In Java, everything extends Object, so something like List<Object> will let you put anything you want into it. I should play with more languages, I didn't realize that was an uncommon feature.
Good, because types and classes are different things despite what languages like Java and my beloved C# would have us to think.
"Object" as in Java is a type for any tagged box type. Primitives, such as "int", don't derive from Object, but can be promoted to a corresponding boxed type, such as "Integer".
The story is a little more complex in C#, where ValueType derives from Object and "value types", both primitive and user-defined, are truly subclasses of Object. Types not derived from ValueType are considered reference types.
In languages where genericity is supported, you don't need a common base class with a pre-defined set of methods, as you can give type constraints.
Java could also have done it with interfaces, but those were the days OO was becoming mainstream and interface (component) based programming wasn't yet well understood.