Java interface with default methods how to#
Java interface with default methods code#

Still, you can mimic the fields in interfaces using weak hash maps to get access to the inherited class “vtable” of fields and methods to delegate to. Something that was needed to retain backward compatibility of JDK libraries when extended with functional methods. The default method is a technological mistake. Perhaps it has but “please don’t use it” kind of. Java does not have multiple implementation inheritance. Why? Because in that case, they were not interfaces but classes. You can have the class somewhere else, or as an inner class inside the interface. Presumably, these implementations will be implemented in the class and the default methods will only delegate. However, you can have instead of String any class and you can implement not only setters and getters but any methods that do something with that class. In this example the only value stored is a String. Private static final WeakHashMap map = new WeakHashMap() Īll you have to do is write at the end of the list of interfaces the class implements HasName and it magically has. And using that you can easily implement the HasName interface. So we need some value that can be paired to an object and the lifetime of the value has to be the same as the lifetime of the object. We can store the value somewhere else, one for each object instance. The only requirement is that whatever is set for an object the same is get. But it is not absolutely necessary that the value is stored IN the object. You can not access that from the interface. These are setters and getters that accessing a String variable that is in the object. How would you implement getName() and setName()? If there is an interface I and class C implements the interface, when a method on a C c object is invoked the variable this is actually the object c. A default method can access the this variable, which is always the object that is implementing the interface and on which behalf the method was invoked. HasName interface with default implementationĭefault methods just deliver default functionality. But now we could do that using default methods. Summary: we could not implement having the name for some of the objects implemented in some spare classes. It was something more complex that would just make the topic even more complicated and believe me: it is going to be complex enough. If it were only names, we could live with it having other classes having names. As we went up the line reaching for the top we realized that those classes and their some other descendants do not have a name, why to force them? To be honest, in real life it was a bit more complex than just having a name. In that case, we just tried to move the HasName upward in the inheritance line, but in some cases, it just did not work.

There were classes that extended already other classes. Actually, it was not working for a long time. Therefore we created a classĪnd each of the classes that had a name, were just extending this class.

Java interface with default methods code#

Methods that were just setters and getters the copy-paste code smell just filled the room unbearable. Sample problemĪ few years ago I worked on an application that used a lot of different types of objects that each had a name. I am also a bit afraid to let some ifrit out of the bottle, but on the other hands, those ifrits just do not stay there anyway. What I write here are some coding technics that can be made using Java 8 but their usability is questionable at least for me. For this very reason, please read these lines at your own risk, and do not imply that this is a pattern to be followed, just as well do not imply the opposite.

Java interface with default methods how to#

This week I look at how to use default methods to do real inheritance, which actually, default methods were not designed for. I was talking about the multiple inheritances of default methods in the last blog article and how they behave during compilation and run time. Warning: you can not make this unseen once you have read