Implementing interfaces in Java

Introduction

Interfaces let you define a set of actions that different classes can do. Implementing interfaces helps make sure classes follow the same rules.

When you want different classes to share the same actions but have different details.

When you want to make sure a class has certain methods before using it.

When you want to write code that works with many types of objects in the same way.

When you want to separate what a class can do from how it does it.

Syntax

Java

interface InterfaceName {
    returnType methodName();
}

class ClassName implements InterfaceName {
    @Override
    public returnType methodName() {
        // method body
    }
}

Use the implements keyword to say a class follows an interface.

All methods in an interface must be implemented in the class.

Examples

This example shows an interface Animal with a method sound. The class Dog implements it and defines the sound.

Java

interface Animal {
    void sound();
}

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

Here, Vehicle interface has two methods. Car class implements both methods.

Java

interface Vehicle {
    void start();
    void stop();
}

class Car implements Vehicle {
    @Override
    public void start() {
        System.out.println("Car started");
    }
    @Override
    public void stop() {
        System.out.println("Car stopped");
    }
}

Sample Program

This program defines an interface Printer with a method print. Two classes implement it differently. The Main class creates objects and calls their print methods.

Java

interface Printer {
    void print();
}

class InkjetPrinter implements Printer {
    @Override
    public void print() {
        System.out.println("Printing with Inkjet Printer");
    }
}

class LaserPrinter implements Printer {
    @Override
    public void print() {
        System.out.println("Printing with Laser Printer");
    }
}

public class Main {
    public static void main(String[] args) {
        Printer inkjet = new InkjetPrinter();
        Printer laser = new LaserPrinter();
        inkjet.print();
        laser.print();
    }
}

Important Notes

Interfaces cannot have method bodies (except default or static methods in newer Java versions).

A class can implement multiple interfaces separated by commas.

Use @Override to help catch mistakes when implementing methods.

Summary

Interfaces define what methods a class must have.

Use implements keyword for a class to follow an interface.

All interface methods must be implemented in the class.

Practice

1. What keyword does a Java class use to follow an interface?

easy

A. uses

B. extends

C. inherits

D. implements

Solution

1. Step 1: Understand Java class and interface relationship

   In Java, a class follows an interface by using a specific keyword to promise it will provide all methods declared in the interface.

2. Step 2: Identify the correct keyword

   The keyword to make a class follow an interface is implements, not extends which is for classes inheriting other classes.

3. Final Answer:

   implements -> Option D

4. Quick Check:

   Class follows interface = implements [OK]

Hint: Remember: classes use implements for interfaces, extends for classes [OK]

Common Mistakes:

• Using extends instead of implements for interfaces
• Confusing inherits keyword which doesn't exist in Java
• Using uses keyword which is invalid

2. Which of the following is the correct syntax to declare a class Car that implements interface Vehicle?

easy

A. class Car extends Vehicle {}

B. class Car implements Vehicle {}

C. interface Car implements Vehicle {}

D. class Car uses Vehicle {}

Solution

1. Step 1: Identify class and interface keywords

   A class is declared with class, and interfaces with interface. Here, Car is a class, Vehicle is an interface.

2. Step 2: Use correct syntax for implementing interface

   The class Car must use implements keyword to follow Vehicle interface. So class Car implements Vehicle {} is correct.

3. Final Answer:

   class Car implements Vehicle {} -> Option B

4. Quick Check:

   Class + implements + Interface = correct syntax [OK]

Hint: Class implements interface with 'implements' keyword [OK]

Common Mistakes:

• Using extends instead of implements for interfaces
• Declaring class as interface
• Using invalid keyword uses

3. What will be the output of this code?

interface Printer {
    void print();
}

class Document implements Printer {
    public void print() {
        System.out.println("Printing document");
    }
}

public class Main {
    public static void main(String[] args) {
        Printer p = new Document();
        p.print();
    }
}

medium

A. Printing document

B. Compilation error: print() not implemented

C. Runtime error

D. No output

Solution

1. Step 1: Check if interface method is implemented

   The interface Printer declares method print(). The class Document implements Printer and provides public void print() method, so no error.

2. Step 2: Trace main method execution

   Main creates Printer reference p to new Document object and calls p.print(). This calls Document's print() which prints "Printing document".

3. Final Answer:

   Printing document -> Option A

4. Quick Check:

   Implemented method runs and prints output [OK]

Hint: Implemented interface methods run normally when called [OK]

Common Mistakes:

• Forgetting to make print() public causes compile error
• Assuming interface methods run automatically without implementation
• Confusing runtime error with compile error

4. Identify the error in this code:

interface Animal {
    void sound();
}

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

medium

A. No error, code is correct

B. Dog should extend Animal, not implement

C. Method sound() must be public in Dog class

D. Interface Animal cannot have methods

Solution

1. Step 1: Check method visibility in interface implementation

   Interface methods are implicitly public. When implementing, the method must be declared public in the class.

2. Step 2: Identify method declaration in Dog class

   Dog's sound() method has default (package-private) visibility, missing public keyword, causing compile error.

3. Final Answer:

   Method sound() must be public in Dog class -> Option C

4. Quick Check:

   Interface methods require public implementation [OK]

Hint: Interface methods must be public in implementing class [OK]

Common Mistakes:

• Omitting public keyword on implemented methods
• Using extends instead of implements for interfaces
• Thinking interface methods can be private

5. Given interface Calculator with methods add(int a, int b) and subtract(int a, int b), which class correctly implements it to return the sum and difference respectively?

hard

A. class Calc implements Calculator {
public int add(int a, int b) { return a + b; }
public int subtract(int a, int b) { return a - b; }
}

B. class Calc implements Calculator {
int add(int a, int b) { return a + b; }
int subtract(int a, int b) { return a - b; }
}

C. class Calc extends Calculator {
public int add(int a, int b) { return a + b; }
public int subtract(int a, int b) { return a - b; }
}

D. class Calc implements Calculator {
public void add(int a, int b) { System.out.println(a + b); }
public void subtract(int a, int b) { System.out.println(a - b); }
}

Solution

1. Step 1: Check method signatures and visibility

   Interface methods are public and return int. So implementing methods must be public and return int with same parameters.

2. Step 2: Analyze each option

   class Calc implements Calculator { public int add(int a, int b) { return a + b; } public int subtract(int a, int b) { return a - b; } } matches signatures exactly with public int return type. class Calc implements Calculator { int add(int a, int b) { return a + b; } int subtract(int a, int b) { return a - b; } } misses public keyword. class Calc extends Calculator { public int add(int a, int b) { return a + b; } public int subtract(int a, int b) { return a - b; } } uses extends which is invalid for interfaces. class Calc implements Calculator { public void add(int a, int b) { System.out.println(a + b); } public void subtract(int a, int b) { System.out.println(a - b); } } changes return type to void, which is incorrect.

3. Final Answer:

   class Calc implements Calculator { public int add(int a, int b) { return a + b; } public int subtract(int a, int b) { return a - b; } } -> Option A

4. Quick Check:

   Match method signatures exactly with public and return type [OK]

Hint: Implemented methods must match interface signatures exactly [OK]

Common Mistakes:

• Omitting public keyword on methods
• Using extends instead of implements for interfaces
• Changing return types or parameters