Abstract Methods and Abstract Classes

Abstract Methods #

An abstract method refers to a method that must be overridden in a child class to be used.
The purpose of declaring and using abstract methods in Java is to ensure that child classes inheriting a class containing abstract methods must implement those abstract methods.
An abstract method only has a declaration; its implementation is not written. (The unimplemented part is overridden and used by child classes.)

abstract 반환타입 메소드이름();

A semicolon is added directly at the end of the declaration to indicate that there is no implementation.

Abstract Classes #

In Java, a class that contains one or more abstract methods is called an abstract class.
Such abstract classes allow defining a set of methods that exhibit polymorphism, a crucial characteristic in object-oriented programming.
That is, if a method that must be used is declared as an abstract method in an abstract class, all classes inheriting this class must redefine this abstract method.

abstract class ClassName{
  // ...
  abstract 반환타입 메소드이름();
}

Since such abstract classes contain abstract methods whose behavior is not defined, instance creation is not possible. An abstract class first requires creating a child class through inheritance, and only after the child class overrides all abstract methods of the abstract class can an instance of the child class be created.

Except for the fact that an abstract class contains abstract methods, it is identical to a regular class in all other aspects. That is, it can also include constructors, fields, and regular methods.

abstract class Animal {
  abstract void cry();
}
class Cat extends Animal{
  void cry() {
    System.out.println("야옹");
  }
}
public class PolymorphismEx02{
  // Animal a = new Animal(); 추상 클래스는 인스턴스 생성 불가능
  Cat c = new Cat();
  
  c.cry(); // 야옹
}

Purpose of Abstract Methods #

To ensure that child classes inheriting a class containing abstract methods must implement those abstract methods.
If implemented as a regular method, users might choose to implement that method or not.