Implementing interfaces in Java - Mini Project: Build & Apply

Implementing interfaces

📖 Scenario: You are creating a simple program to manage different types of vehicles. Each vehicle can start and stop. You will use an interface to define these actions and then create classes that implement this interface.

🎯 Goal: Build a Java program that defines an interface called Vehicle with methods start() and stop(). Then create two classes, Car and Bike, that implement the Vehicle interface. Finally, create objects of these classes and call their methods.

📋 What You'll Learn

Create an interface named Vehicle with two methods: start() and stop().

Create a class Car that implements the Vehicle interface.

Create a class Bike that implements the Vehicle interface.

In each class, provide simple print statements inside start() and stop() methods to show which vehicle is starting or stopping.

Create objects of Car and Bike and call their start() and stop() methods.

💡 Why This Matters

🌍 Real World

Interfaces are used in real-world Java programs to define common behaviors that different classes can share, like vehicles starting and stopping.

💼 Career

Understanding interfaces is essential for Java developers because they enable flexible and maintainable code design, especially in large projects and frameworks.

1

Create the Vehicle interface

Create an interface called Vehicle with two methods: void start() and void stop().

Java

// Your code here
// Define interface Vehicle with methods start() and stop()

Hint

An interface in Java uses the keyword interface. Methods inside interfaces do not have a body.

2

Create the Car class implementing Vehicle

Create a class called Car that implements the Vehicle interface. Implement the start() method to print "Car is starting" and the stop() method to print "Car is stopping".

Java

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

// Your code here
// Define class Car implementing Vehicle with start() and stop() methods

Hint

Use implements Vehicle after the class name. Implement both methods with public void and add print statements inside.

3

Create the Bike class implementing Vehicle

Create a class called Bike that implements the Vehicle interface. Implement the start() method to print "Bike is starting" and the stop() method to print "Bike is stopping".

Java

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

public class Car implements Vehicle {
    public void start() {
        System.out.println("Car is starting");
    }
    public void stop() {
        System.out.println("Car is stopping");
    }
}

// Your code here
// Define class Bike implementing Vehicle with start() and stop() methods

Hint

Similar to the Car class, implement the Vehicle interface and add print statements for Bike.

4

Create objects and call methods

In a Main class with a main method, create an object of Car named myCar and an object of Bike named myBike. Call start() and stop() on both objects.

Java

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

public class Car implements Vehicle {
    public void start() {
        System.out.println("Car is starting");
    }
    public void stop() {
        System.out.println("Car is stopping");
    }
}

public class Bike implements Vehicle {
    public void start() {
        System.out.println("Bike is starting");
    }
    public void stop() {
        System.out.println("Bike is stopping");
    }
}

// Your code here
// Create Main class with main method, create myCar and myBike objects, call start() and stop() on both

Hint

Remember to create a Main class with a public static void main(String[] args) method. Create objects and call methods in order.

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