public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)


import java.util.Date;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicLong;

public class ComputIfAbsentTest {
	
	private static Map<String,AtomicLong> map = new ConcurrentHashMap<String,AtomicLong>();
	
	private static void add(String key,long value){
		AtomicLong al = map.get(key);
		if(al==null){
			al = new AtomicLong();
			map.put(key, al);
		}
		al.addAndGet(value);
	}
	
	private static void addUseComputeIfAbsent(String key,long value){
		map.computeIfAbsent(key, k-> new AtomicLong()).addAndGet(value);
	}
	
	public static void main(String[] args) throws Exception {
		System.out.println("ComputIfAbsentTest start "+new Date());		
		add("a",2);
		addUseComputeIfAbsent("b",3);	
		System.out.println(map);
	}
	
}