Cannot instantiate the type Set

问题:

I am trying to create a Set of Strings which is filled with the keys from a Hashtable so a for-each loop can iterate through the Set and put defaults in a Hashtable. I am still learning Java but the way I am trying to do it isn't valid syntax. Could someone please demonstrate the proper way of doing this and explain why my way doesn't work and theirs does.

private Hashtable<String, String> defaultConfig() {
    Hashtable<String, String> tbl = new Hashtable<String, String>();
    tbl.put("nginx-servers","/etc/nginx/servers");
    tbl.put("fpm-servers","/etc/fpm/");
    tbl.put("fpm-portavail","9001");
    tbl.put("webalizer-script","/usr/local/bin/webalizer.sh");
    tbl.put("sys-useradd","/sbin/useradd");
    tbl.put("sys-nginx","/usr/sbin/nginx");
    tbl.put("sys-fpmrc","/etc/rc.d/php_fpm");
    tbl.put("www-sites","/var/www/sites/");
    tbl.put("www-group","www"); 
    return tbl;
}

//This sets missing configuration options to their defaults.
private void fixMissing(Hashtable<String, String> tbl) {
    Hashtable<String, String> defaults = new Hashtable<String, String>(defaultConfig());
    //The part in error is below...
    Set<String> keys = new Set<String>(defaults.keySet());

    for (String k : keys) {
        if (!tbl.containsKey(k)) {
            tbl.put(k, defaults.get(k));
        }
    }
}

回答1:

Set is not a class, it is an interface.

So basically you can instantiate only class implementing Set (HashSet, LinkedHashSet orTreeSet)

For instance :

Set<String> mySet = new HashSet<String>();

回答2:

Set is an interface. You cannot instantiate an interface, only classes which implement that interface.

The interface specifies behaviour, and that behaviour can be implemented in different ways by different types. If you think about it like that, it makes no sense to instantiate an interface because it's specifying what a thing must do, not how it does it.

回答3:

HashMap's keySet() method already creates the set you need, so simply:

Set<String> keys = defaults.keySet();

This is a view of the keys in defaults, so its contents will change when changes are made to the underlying (defaults) map. Changes made to keys will be reflected in the map, as well, but you can only remove...not add...keys from the map.

If you need a copy of the keys that doesn't interact with the original map, then use one of the types suggested, as in:

Set<String> keys = new HashSet( defaults.keySet() );