When to use or how to use Multiple Bound in generics

Question

I am new to generics and learning generics from hear https://docs.oracle.com/javase/tutorial/java/generics/bounded.html

I am learning about Multiple Bounds what I understood is you can specify class like follows

class D <T extends A & B & C> { /* ... */ }
D<A> d = new D<>();

only if A does implements B and C both other wise compile time error will ocur also B and C should be Interface other wise //interface is expeced compile time error will occurs

I am not talking about wildcards

My problem is I am not getting any real programing use of this. I am finding a way/example how can i use Multiple bound generics while codding.

When should I use it?

thanks

Answer 1

Consider the following snippets:

class SpineWarmCollection <T extends Vertebrate & Warmblooded> { /* ... */ }

class Mammal extends Vertebrate implements Warmblooded {}

class Bird extends Vertebrate implements Warmblooded {}

class Reptile extends Vertebrate {}

SpineWarmCollection<Mammal> mammalCollection = new SpineWarmCollection<>();

SpineWarmCollection<Bird> birdCollection = new SpineWarmCollection<>();

SpineWarmCollection<Reptile> reptileCollection = new SpineWarmCollection<>(); // Generates a compile error, since Reptiles are not warmblooded.

Vertebrate is a class in animal taxonomy; however, warmbloodedness is a trait. There's no single ancestor class for warmblooded-ness, since both Mammals and Birds are warmblooded, but their common ancestor, Vertebrate, is not.

Since T can only be a class that extends Vertebrate and Warmblooded, the generic can access any methods declared in Vertebrate and Warmblooded.

You don't even need a class. T could extend interfaces only, which would allow a generic to be used by any sets of classes that implement the interfaces, even of those sets of classes are completely unrelated to one another.