Java doesn’t have extension methods. Default methods are not extension methods. Let’s look at each feature.
Default methods
Both Java and C# support this feature
Problems solved:
- Many objects may implement the same interface and all of them may use the same implementation for a method. A base class could solve this issue but only if the interface implementors don’t already have a base class as neither java nor C# support multiple inheritance.
- An API would like to add a method to an interface without breaking the API consumers. Adding a method with a default implementation solves this.
Java’s or C#’s default methods are a feature to add a default implementation to an interface. So objects that extend an interface don’t have to implement the method, they could just use the default method.
interface IA { default public int AddOne(int i) { return i + 1; } }
Any object that implements IA doesn’t have to implement AddOne because there is a default method that would be used.
public class MyClass implements IA { /* No AddOne implementation needed */ }
C# now has this feature in C# 8 (or .Net 5)
C#’s Extension Method
Problems solved:
- Ability to add methods to sealed classes.
- Ability to add methods to classes from third-party libraries without forcing inheritance.
- Ability to add methods to model classes in environments where methods in model classes are not allowed for convention reasons.
- The ability for IntelliSense to present these methods to you.
Example: The type string is a sealed class in C#. You cannot inherit from string as it is sealed. But you can add methods you can call from a string.
var a = "mystring";
a.MyExtensionMethed()
Java lacks this feature and would be greatly improved by adding this feature.
Conclusion
There is nothing even similar about Java’s default methods and C#’s extension method features. They are completely different and solve completely different problems.