The role of superclass constructors in an object’s life

All the constructors in an object’s inheritance tree must run when you make a new object.

Let that sink in.

That means every superclass has a constructor (because every class has a constructor), and each constructor up the hierarchy runs at the time an object of a subclass is created.

Saying new is a Big Deal. It starts the whole constructor chain reaction. And yes, even abstract classes have constructors. Although you can never say new on an abstract class, an abstract class is still a superclass, so its constructor runs when someone makes an instance of a concrete subclass.

The super constructors run to build out the superclass parts of the object. Remember, a subclass might inherit methods that depend on superclass state (in other words, the value of instance variables in the superclass). For an object to be fully-formed, all the superclass parts of itself must be fully-formed, and that’s why the super constructor must run. All instance variables from every class in the inheritance tree have to be declared and initialized. Even if Animal has instance variables that Hippo doesn’t inherit (if the variables are private, for example), the Hippo still depends on the Animal methods that use those variables.