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There are methods that a subclass cannot override. For example, in Java, a method that is declared final in the super class cannot be overridden. Methods that are declared private or static cannot be overridden either because they are implicitly final. It is also impossible for a class that is declared final to become a super class. [9]
In object-oriented programming, a covariant return type of a method is one that can be replaced by a "narrower" (derived) type when the method is overridden in a subclass. A notable language in which this is a fairly common paradigm is C++. C# supports return type covariance as of version 9.0. [1]
Method overloading, on the other hand, refers to differentiating the code used to handle a message based on the parameters of the method. If one views the receiving object as the first parameter in any method then overriding is just a special case of overloading where the selection is based only on the first argument.
In C#, a class is a reference type while a struct (concept derived from the struct in C language) is a value type. [5] Hence an instance derived from a class definition is an object while an instance derived from a struct definition is said to be a value object (to be precise a struct can be made immutable to represent a value object declaring attributes as readonly [6]).
For instance, in C#, the base method or property can only be overridden in a subclass if it is marked with the virtual, abstract, or override modifier, while in programming languages such as Java, different methods can be called to override other methods. [15] An alternative to overriding is hiding the inherited code.
In OCaml, parent classes are specified individually in the body of the class definition. Methods (and attributes) are inherited in the same order, with each newly inherited method overriding any existing methods. OCaml chooses the last matching definition of a class inheritance list to resolve which method implementation to use under ambiguities.
Instance methods – belong to individual objects, and have access to instance variables for the specific object they are called on, inputs, and class variables; Depending on the definition of the language, subclasses may or may not be able to override the methods defined by superclasses.
Java, C++ (Since C++11) and D allow inheritance or overriding a class method to be prohibited by labeling a declaration of a class or method, respectively, with the keyword "final". In the book Effective Java, author Joshua Bloch writes (in item 17) that programmers should "Design and document for inheritance or else prohibit it".