Question

I have the following code

public class Test{

private static final String key = "key";

public static void main(String[] a){
    ConcurrentHashMap<String,String > map = new ConcurrentHashMap<String,String>();
    System.out.println(map.replace(key,"1"));
    System.out.println(map.replace(key,"2"));
}

}

The output is null both times. Isn’t it supposed to be 1 second time?

Answer 1

The doc says:

Replace entry for key only if
currently mapped to some value. Acts
as

if ((map.containsKey(key)) 
    return map.put(key, value);   
else
    return null;

Hence no, the first replace doesn’t do the put.