Difference between Class and Interface

Interface in Java

An interface as a collection of abstract methods that can be implemented by a class. It only specifies the method signature(name, parameters, and return type) but not its implementation. The specified methods are then implemented in a class. Therefore an interface can have methods and variables. However, by default, the methods declared in an interface are abstract.

Properties of an Interface

• Abstract method: An interface helps in declaring one or more than one abstract methods. These methods have no implementation. The class that implements an interface must have implementations of all the declared methods in an interface
   
• Multiple Inheritance: An interface can extend multiple interfaces. This property allows it to inherit properties and methods from multiple sources
   
• Method Signatures: An interface contains method signatures. These are just declarations of methods without implementation
   
• Access Modifiers: The access modifier determines the visibility of the class or its members. It can be public, protected, or private
   

Syntax:

<access_modifer> interface <interface_name> {
        //method signatures

        <access_modifier> <return_type> <method_name> ([parameters]);
}

• The access modifier determines the visibility of the class or its members. It can be public, protected, or private
   
• The <interface_name> is the name of the interface
   
• Method signatures are the declarations of methods that must be implemented by any class that uses the interface
   
• The method signatures do not have a method body. They are only declarations of methods that must be implemented by a class that uses the interface
   

Example: 

• Java

Java

public class Main {
    public static void main(String[] args) {
        NinjaStudent myNinja1 = new Ninja1("Ninja1", 15);
        myNinja1.NinjaGoals();
        myNinja1.NinjaSkills();
    }

    public interface NinjaStudent {
        public void NinjaGoals();
        public void NinjaSkills();
    }

    public static class Ninja1 implements NinjaStudent {
        private String name;
        private int age;

        public Ninja1(String name, int age) {
            this.name = name;
            this.age = age;
        }

        @Override
        public void NinjaGoals() {
            System.out.println("These are ninja's goals");
        }

        @Override
        public void NinjaSkills() {
            System.out.println("These are ninja's skills");
        }
    }
}

You can also try this code with Online Java Compiler

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Output

Explanation

We have created a NinjaStudent interface that defines two methods NinjaSkills() and NinjaGoals(). We then create a Ninja1 class that implements the NinjaStudent interface and provides its implementation for the two methods. We then created a new Ninja1 object using the NinjaStudent interface and called its NinjaSkills() and NinjaGoals() methods.

Also see, Java Ioexception

Differences Between a Class and an Interface

This comparison table shows the difference between class and interface.

Also see, Difference Between Rank and Dense Rank

Read more, how to run java program

When to Use Class vs Interface

Best Use Cases for Classes

Classes are ideal when you need to implement concrete behavior, manage internal states, or create complete objects with both data and functionality. They are suited for modeling real-world entities such as User, Product, or BankAccount, where attributes (like name, price, or balance) and behaviors (like deposit, withdraw, or purchase) are encapsulated. Classes also support inheritance, allowing code reuse and specialization. When you require object instantiation and full behavior implementation, using classes is the preferred approach.

Best Use Cases for Interfaces

Interfaces are best used when defining a set of behaviors without dictating how they're implemented. They enable multiple inheritance of type in Java, helping classes share functionality while staying loosely coupled. Interfaces shine in scenarios like defining Comparable for sorting, Runnable for threading, or List for collections. They allow different classes to implement the same set of methods differently, promoting flexibility and separation of concerns in design.

Interface as a Design Contract

Interfaces act as a design contract by specifying what methods a class must implement without prescribing how. This abstraction encourages loose coupling between components, making applications easier to test, extend, and maintain. For example, in Java frameworks like Spring, interfaces allow developers to swap implementations without altering the client code, improving scalability. Interments enable modular architecture by enforcing consistent behavior across classes, making them vital in large systems and API design. Their use fosters a clean separation between specification and implementation, which is a core principle of object-oriented design.

Frequently Asked Questions

What is the difference between type, interface, and class?

A type defines a variable's nature, an interface specifies method contracts without implementation, and a class implements methods and provides object structure.

What is an interface in Java?

An interface in Java is a blueprint defining abstract methods and constants, allowing classes to implement and ensure specific behaviors without inheritance.

Can an interface have a class in Java?

No, an interface cannot have a class, but it can have nested static classes for utility purposes without affecting the interface's abstract nature.

Can a class implement multiple interfaces?

Yes, Java allows a class to implement multiple interfaces using a comma-separated list, like class A implements Interface1, Interface2. This supports flexibility and avoids the complexities of multiple class inheritance.

Can an interface extend another interface?

Yes, an interface can extend one or more interfaces to inherit method signatures and ensure consistent behavior. For example:

interface B extends A { }

Is an interface 100% abstract?

Traditionally, interfaces were fully abstract, but since Java 8, they can include default and static methods, allowing some concrete behavior as well.

Conclusion

This article discussed some of the differences between class and interface by briefly discussing their concept, syntax, and properties. We also looked into some examples to understand the same. Understanding the distinctions between classes and interfaces is crucial for effective object-oriented programming. While classes provide a blueprint for objects with shared attributes and behavior, interfaces enforce a specific set of methods that classes must implement. You can read more such articles on our platform, Code360.

• Class and Objects in Java
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• Introduction to Java
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