How to instantiate an abstract class in Java?

An abstract class is a class declared in Java through the "Abstract" keyword. Abstract classes are a concept in one of the four principles of object-oriented programming (OOP) called "inheritance." Inheritance refers to a feature of Java classes in which a class called a "subclass" inherits all the properties of the class. The parent class is often called the "superclass".

In Java, an abstract class refers to a base super class that other subclasses can inherit from. It can contain both abstract and non-abstract methods.

algorithm

• Step 1 - Identify methods in the class that have default implementation or no implementation.

• Step 2 - Remove the implementation of these methods.

• Step 3 - Add the abstract keyword to the class declaration.

• Step 4 - Add the abstract keyword to the method declaration modified in step 2.

• Step 5 - If the class has any instance variables that need to be initialized, add a constructor to initialize them.

• Step 6 - Update any subclasses of the abstract class to implement abstract methods or to make themselves abstract.

grammar

Let’s look at the syntax for instantiating abstract classes in Java -

// Abstract Class
abstract class Shape {
   public abstract void draw();
}

method

Because abstract classes are incomplete classes, they cannot be instantiated directly using the "new" keyword.

• Concrete Subclass - In order to correctly instantiate an ambiguous or incomplete abstract class, you may choose to use a concrete subclass. By seamlessly extending from this parent abstraction and implementing each of its method requirements, users can successfully create and implement this newly instantiated subclass without errors or inconsistencies in its operation.

• Lambda Expression - To create an object from an abstract class, you have another option - use a lambda expression that provides implementation for all its abstractions. This lambda creation is then assigned to a compatible functional interface variable based on these signatures.

Instantiate abstract class

Let's look at a sample code snippet to understand the use of abstract classes. The first scenario provides code with non-abstract classes.

Example

class Shape {
   public void printName() {  
      System.out.println("I'm a shape"); 
   }
    
   public float area() {  
      return 0;  
   }
    
   public void printDetails() {  
      this.printName();
      System.out.println("... and my area is " + this.area());
   }
}

class Circle extends Shape {  
   private float radius;
    
   public Circle(float radius) {  
      this.radius = radius;
   }
    
   public void printName() {        
      System.out.println("I'm a circle");  
   }  
    
   public float area() {  
      return (float) (Math.PI * Math.pow(radius, 2));  
   }
}

class Rectangle extends Shape {  
   private float length;
   private float width;  
    
   public Rectangle(float length, float width) {
      this.length = length;  
      this.width = width;  
   }
    
   public void printName() {  
      System.out.println("I'm a rectangle");   
   } 
   
   public float area() {  
      return length * width;  
   }
}

public class Main {     
   public static void main(String[] args) {  
      Shape[] shapes = { new Circle(3.5f), new Rectangle(4.0f, 5.0f) };  
      for (Shape shape : shapes) {
         shape.printDetails();
      }
   }
}

Output

I'm a circle
... and my area is 38.48451
I'm a rectangle
... and my area is 20.0

Both the Circle and Rectangle classes inherit the printName(), area() and printDetails() methods from the "Shape" superclass. However, neither class overrides the area() method to provide its own implementation.

By calling the printDetails() method of the Circle object, the output will be "I am a circle...and my area is 38.48451". Likewise, calling the printDetails() method on a Rectangle object will print "I am a rectangle...and my area is 20.0". This ensures that the output reflects the correct shape and its corresponding region according to the specific implementation provided in each class

Example 1: Concrete subclass

// With abstract class
abstract class Shape {
   public abstract void printName();
   public abstract float area();
   public void printDetails() {
      this.printName();
      System.out.println("... and my area is " + this.area());
   }
}

// Concrete class
class Circle extends Shape {
   private float radius;
   public Circle(float radius) {
      this.radius = radius;
   }
   public void printName() {
      System.out.print("I'm a circle");
   }
   public float area() {
      return (float) (Math.PI * Math.pow(radius, 2));
   }
}

// Concrete class
class Rectangle extends Shape {
   private float length;
   private float width;
   public Rectangle(float length, float width) {
      this.length = length;
      this.width = width;
   }
   public void printName() {
      System.out.print("I'm a rectangle");
   }
   public float area() {
      return length * width;
   }
}

// Main class
public class Main {
   public static void main(String[] args) {
      Shape[] shapes = { new Circle(10), new Rectangle(5, 10) };
      for (Shape shape : shapes) {
         shape.printDetails();
      }
   }
}

Output

I'm a circle... and my area is 314.15927
I'm a rectangle... and my area is 50.0

In the updated code above, the Circle and Rectangle classes implement the abstract methods printName() and area() defined in the "Shape" abstract class. The printDetails() method in the Shape class can use these methods to print out the shape name and its respective region.

By making Shape an abstract class and defining abstract methods, we ensure that any class that extends the Shape class must provide its own implementation for the printName() and area() methods.

Example 2: Lambda expression

interface Nameable {
   String getName();
}
 
abstract class Shape {
   private Nameable nameable;
 
   public Shape(Nameable nameable) {
      this.nameable = nameable;
   }
 
   public abstract float area();
 
   public void printDetails() {
      System.out.println("I'm a " + nameable.getName() + " ... and my area is " + this.area());
   }
}
 
class Circle extends Shape {
   private float radius;
 
   public Circle(float radius) {
      super(() -> "circle");
      this.radius = radius;
   }
 
   @Override
   public float area() {
      return (float) (Math.PI * Math.pow(radius, 2));
   }
}
 
class Rectangle extends Shape {
   private float width;
   private float height;
 
   public Rectangle(float width, float height) {
      super(() -> "rectangle");
      this.width = width;
      this.height = height;
   }
 
   @Override
   public float area() {
      return width * height;
   }
}
 
public class Main {
   public static void main(String[] args) {
      Shape[] shapes = { new Circle(10), new Rectangle(5, 10) };
      for (Shape shape : shapes) {
         shape.printDetails();
      }
   }
}

Output

I'm a circle ... and my area is 314.15927
I'm a rectangle ... and my area is 50.0

In the latest update to this code, we have introduced an improved way of specifying Shape as an abstract class while internalizing its getName() function. A further improvement involves integrating the printName method, which successfully utilizes the data from getName() to display the name of each corresponding shape. For Circle and Rectangle subclasses - they now override the corresponding getName using a lambda expression so that the expected form is accurately identified.

in conclusion

In short, an abstract class can only be instantiated through its base subclass, not directly. This is an inherited concept.

The main reason behind this is that abstract classes are not complete implementations of their methods and objects, but are used by subclasses to inherit them.

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