Java enum Inheritance and Interface

February 8, 2023

What is Java enums?

In Java, an enumeration is a special kind of class that represents a fixed set of values. Each value in an enum is called an enumeration constant, and it’s defined as a static final instance of the enum type. Enums are used to define a fixed set of values that a variable of that type can take on.

Enums are inherently final classes and cannot be subclassed. They cannot extend other enums or classes.

To know more about Java enum in java read the complete article.

Java enum inheritance and Interface:

In Java, enums are a special kind of class that represent a fixed set of values, and they cannot be inherited by another class in the traditional sense. This means that an enum cannot extend another class. However, enum class can inherit another enum class.

In Java, an interface is a set of method signatures that a class can implement. An enum, just like any other class, can implement one or more interfaces. When an enum implements an interface, it must provide an implementation for all of the interface’s methods. The enum can also define additional methods of its own.

Note: By implementing an interface, an enum can inherit the methods of the interface.

Note: Enums can be used as an alternative to constants in java, it will make your code more readable and maintainable.

enhanced-for-loop

enumset in java

enum-constructor

encapsulation in Java

do-while-loop in java

Let’s look at a Java enum where the Java enum constructor is used to perform an operation on the given code.

Example 1 : Inheriting a Class from an enum Class

Run

enum ParentEnum {
    CONSTANT1, CONSTANT2, CONSTANT3;
}

 enum ChildEnum {
    CONSTANT4(ParentEnum.CONSTANT1), CONSTANT5(ParentEnum.CONSTANT2);

    private final ParentEnum parentEnum;

    ChildEnum(ParentEnum parentEnum) {
        this.parentEnum = parentEnum;
    }

    public ParentEnum getParentEnum() {
        return parentEnum;
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println("Parent Enum constants: ");
        for (ParentEnum constant : ParentEnum.values()) {
            System.out.println(constant);
        }
        System.out.println("Child Enum constants: ");
        for (ChildEnum constant : ChildEnum.values()) {
            System.out.println(constant + " - Parent Enum : " + constant.getParentEnum());
        }
    }
}

Output

Parent Enum constants: 
CONSTANT1
CONSTANT2
CONSTANT3
Child Enum constants: 
CONSTANT4 - Parent Enum : CONSTANT1
CONSTANT5 - Parent Enum : CONSTANT2

Explanation: In this example, the ParentEnum is defined with 3 constants and the ChildEnum defined 2 additional constants and a constructors that takes a ParentEnum as an argument. The main class iterates through the constants of the ParentEnum and ChildEnum using the values() method and prints the constants to the console. Since the constructor is private, we cannot access it from outside the class. However, we can use enum constants to call the constructor.

Example 2 : Java enum interface Method

Run

public interface Shape {
    double getArea();
}
enum Square implements Shape {
    SMALL(1), MEDIUM(2), LARGE(3);
    private double side;
    Square(double side) {
        this.side = side;
    }
    public double getArea() {
        return side * side;
    }
}
public class Main {
    public static void main(String[] args) {
        Shape shape = Square.SMALL;
        System.out.println("Area of square: " + shape.getArea());
    }
}

Output

Area of square: 1.0

Explanation: In this example, I've defined an interface Shape that contains a single method getArea(). The Square enum implements this interface and provides its own implementation of the getArea() method. In the main class, I've created a variable of the Shape interface type and assigned an instance of the Square enum. Then I've used the getArea() method to get the area of the square. It's worth noting that the Shape interface should be declared before the Square enum in the code and enums are final classes and cannot be inherited by other classes in the traditional sense, but it can implement interfaces and provide methods in enums.

Example 3: Java enum inheritence

Run

public interface Shape {
    double getArea();
}

public enum Square implements Shape {
    SMALL(1), MEDIUM(2), LARGE(3);

    private double side;

    Square(double side) {
        this.side = side;
    }

    public double getArea() {
        return side * side;
    }
}

public enum Circle implements Shape {
    SMALL(1), MEDIUM(2), LARGE(3);

    private double radius;
    private final double PI = 3.14;

    Circle(double radius) {
        this.radius = radius;
    }

    public double getArea() {
        return PI * radius * radius;
    }
}

public class Main {
    public static void main(String[] args) {
        Shape[] shapes = new Shape[]{Square.SMALL, Square.MEDIUM, Square.LARGE, Circle.SMALL, Circle.MEDIUM, Circle.LARGE};
        for (Shape shape : shapes) {
            System.out.println(shape + " area: " + shape.getArea());
        }
    }
}

Output

SMALL area: 1.0
MEDIUM area: 4.0
LARGE area: 9.0
SMALL area: 3.14
MEDIUM area: 12.56
LARGE area: 28.26

Explanation: This code defines an interface Shape with a single method getArea(). It then defines two enums Square and Circle that implement the Shape interface and provide their own implementation of the getArea() method. Finally, the main class creates an array of Shape objects that contains instances of the Square and Circle enums, and it iterates through the array to print the area of each shape to the console using the getArea() method.

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Java enum Inheritance and Interface

What is Java enums?

Java enum inheritance and Interface:

Example 1 : Inheriting a Class from an enum Class

Output

Example 2 : Java enum interface Method

Output

Example 3: Java enum inheritence

Output

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