here as there is no value with "theInitiatedKey" in the map
// The same newly allocated by the mapping function value is returned
ret = map.computeIfAbsent("theInitiatedKey", MapInit::valueInitiator);
System.out.println(ret);
System.out.println(map);
// The "mapping function" isn't called here as there is the value with "theInitiatedKey" in the map
// Instead, the value with "theInitiatedKey" is acquired from the map and returned
ret = map.computeIfAbsent("theInitiatedKey", MapInit::valueInitiator);
System.out.println(ret);
System.out.println(map);
}
private static String valueInitiator(String key) {
// The function is called if only the value with this key is absent from the map
System.out.println("Initiating 'theInitiatedValue'");
return "theInitiatedValue";
}
}
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Java2html
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The output:
theValue
{theKey=theValue}
theValue
{theKey=theValue}
Initiating 'theInitiatedValue'
theInitiatedValue
{theKey=theValue, theInitiatedKey=theInitiatedValue}
theInitiatedValue
{theKey=theValue, theInitiatedKey=theInitiatedValue}
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Inspired by the StackOverflow's question
Java map.get(key) - automatically do put(key) and return if key doesn't exist?. Thanks for the solution to Roger Lindsjö.
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