What are Generics

Generics in Java are bit of code that is placed in angle brakets (< >) during declarations. Generics are mainly used to help developers create type-safe collections by allowing types to be parameters, when defining classes, interfaces, and methods. This allows a developer to reuse the same code with different inputs.

What does a generic look like?

Dog greatDane = new Dog();

ArrayList<Dog> allOfOurDogs = new Arraylist<Dog>();

allOfOurDogs.add(greatDane);

The line of code above is creating an ArrayList and specifying the type of object that it will hold. This is a pretty big deal, and is the main power of generics. Using generics in this example gives a developer confidence that the objects added to the allOfOurDogs ArrayList will always come out as Dog references.

To truly appriciate what that means, you have to first understand that ArrayLists on their own can normally take ANY object, of any type.

Dog greatDane = new Dog();
Cat maineCoon = new Cat();
int ten = 10;
boolean thisIsTrue = true;

ArrayList aBunchOfNonsense = new ArrayList();

aBunchOfNonsense.add(greatDane);
aBunchOfNonsense.add(maineCoon);
aBunchOfNonsense.add(ten);
aBunchOfNonsense.add(boolean);

Everything in the example above is perfectly legal in Java, and there isn’t a rule that says you must use genertics, but you might run into some surprises when something gets added to the ArrayList that you didn’t expect.

Summary

Generics provide compile time safety, by specifying the type of Object reference that should be expected with a parameter, class, or method.