Why does java.util.Properties implement Map and not Map

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Because they did it in a hurry in the early days of Java, and didn’t realise what the implications would be four versions later.

Generics were supposed to be part of the design of Java from the beginning, but the feature was dropped as being too complicated and, at the time, unnecessary. As a result, lots of code in the standard libraries is written with the assumption of non-generic collections. It took the prototype language “Pizza” from Martin Odersky to show how they could be done fairly well while maintaining near perfect backwards compatibility, with both Java code and bytecode. The prototype led to Java 5, in which the collections classes were retrofitted with generics in a way that allowed old code to keep working.

Unfortunately, if they were to retroactively make Properties inherit from Map<String, String>, then the following previously valid code would stop working:

Map<Object, Object> x = new Properties()
x.put("flag", true)

Why anybody would do that is beyond me, but Sun’s commitment to backwards compatibility in Java has gone beyond heroic into the pointless.

What’s now appreciated by most educated observers is that Properties should never have inherited from Map at all. It should instead wrap around Map, exposing only those features of Map that make sense.

Since reinventing Java, Martin Odersky has gone on to create the new Scala language, which is cleaner, inherits fewer mistakes, and breaks new ground in a number of areas. If you’re finding Java’s niggles annoying, take a look at it.

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