Pulling values from a Java Properties file in order?

Question

I have a properties file where the order of the values is important. I want to be able to iterate through the properties file and output the values based on the order of the original file.

However, since the Properties file is backed by, correct me if I'm wrong, a Map that does not maintain insertion order, the iterator returns the values in the wrong order.

Here is the code I'm using

    Enumeration names = propfile.propertyNames();
    while (names.hasMoreElements()) {
        String name = (String) names.nextElement();
        //do stuff
    }

Is there anyway to get the Properties back in order short of writting my own custom file parser?

Answer 1

Nope - maps are inherently "unordered".

You could possibly create your own subclass of Properties which overrode setProperty and possibly put, but it would probably get very implementation-specific... Properties is a prime example of bad encapsulation. When I last wrote an extended version (about 10 years ago!) it ended up being hideous and definitely sensitive to the implementation details of Properties.

Answer 2

If you can alter the property names your could prefix them with a numeral or other sortable prefix and then sort the Properties KeySet.

Answer 3

Apache Commons Configuration might do the trick for you. I haven't tested this myself, but I checked their sources and looks like property keys are backed by LinkedList in AbstractFileConfiguration class:

public Iterator getKeys()
{
    reload();
    List keyList = new LinkedList();
    enterNoReload();
    try
    {
        for (Iterator it = super.getKeys(); it.hasNext();)
        {
            keyList.add(it.next());
        }

        return keyList.iterator();
    }
    finally
    {
        exitNoReload();
    }
}

Answer 4

It's quite simple. You just have to extend Properties and override both put() and keys() methods.

E.g.

class LinkedProperties extends Properties {

    private final LinkedHashSet<Object> keys = new LinkedHashSet<Object>();

    public Enumeration<Object> keys() {
        return Collections.<Object>enumeration(keys);
    }

    public Object put(Object key, Object value) {
        keys.add(key);
        return super.put(key, value);
    }
}

Answer 5

You must inherit also keySet() if you want to export Properties as XML:

public Set<Object> keySet() { return keys; }