Interface Example in Java

Introduction

Interface is a classic Java console program that demonstrates the concept with complete source code and sample output. Object-oriented programming models real entities with classes, objects, inheritance and polymorphism.

This tutorial walks through the program line by line, explains how the logic works, and highlights best practices you can apply in your own code.

Definition

An interface in Java programming language is an abstract type that is used to specify an interface (in the generic sense of the term) that classes must implement. Interfaces are declared using the interface keyword, and may only contain method signature and constant declarations. All methods of an Interface do not contain implementation. Interfaces cannot be instantiated, but rather are implemented. A class that implements an interface must implement all of the methods described in the interface, or be an abstract class.

Syntax

interface Interface_name{
   //Do something with any number of final, static fields or any number of abstract methods
}

Interface Example Program

interface Main{
	public void display();
	public void disp();
}
class InterfaceDemo implements Main{
	public void display(){
		System.out.println("First implementation");
	}
	public void disp(){
		System.out.println("Second implementation");
	}
	public static void main(String arg[]){
		Main obj = new InterfaceDemo();
		obj. display();
		obj.disp();
	}
}

Sample Output

First implementation
Second implementation

When to use

Use OOP examples when modelling entities with state and behaviour in larger applications.

How it works

1. Execution begins in the main method — the JVM calls this method when you run the class.

2. A println / print call writes text to the console — part of the sample output below.

3. A println / print call writes text to the console — part of the sample output below.

4. Main obj = new InterfaceDemo(); updates a variable used in the calculation or output.

5. Compare your console output with the sample output for Interface to confirm the program behaves correctly.

Best Practices

• Use meaningful variable and class names that describe their purpose.
• Compile and run the program locally — modify values to see how output changes.
• Read compiler errors carefully; they usually point to the exact line to fix.

Common Mistakes

• Copying code without understanding each line — practice by changing one statement at a time.
• Mismatching the public class name and the .java filename.
• Forgetting semicolons at the end of statements.