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Java Basic

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Text lesson

abstract

Example

An abstract method, belonging to an abstract class, lacks a body, and its implementation is delegated to the subclass:

// Code from filename: Main.java
// abstract class
abstract class Main {
  public String fname = "John";
  public int age = 24;
  public abstract void study(); // abstract method
}          

// Subclass (inherit from Main)
class Student extends Main {
  public int graduationYear = 2018;
  public void study() { // the body of the abstract method is provided here
    System.out.println("Studying all day long");
  }
}
// End code from filename: Main.java

// Code from filename: Second.java
class Second {
  public static void main(String[] args) {
  // create an object of the Student class (which inherits attributes and methods from Main)
    Student myObj = new Student();

    System.out.println("Name: " + myObj.fname);
    System.out.println("Age: " + myObj.age);
    System.out.println("Graduation Year: " + myObj.graduationYear);
    myObj.study(); // call abstract method
  }
}

Definition and Usage

The “abstract” keyword, serving as a non-access modifier, is utilized for both classes and methods.

Class: An abstract class is a constrained type that cannot be instantiated directly; instead, it must be inherited by another class to be utilized.

Method: An abstract method is exclusive to an abstract class, devoid of implementation, with its functionality inherited and provided by a subclass.