import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ForkJoinPool;
public class ConcurrentHashMapDemo extends ConcurrentUtils {
/*
* Java 8 introduces three kinds of parallel operations: forEach, search and
* reduce. Each of those operations are available in four forms accepting
* functions with keys, values, entries and key-value pair arguments.
*
* All of those methods use a common first argument called
* parallelismThreshold. This threshold indicates the minimum collection
* size when the operation should be executed in parallel. E.g. if you pass
* a threshold of 500 and the actual size of the map is 499 the operation
* will be performed sequentially on a single thread.
*/
public static void main(String[] args) {
// Can change: -Djava.util.concurrent.ForkJoinPool.common.parallelism=5
System.out.println(ForkJoinPool.getCommonPoolParallelism()); // 3
ConcurrentHashMap map = new ConcurrentHashMap<>();
map.put("foo", "bar");
map.put("han", "solo");
map.put("r2", "d2");
map.put("c3", "p0");
/*
* use a threshold of one to always force parallel execution for
* demonstrating purposes.
*/
map.forEach(1, (key, value) -> System.out.printf(
"key: %s; value: %s; thread: %s\n", key, value, Thread
.currentThread().getName()));
// single thread
System.out.println("single thread");
map.forEach((key, value) -> System.out.printf(
"key: %s; value: %s; thread: %s\n", key, value, Thread
.currentThread().getName()));
System.out.println("search foo:");
/*
* If multiple entries of the map match the given search function the
* result may be non-deterministic.
*/
String result = map.search(1, (key, value) ->
{
System.out.println(Thread.currentThread().getName());
if ("foo".equals(key)) {
return value;
}
return null;
});
System.out.println("Result: " + result); // bar
System.out.println("searchValues:");
result = map.searchValues(1, value -> {
System.out.println(Thread.currentThread().getName());
if (value.length() > 3) {
return value;
}
return null;
});
System.out.println("Result: " + result);
System.out.println("Reduce:");
result = map.reduce(1,
(key, value) -> {
System.out.println("Transform: " + Thread.currentThread().getName());
return key + "=" + value;
},
(s1, s2) -> {
System.out.println("Reduce: " + Thread.currentThread().getName());
return s1 + ", " + s2;
});
System.out.println("Result: " + result);
}
}