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copied!<p><code>java.util.Properties</code> implements the <code>java.util.Map</code> interface, and so you <em>can</em> just treat it as such, and use methods like <code>putAll</code> to add the contents of another <code>Map</code>.</p> <p>However, if you treat it like a Map, you need to be very careful with this:</p> <pre><code>new Properties(defaultProperties); </code></pre> <p>This often catches people out, because it <em>looks</em> like a copy constructor, but it isn't. If you use that constructor, and then call something like <code>keySet()</code> (inherited from its <code>Hashtable</code> superclass), you'll get an empty set, because the <code>Map</code> methods of <code>Properties</code> do not take account of the default <code>Properties</code> object that you passed into the constructor. The defaults are only recognised if you use the methods defined in <code>Properties</code> itself, such as <code>getProperty</code> and <code>propertyNames</code>, among others.</p> <p>So if you need to merge two Properties objects, it is safer to do this:</p> <pre><code>Properties merged = new Properties(); merged.putAll(properties1); merged.putAll(properties2); </code></pre> <p>This will give you more predictable results, rather than arbitrarily labelling one of them as the "default" property set.</p> <p>Normally, I would recommend not treating <code>Properties</code> as a <code>Map</code>, because that was (in my opinion) an implementation mistake from the early days of Java (Properties should have contained a <code>Hashtable</code>, not extended it - that was lazy design), but the anemic interface defined in <code>Properties</code> itself doesn't give us many options.</p>

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