need help with Collections

[Nick Kocher]

Hi,
I need a method that would change the keys and values of the Properties instance so that keys become values and vice versa.

I wrote my own version, but I think there must be already such a method somethere in Collections and I want to use it.

Properties pp = new Properties();
//Inverse Values and keys 
    public void inverseValues(){
      set = pp.keySet();
      Iterator itr = set.iterator();
      while(itr.hasNext()) {
        key = (String) itr.next();
        value = (String) pp.get(key);
        pp.remove(key);
        pp.put(value, key);
      }
    }  

[Stefan Krompass]

Hi,

there is no such method in the API since it would only work for a subset of all properties. Imagine the following case:

key1=value1
key2=value1

When your values become keys, they are no longer unique.

Stefan

[David Harkness]

Originally posted by Nick Kocher:
I wrote my own version, but I think there must be already such a method somethere in Collections and I want to use it.

There is no such API method that I am aware of, for it's certainly an odd thing to do to a Map/Properties (not that it couldn't be useful). Also, it has the problem that Stefan mentioned.

However, you can at least improve your method to iterate over the paired Map.Entry objects. I haven't looked at the Properties source, so I'm assuming it implements the Map interface. Finally, I recommend building a new Collection rather than modifying the one in-place.

public Properties inverseValues(Properties props)
{
    Properties newProps = new Properties();
    for ( Iterator itr = props.entrySet().iterator() ; itr.hasNext() ; )
    {
        Map.Entry entry = (Map.Entry) itr.next();
        newProps.put(entry.getValue(), entry.getKey());
    }
    return newProps;
}