Why does setProperty in Java return Object?

Question

The Properies class in Java SE 6 has a method called setProperty(String key, String value), which returns an Object. Furthermore, the previous Object stored for this key, or NULL if none exists. Since setProperty(String key, String value) can only take a String as value, why doesn't that method return a String?

Answer 1

Unfortunately class java.util.Properties was introduced into java 1.0, many years before generics. Properties extends Hashtable that can store any type of data. So, you can do the following:

Properties props = new Properties();
props.put("key", new Object());  // use Hashtable's put method
props.setProperty("key", "value"); // use Proerties' setProperty method

In this example setProperty must return the previous value stored in this entry, i.e. Object. But it is not String! To avoid ClassCastException the JDK creators had to define setProperty() as method that returns Object.

BTW even now class Properties implements Map<Object, Object> instead of Map<String, String> for backwards compatibility.

Answer 2

Because Properties was misconceived from the start by extending Hashtable, which can store anything. The design thus didn't respect the Liskov substitution principle: everything a base class can do, a subclass must be able to do.

Since Properties extends Hashtable, you can in fact store any kind of Object in it.