[Java] Safe way of exposing keySet().
- by Jake
This must be a fairly common occurrence where I have a map and wish to thread-safely expose its key set:
public MyClass {
Map<String,String> map = // ...
public final Set<String> keys() {
// returns key set
}
}
Now, if my "map" is not thread-safe, this is not safe:
public final Set<String> keys() {
return map.keySet();
}
And neither is:
public final Set<String> keys() {
return Collections.unmodifiableSet(map.keySet());
}
So I need to create a copy, such as:
public final Set<String> keys() {
return new HashSet(map.keySet());
}
However, this doesn't seem safe either because that constructor traverses the elements of the parameter and add()s them. So while this copying is going on, a ConcurrentModificationException can happen.
So then:
public final Set<String> keys() {
synchronized(map) {
return new HashSet(map.keySet());
}
}
seems like the solution. Does this look right?
