JAVA8 Map接口  新增方法 computeIfAbsent
public V computeIfAbsent(K key, Function< ? super K,? extends V> mappingFunction)


ConcurrentHashMap computeIfAbsent 线程安全
computeIfAbsent 不要修改集合 （put 等操作） ，可能导致死循环




import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.atomic.AtomicLong;
import java.util.function.Function;

public class ComputeIfAbsentTest {
    static Map<String,AtomicLong> map = new HashMap<>();
    public static void main(String[] args) throws Exception {
        String key = "a";
        System.out.println(increment(key)); 
        System.out.println(increment(key));
        System.out.println(increment(key));
        
        Function<String,AtomicLong> nullResult = (k) -> {return null;};
        Function<String,AtomicLong> errorFunction = (k) -> {throw new RuntimeException("error");};
        
        key = "b";
        // null 值 不会 put 进去
        map.computeIfAbsent(key, nullResult);   
        try {
            key = "c";
            map.computeIfAbsent(key, errorFunction);
        }catch(Throwable e) {
            System.out.println(e);
        }
        System.out.println(map);
    }
    
    private static long increment(String key) {
        if(key==null) {
            return 0;
        }
        AtomicLong count = map.computeIfAbsent(key, (k)-> new AtomicLong(0));
        return count.incrementAndGet(); 
    }
}