Interface declaration in Java

Introduction

An interface in Java is like a contract that says what methods a class must have, without saying how they work.

When you want different classes to share the same set of actions but do them differently.

When you want to make sure a class follows certain rules or behaviors.

When you want to separate what something does from how it does it.

When you want to use multiple inheritance of type, since Java classes can only extend one class.

When you want to design flexible and reusable code.

Syntax

Java

public interface InterfaceName {
    // method signatures
    returnType methodName(parameters);
}

Interfaces only declare methods; they do not provide method bodies (except default or static methods in newer Java versions).

All methods in an interface are implicitly public and abstract.

Examples

This interface says any class that is an Animal must have eat and sleep methods.

Java

public interface Animal {
    void eat();
    void sleep();
}

Here, Vehicle interface requires a getSpeed method that returns an integer.

Java

interface Vehicle {
    int getSpeed();
}

This interface has a default method info with a body, which classes can use or override.

Java

public interface Drawable {
    void draw();
    default void info() {
        System.out.println("Drawable object");
    }
}

Sample Program

This program defines an interface Printer with a print method. ConsolePrinter class implements Printer and provides the print method. The main method creates a ConsolePrinter and calls print.

Java

public interface Printer {
    void print(String message);
}

public class ConsolePrinter implements Printer {
    public void print(String message) {
        System.out.println(message);
    }
}

public class Main {
    public static void main(String[] args) {
        Printer printer = new ConsolePrinter();
        printer.print("Hello, Interface!");
    }
}

Important Notes

Interfaces cannot have instance fields (variables), only constants (static final).

A class can implement multiple interfaces, helping with flexible design.

Since Java 8, interfaces can have default and static methods with bodies.

Summary

Interfaces define what methods a class must have, without how they work.

Use interfaces to ensure classes follow certain behaviors.

Interfaces help write flexible and reusable code by separating method declaration from implementation.

Practice

1. What is the main purpose of an interface in Java?

easy

A. To inherit code from multiple classes

B. To store data values like variables

C. To create objects directly

D. To declare methods that a class must implement without providing their body

Solution

1. Step 1: Understand what an interface declares

   An interface only declares method signatures without any implementation.

2. Step 2: Compare with other options

   Interfaces do not store data, create objects, or inherit code from classes.

3. Final Answer:

   To declare methods that a class must implement without providing their body -> Option D

4. Quick Check:

   Interface purpose = declare methods only [OK]

Hint: Interfaces declare methods without bodies [OK]

Common Mistakes:

• Thinking interfaces can store variables
• Confusing interfaces with classes
• Believing interfaces create objects

2. Which of the following is the correct way to declare an interface named Vehicle in Java?

easy

A. interface Vehicle {}

B. class Vehicle interface {}

C. interface Vehicle() {}

D. Vehicle interface {}

Solution

1. Step 1: Recall Java interface syntax

   Interfaces are declared with the keyword interface followed by the name and curly braces.

2. Step 2: Check each option

   interface Vehicle {} matches correct syntax: interface Vehicle {}. Others have syntax errors like misplaced parentheses or keywords.

3. Final Answer:

   interface Vehicle {} -> Option A

4. Quick Check:

   Correct interface syntax = interface Name {} [OK]

Hint: Use 'interface Name {}' to declare interfaces [OK]

Common Mistakes:

• Adding parentheses after interface name
• Mixing class and interface keywords
• Omitting the 'interface' keyword

3. What will be the output of the following code?

interface Animal {
    void sound();
}

class Dog implements Animal {
    public void sound() {
        System.out.println("Bark");
    }
}

public class Test {
    public static void main(String[] args) {
        Animal a = new Dog();
        a.sound();
    }
}

medium

A. Compilation error

B. sound

C. Bark

D. No output

Solution

1. Step 1: Understand interface implementation

   The class Dog implements Animal and provides the method sound() which prints "Bark".

2. Step 2: Trace the main method execution

   In main, an Animal reference points to a Dog object, calling sound() prints "Bark".

3. Final Answer:

   Bark -> Option C

4. Quick Check:

   Interface method called prints 'Bark' [OK]

Hint: Implemented method runs when called via interface reference [OK]

Common Mistakes:

• Expecting interface to print something
• Missing 'public' in method implementation causing error
• Thinking no output occurs

4. Identify the error in this interface declaration:

interface Calculator {
    int add(int a, int b) {
        return a + b;
    }
}

medium

A. Interfaces cannot have method bodies unless default or static

B. Method name 'add' is invalid in interfaces

C. Return type 'int' is not allowed in interfaces

D. Interface name must start with lowercase

Solution

1. Step 1: Check method body rules in interfaces

   In Java, interface methods cannot have bodies unless marked as default or static.

2. Step 2: Analyze the given method

   The method add has a body but no default or static keyword, causing a syntax error.

3. Final Answer:

   Interfaces cannot have method bodies unless default or static -> Option A

4. Quick Check:

   Method bodies in interfaces need default/static [OK]

Hint: Interface methods need default/static for bodies [OK]

Common Mistakes:

• Adding method bodies without default/static
• Thinking method names are restricted
• Ignoring Java naming conventions

5. Given two interfaces:

interface Printable {
    void print();
}

interface Showable {
    void show();
}

class Document implements Printable, Showable {
    public void print() {
        System.out.println("Printing document");
    }
    public void show() {
        System.out.println("Showing document");
    }
}

public class Test {
    public static void main(String[] args) {
        Document doc = new Document();
        doc.print();
        doc.show();
    }
}

What is the output when running Test?

hard

A. Compilation error due to multiple interfaces

B. Printing document\nShowing document

C. Printing document only

D. No output

Solution

1. Step 1: Understand multiple interface implementation

   Java allows a class to implement multiple interfaces and must provide all their methods.

2. Step 2: Trace method calls in main

   The Document class implements both methods. Calling print() and show() prints both messages.

3. Final Answer:

   Printing document\nShowing document -> Option B

4. Quick Check:

   Multiple interfaces implemented, all methods run [OK]

Hint: Classes can implement many interfaces, all methods must be defined [OK]

Common Mistakes:

• Thinking multiple interfaces cause errors
• Forgetting to implement all interface methods
• Expecting only one method to run