Java: OOP Summary

Objects

• All objects are allocated on the heap.
• Objects consist of instance fields for the class of the object, plus instance fields for its superclasses.
• Every object is created by a class constructor. Interfaces don't have constructors.

Reference variables

• All non-primitive variables contain only references to objects on the heap. They never contain the object. C++ object variables may be either, but Java supports only object references.

Assignment

• Reference variable assignment is fast because only the reference is copied (currently 4 bytes).
• Making a copy of an object requires using clone().
• Assigning up the inheritance hierarchy is always possible without a cast. (is-a is obvious)
• Assigning down the inheritance hierarchy is only possible with a cast. Downcasting makes a run-time check to be sure it's legal. (is-a is possible)
• Assignment to an interface type variable from a reference type implements that interface is possible without a cast. (is-a is obvious).
• Assignment to an interface type variable from a reference type that does not implement that interface requires an explicit cast which causes a run-time check to make sure that the object's class really does implement this interface. (is-a is possible)
• null can be assigned to any reference type.

Inheritance and interfaces

• Every class has one superclass, except Object, which is the root class for all other classes.
• A class may implement many interfaces, but extend only one class.

Overriding and polymorphism

• Methods (non-final) may be redefined in a subclass.
• The @Override annotation provides compiler checking.
• Polymorphism: When a method is called, the overridden method corresponding to the actual object type is called, regardless of reference variable type.