Java: shortest way to reverse Properties

Question

I java a java.util.Properties object and I want to obtain another one with the same pairs but keys are converted to values and viceversa.

If there are collision (i.e. there are two equal values) then just pick as value an arbitrary key.

What is the shortest way to do it.

Feel free to use libraries, commons-collections, or whatever.

Answer 1

A Properties object is a Hashtable object, so you should be able to do something like:

Hashtable<String, String> reversedProps = new Hashtable<String, String>();

for (String key : props.keySet()) {
    reversedProps.put(props.get(key), key);
}

Result: 3 lines of code.

This code is untested, but it should give you the idea.

Answer 2

Something like:

Properties fowards = new Properties();
fowards.load(new FileInputStream("local.properties"));

Properties backwards = new Properties();

for (String propertyName : fowards.stringPropertyNames())
{
  backwards.setProperty(forwards.get(propertyName), propertyName);
}

Answer 3

You can consider using a BiMap by google collections which is essentially a reversable Map. It guarantees uniquness of keys as well as values.

Check it out here. This is the API