Implementing interfaces in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of interface method implementation

What is the output of this Java program that implements an interface?

Java

interface Speaker {
    void speak();
}

class Person implements Speaker {
    public void speak() {
        System.out.println("Hello from Person");
    }
}

public class Main {
    public static void main(String[] args) {
        Speaker s = new Person();
        s.speak();
    }
}

A Compilation error: speak() must be public

B Hello from Person

C Runtime error: NullPointerException

D No output

💡 Hint

Check if the method speak() is correctly implemented with the right access modifier.

The class Person implements the interface Speaker and correctly defines the speak() method as public. The program prints the message from the speak() method.

❓ Predict Output

intermediate

Interface reference to different implementations

What will be printed when running this Java code?

Java

interface Animal {
    void sound();
}

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

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

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

A

Woof
Meow

B

Woof
Woof

C

Meow
Woof

D Compilation error: cannot assign Cat to Animal

💡 Hint

Look at how the interface reference is assigned to different objects.

The interface reference 'a' points first to a Dog object, then to a Cat object. Calling sound() prints the respective sounds.

🔧 Debug

advanced

Why does this interface implementation cause a compile error?

Identify the cause of the compile error in this code snippet.

Java

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

class SimpleCalculator implements Calculator {
    public int add(int x, int y) {
        return x + y;
    }
}

A Method add() in SimpleCalculator must be declared public

B Interface Calculator cannot have method with parameters

C Class SimpleCalculator must be abstract

D Method add() must return void

💡 Hint

Check the access modifier of the implemented method compared to the interface method.

Interface methods are implicitly public. The implementing method must also be public, otherwise it causes a compile error.

📝 Syntax

advanced

Which code snippet correctly implements multiple interfaces?

Select the code snippet that correctly implements two interfaces in one class.

A

class Multi implements (A, B) {
    public void methodA() {}
    public void methodB() {}
}

B

class Multi implements A & B {
    public void methodA() {}
    public void methodB() {}
}

C

class Multi implements A, B {
    public void methodA() {}
    public void methodB() {}
}

D

class Multi implements A; B {
    public void methodA() {}
    public void methodB() {}
}

💡 Hint

Remember the syntax for implementing multiple interfaces uses commas.

In Java, multiple interfaces are implemented by separating their names with commas, not with & or parentheses or semicolons.

🚀 Application

expert

Determine the output of interface default method override

What is the output of this Java program that uses interface default methods and overrides?

Java

interface Printer {
    default void print() {
        System.out.println("Printing from Printer");
    }
}

interface Scanner {
    default void print() {
        System.out.println("Printing from Scanner");
    }
}

class MultiFunctionMachine implements Printer, Scanner {
    public void print() {
        Printer.super.print();
        Scanner.super.print();
        System.out.println("Printing from MultiFunctionMachine");
    }
}

public class Main {
    public static void main(String[] args) {
        MultiFunctionMachine m = new MultiFunctionMachine();
        m.print();
    }
}

A Compilation error: print() method conflict

B Printing from MultiFunctionMachine

C

Printing from Printer
Printing from MultiFunctionMachine

D

Printing from Printer
Printing from Scanner
Printing from MultiFunctionMachine

💡 Hint

Look at how the class calls the default methods from both interfaces explicitly.

The class overrides print() and calls both interface default methods using InterfaceName.super.method(), then prints its own message.