Good way to clear nested Maps in Java

问题:

public class MyCache {

    AbstractMap<String, AbstractMap<String, Element>> cache = new TreeMap<String, AbstractMap<String,
            Element>>();

    public Boolean putElement(String targetNamespace, Element element) {
        ...
    }

    public void clear() {
        cache.clear();
    }

    // is it better this way?

    public void deepClear() {
        for(AbstractMap<String, Element> innerMap : cache.values()) {
            innerMap.clear();
        }
        cache.clear();
    }
}

Is it necessary to iterate over the values of the root map and clear all the maps nested in the root first, or is it enough to clear the outermost map? My main question is, if there is any difference in the memory consumption of the JVM between these two methods?

回答1:

The two approaches are identical. Assuming no objects (beside the maps themselves) hold references to the nested maps, once the first map is cleared, all the maps it contained are eligible to be garbage collected and so are, consequently, all objects that those maps held references to.