Interface declaration in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of interface method call

What is the output of this Java program that uses an interface and a class implementing it?

Java

interface Greet {
    String sayHello();
}

class EnglishGreeting implements Greet {
    public String sayHello() {
        return "Hello!";
    }
}

public class Main {
    public static void main(String[] args) {
        Greet greet = new EnglishGreeting();
        System.out.println(greet.sayHello());
    }
}

A Runtime error

B sayHello

C Compilation error

D Hello!

💡 Hint

Look at how the interface method is implemented and called.

The class EnglishGreeting implements the interface Greet and overrides the sayHello method to return "Hello!". The main method creates an instance of EnglishGreeting and calls sayHello, printing "Hello!".

🧠 Conceptual

intermediate

Interface method modifiers

Which of the following statements about methods declared in a Java interface is true?

A Interface methods are implicitly public and abstract unless declared default or static.

B Interface methods cannot be overridden by implementing classes.

C Interface methods must always have a body.

D Interface methods can be private by default.

💡 Hint

Think about the default visibility and abstract nature of interface methods.

By default, interface methods are public and abstract unless they are declared as default or static methods with a body. Private methods are allowed only with explicit private modifier (Java 9+), but not by default.

🔧 Debug

advanced

Identify the compilation error in interface implementation

What error will this code produce when compiled?

Java

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

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

public class Main {
    public static void main(String[] args) {
        Calculator calc = new SimpleCalculator();
        System.out.println(calc.add(5, 3));
    }
}

A Compilation error: method add must override interface method

B No compilation error; program prints 8

C Compilation error: subtract method must be implemented

D Runtime error: ClassCastException

💡 Hint

Check if the method signatures match between interface and class.

The class SimpleCalculator correctly implements the add method from Calculator interface. The subtract method is extra and allowed. The program compiles and prints 8.

📝 Syntax

advanced

Correct interface declaration syntax

Which option shows the correct way to declare an interface with one method in Java?

A interface Worker { void work(); }

B interface Worker { void work() {} }

C interface Worker { public void work() {} }

D interface Worker { abstract void work() {} }

💡 Hint

Interface methods do not have a body unless default or static.

In Java, interface methods are abstract by default and cannot have a body unless declared default or static. Option A declares an abstract method correctly without a body.

🚀 Application

expert

Interface with multiple inheritance

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

Java

interface A {
    default String message() { return "A"; }
}
interface B {
    default String message() { return "B"; }
}
class C implements A, B {
    public String message() {
        return A.super.message() + B.super.message();
    }
}
public class Main {
    public static void main(String[] args) {
        C obj = new C();
        System.out.println(obj.message());
    }
}

A Compilation error due to conflicting default methods

B BA

C AB

D Runtime error

💡 Hint

Look at how class C resolves the conflict by calling both interface methods explicitly.

Class C overrides the message method and explicitly calls both A's and B's default methods using A.super.message() and B.super.message(), concatenating their results to "AB".