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.

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