Merging overrides with defaults

[Alec Swan]

I have a property file containing default configurations in a key=value
format. I also have another file with the same format which contains
overrides for some of the properties. I need to apply the overrides to the
default configuration while preserving the format and comments. I also need
to write the merged content to a string.

I am using the following code, but the mergedContent contains the default
content and no overrides. I tried reversing the order in which I add
defaultConfig and overridingConfig to CompositeConfiguration, but still no
luck.

            PropertiesConfiguration mergedConfig = new
PropertiesConfiguration();
            CompositeConfiguration compositeConfig = new
CompositeConfiguration(defaultConfig);
            mergedConfig = defaultConfig;

            compositeConfig.addConfiguration(overridingConfig);

            StringWriter writer = new StringWriter();
            mergedConfig.save(writer);
            String mergedContent = writer.toString();

What's wrong with the code above?

Thanks.