These three concepts are quite confusing. But I think the overload and overwrite are fake concepts. In Java and
Scala, there even doesn’t exist the concept of overwrite. overload can be considered as normal functions, the only
special thing is that normal functions have the same function name. All the things are determined at compile time.
While override is very useful for polymorphism. The called method is determined at runtime. I think the concept of
overwrite may come from the use of keyword virtual in C++. When the base class method is assigned with virtual,
then override takes effects, which calls late binding. Or the methods will be early binded, and there is no
polymorphic behavior.
In C++
override: subclass method overrides base class method means:
- in different range (in derived class and base class)
- the same function name
- the same function signature
- the return type conforms covariance
- the base class method is virtual
overload: function overloading means:
- the same range (in the same class)
- the same function name
- different function signature
overwrite: subclass method hides base class method means:
- in different range (in derived class and base class)
- the same function name
- two cases on parameters (
signature?):- the same parameters, the base class method is not virtual
- different parameters
In Java
override: subclass method overrides base class method means:
- in different range (in derived class and base class)
- the same function name
- the same function signature
- the return type conforms covariance
In Java, when you override a method, you could add @Override annotation on that method, this will let the compiler to
help you check out whether you actually override a method or just mistake or misspell something.
overload: function overloading means:
- the same range (in the same class)
- the same function name
- different function signature
In Scala
Similar case with Java, but no restrictions on Operator Overloading.
Scala provides override modifier for the use of @Overrride annotation in Java, different from the case in Java, using
override modifier is not optional.
Summation
The concept overwrite introduced in C++ is really a result of using virtural keyword.
While in Java and Scala, override is enabled by default. And Java provides@Override annotation(optional) and Scala provides
override modifier(must) to help avoid some common bugs at compile time. override makes polymorphism very powerful.
overload is a concept in the case of different functions sharing the same function name. I think this case takes no
difference with the case that you declare different functions with different function names. Because only function name
can not determine the function signature or function type. While functions are picked by their functions signature, and
override is a mechanism based on function signature restricted by function type.
