When to use Properties and when Map in Java?

Question

Difference between Map and Properties as both have key-value pair.

Answer 1

Use a class that's implementing the Map interface, but isn't the Properties class. The Properties class is broken. Since it's a Hashtable underneath, there are methods that would break the otherwise consistent String-to-String mapping. Its invariant is very easily broken (both mistakenly and on purpose). Use anything else implementing the Map interface. If you're aiming for thread safety - use Collections.synchronizedMap(myUnsyncdMap) to create one.

Ideally, there is no difference between the two classes other than 1 or two methods mentioned in the other answers. However the Properties implementation is actually bizarre - other classes, deriving from the Map interface, are much safer, provide better statical typing and are probably faster (depending on what you need). Taking a quick look at the implementation (specifically which class it derives from) should be enough to convince you to avoid it :)

Answer 2

Properties is a Facade for Map<String,String> + some I/O methods.

Do you need the I/O methods ? use it : don't.

Answer 3

A per key default system is another feature of Properties not present in Hashtable is . E.g.:

Properties p0 = new Properties();
p0.setProperty("a", "0");
p0.setProperty("b", "1");
Properties p1 = new Properties(p0);
p1.setProperty("a", "10");
assert p1.getProperty("a").equals("10");
assert p1.getProperty("b").equals("1");
assert p1.getProperty("c") == null;

Defaults are searched recursively in parent properties.