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Abstract Classes

An abstract class is a special type of class in which you can define abstract methods.

An abstract class can be either extended or implemented.

The reason you may want to use abstract methods/classes is to ensure all concrete child classes implement those methods. It represents methods that the concrete class must instantiate.

⚠️ Abstract methods are not allowed in concrete classes.

The reason for this is that concrete classes can be instantiated directly, and then there won't be an implementation for that method.

Declaration 

abstract class A {
	// no args or return types
	abstract f foo_1;

	// args but no return types
	abstract f foo_2 (arg: number);

	// return types but no args
	abstract f foo_3 -> bool;

	// args and return types
	abstract f foo_4 (arg: number) -> bool;
}

Then in your concrete class

class IAmExtending extends A {
	f foo_1 {
		// must implement
	}

	f foo_2 (arg: number) {
		// must implement
	}

	f foo_3 -> bool {
		// must implement
	}

	f foo_4 (arg: number) -> bool {
		// must implement
	}
}

class IAmImplementing implements A {
	f foo_1 {
		// must implement
	}

	f foo_2 (arg: number) {
		// must implement
	}

	f foo_3 -> bool {
		// must implement
	}

	f foo_4 (arg: number) -> bool {
		// must implement
	}
}

Instantiating

⚠️ Abstract classes cannot be instantiated.

abstract class A { ... }
class B extends A { ... }
class C implements A { ... }

A.create();  ❌
B.create();  ✅
C.create();  ✅