abstract class

Learn abstract classes.

What is an abstract class?

A class marked abstract cannot be instantiated directly; it only serves as a base class for subclasses. Subclasses inherit from it with extends. abstract methods are declared without a body and must be implemented in the subclass.

abstract class Shape {
  abstract area(): number;
}
class Circle extends Shape {
  area() { return 3.14 * 2 * 2; }
}

Abstract method and concrete method

An abstract class can contain both abstract methods (no body, implemented in subclass) and concrete methods (have a body, inherited by subclass). The subclass must implement only abstract methods; concrete methods may be overridden optionally.

abstract class Animal {
  abstract makeSound(): void;
  introduce() { console.log("I am an animal"); }
}

Exercise Compute square area

Define abstract class Sekil with abstract alan(): number inside. Then make Kare extend Sekil, take a kenar field, and implement alan returning kenar * kenar. Print new Kare(4).alan() with console.log.

Show example solution

abstract class Sekil {
  abstract alan(): number;
}
class Kare extends Sekil {
  kenar: number;
  constructor(kenar: number) { super(); this.kenar = kenar; }
  alan(): number { return this.kenar * this.kenar; }
}
console.log(new Kare(4).alan());

Test yourself

Which of the following is true for an abstract class?
- Can contain only static members
- Cannot be instantiated directly with new
- Can have only one subclass
- Methods cannot be defined inside it
Show answer

Cannot be instantiated directly with new — Abstract classes cannot be instantiated directly; subclasses inherit with extends and implement abstract methods.

Abstract class declaration

 ____ class Sekil {
  abstract alan(): number;
}

Show answer

 abstract class Sekil {
  abstract alan(): number;
}

Abstract class parts

abstract class

Base class declaration
abstract metot

Bodyless, implemented in subclass
extends

Subclass inherits
somut metot

Has body, inherited by subclass

Next lesson: implements

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