Why interfaces are used in Java - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of Interfaces in Java

Why do Java programmers use interfaces in their code?

A To allow unrelated classes to implement the same set of methods, enabling polymorphism.

B To store data persistently on disk.

C To create objects directly without using classes.

D To increase the speed of program execution by compiling to machine code.

💡 Hint

Think about how different classes can share behavior without sharing a parent class.

Interfaces define a contract of methods that different classes can implement. This allows unrelated classes to be treated the same way, enabling polymorphism and flexible code design.

❓ Predict Output

intermediate

Output of Interface Implementation

What is the output of this Java code?

Java

interface Animal {
    void sound();
}

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

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

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

A

Bark
Bark

B

Meow
Bark

C Compilation error due to missing method implementation

D

Bark
Meow

💡 Hint

Look at which class instance is assigned to the interface variable before calling sound().

The interface variable a first refers to a Dog object, so sound() prints "Bark". Then it refers to a Cat object, so sound() prints "Meow".

🔧 Debug

advanced

Why does this interface code cause a compilation error?

This code tries to implement an interface but causes a compilation error. What is the reason?

Java

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

class Car implements Vehicle {
    public void start() {
        System.out.println("Car started");
    }
    // Missing stop() method implementation
}

public class Test {
    public static void main(String[] args) {
        Vehicle v = new Car();
        v.start();
        v.stop();
    }
}

A Interfaces cannot be implemented by classes.

B The class Car does not implement all methods declared in the Vehicle interface.

C The main method is missing the throws declaration.

D The interface Vehicle cannot have more than one method.

💡 Hint

Check if Car has all methods that Vehicle requires.

In Java, a class that implements an interface must provide concrete implementations for all its methods. Car misses the stop() method, causing a compilation error.

📝 Syntax

advanced

Identify the Syntax Error in Interface Declaration

Which option contains a syntax error in the interface declaration?

A

interface Shape {
    double area() {}
}

B

interface Shape {
    double area();
}

C

interface Shape {
    double area();
    void draw();
}

D

interface Shape {
    void resize(int size);
}

💡 Hint

Interfaces cannot have method bodies unless they are default or static methods.

In option A, the method area() has an empty body {} which is not allowed in interfaces unless marked as default or static.

🚀 Application

expert

Using Interfaces for Multiple Inheritance

Given these interfaces and classes, what will be the output when running the main method?

Java

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();
    }
}

A

Showing document
Printing document

B Compilation error due to multiple inheritance

C

Printing document
Showing document

D Runtime error due to method conflict

💡 Hint

Interfaces allow a class to inherit from multiple sources without conflict if methods are implemented.

Java allows multiple inheritance of type through interfaces. Document implements both Printable and Showable and provides both methods, so both print statements run in order.