Concrete class in Java

A concrete class is a class that has an implementation for all of its methods. They cannot have any unimplemented methods. It can also extend an abstract class or implement an interface as long as it implements all their methods. It is a complete class and can be instantiated.

In other words, we can say that any class which is not abstract is a concrete class.

Necessary condition for a concrete class: There must be an implementation for each and every method.

Example: The image below shows three classes Shape, Rectangle and Circle. Shape is abstract whereas Rectangle and Circle are concrete and inherit Shape. This is because Rectangle and Circle implement area() method.

Example 1: The below code shows a simple concrete class:

// Java program to illustrate concrete class

// Concrete Class
class Main {

    // this method calculates
    // product of two numbers
    static int product(int a, int b)
    {
        return a * b;
    }

    // this method calculates
    // sum of two numbers
    static int sum(int a, int b)
    {
        return a + b;
    }

    // main method
    public static void main(String args[])
    {
        int p = product(5, 10);
        int s = sum(5, 10);

        // print product
        System.out.println("Product: " + p);

        // print sum
        System.out.println("Sum: " + s);
    }
}

Output:

Product: 50
Sum: 15

Example 2: The code below illustrates a concrete class which extends an abstract class. The method product() in interface X is implemented by class Product but it does not implement method sum(), therefore it has to be abstract. Whereas class Main implements the unimplemented method sum(), therefore there are no unimplemented methods. Hence, it is a concrete class.

// Java program to illustrate concrete class

// This is an interface
interface X {
    int product(int a, int b);
    int sum(int a, int b);
}

// This is an abstract class
abstract class Product implements X {

    // this method calculates
    // product of two numbers
    public int product(int a, int b)
    {
        return a * b;
    }
}

// This is a concrete class that implements
class Main extends Product {

    // this method calculates
    // sum of two numbers
    public int sum(int a, int b)
    {
        return a + b;
    }

    // main method
    public static void main(String args[])
    {
        Main ob = new Main();
        int p = ob.product(5, 10);
        int s = ob.sum(5, 10);

        // print product
        System.out.println("Product: " + p);

        // print sum
        System.out.println("Sum: " + s);
    }
}

Output:

Product: 50
Sum: 15