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".

Practice

1. What is the main purpose of an interface in Java?

easy

A. To inherit code from multiple classes

B. To store data values like variables

C. To create objects directly

D. To declare methods that a class must implement without providing their body

Solution

1. Step 1: Understand what an interface declares

   An interface only declares method signatures without any implementation.

2. Step 2: Compare with other options

   Interfaces do not store data, create objects, or inherit code from classes.

3. Final Answer:

   To declare methods that a class must implement without providing their body -> Option D

4. Quick Check:

   Interface purpose = declare methods only [OK]

Hint: Interfaces declare methods without bodies [OK]

Common Mistakes:

• Thinking interfaces can store variables
• Confusing interfaces with classes
• Believing interfaces create objects

2. Which of the following is the correct way to declare an interface named Vehicle in Java?

easy

A. interface Vehicle {}

B. class Vehicle interface {}

C. interface Vehicle() {}

D. Vehicle interface {}

Solution

1. Step 1: Recall Java interface syntax

   Interfaces are declared with the keyword interface followed by the name and curly braces.

2. Step 2: Check each option

   interface Vehicle {} matches correct syntax: interface Vehicle {}. Others have syntax errors like misplaced parentheses or keywords.

3. Final Answer:

   interface Vehicle {} -> Option A

4. Quick Check:

   Correct interface syntax = interface Name {} [OK]

Hint: Use 'interface Name {}' to declare interfaces [OK]

Common Mistakes:

• Adding parentheses after interface name
• Mixing class and interface keywords
• Omitting the 'interface' keyword

3. What will be the output of the following code?

interface Animal {
    void sound();
}

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

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

medium

A. Compilation error

B. sound

C. Bark

D. No output

Solution

1. Step 1: Understand interface implementation

   The class Dog implements Animal and provides the method sound() which prints "Bark".

2. Step 2: Trace the main method execution

   In main, an Animal reference points to a Dog object, calling sound() prints "Bark".

3. Final Answer:

   Bark -> Option C

4. Quick Check:

   Interface method called prints 'Bark' [OK]

Hint: Implemented method runs when called via interface reference [OK]

Common Mistakes:

• Expecting interface to print something
• Missing 'public' in method implementation causing error
• Thinking no output occurs

4. Identify the error in this interface declaration:

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

medium

A. Interfaces cannot have method bodies unless default or static

B. Method name 'add' is invalid in interfaces

C. Return type 'int' is not allowed in interfaces

D. Interface name must start with lowercase

Solution

1. Step 1: Check method body rules in interfaces

   In Java, interface methods cannot have bodies unless marked as default or static.

2. Step 2: Analyze the given method

   The method add has a body but no default or static keyword, causing a syntax error.

3. Final Answer:

   Interfaces cannot have method bodies unless default or static -> Option A

4. Quick Check:

   Method bodies in interfaces need default/static [OK]

Hint: Interface methods need default/static for bodies [OK]

Common Mistakes:

• Adding method bodies without default/static
• Thinking method names are restricted
• Ignoring Java naming conventions

5. Given two interfaces:

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();
    }
}

What is the output when running Test?

hard

A. Compilation error due to multiple interfaces

B. Printing document\nShowing document

C. Printing document only

D. No output

Solution

1. Step 1: Understand multiple interface implementation

   Java allows a class to implement multiple interfaces and must provide all their methods.

2. Step 2: Trace method calls in main

   The Document class implements both methods. Calling print() and show() prints both messages.

3. Final Answer:

   Printing document\nShowing document -> Option B

4. Quick Check:

   Multiple interfaces implemented, all methods run [OK]

Hint: Classes can implement many interfaces, all methods must be defined [OK]

Common Mistakes:

• Thinking multiple interfaces cause errors
• Forgetting to implement all interface methods
• Expecting only one method to run