Implementing interfaces in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an interface named Printable.

Java

public interface [1] {
    void print();
}

Drag options to blanks, or click blank then click option'

APrintInterface

BPrint

CPrinter

DPrintable

Attempts:

3 left

💡 Hint

Common Mistakes

Using a class name instead of an interface name.

Misspelling the interface name.

The interface name must match the declaration. Here, Printable is the correct name.

2fill in blank

medium

Complete the code to make class Document implement the Printable interface.

Java

public class Document [1] Printable {
    public void print() {
        System.out.println("Printing document...");
    }
}

Drag options to blanks, or click blank then click option'

Ainherits

Bextends

Cimplements

Duses

Attempts:

3 left

💡 Hint

Common Mistakes

Using extends instead of implements.

Forgetting to implement all interface methods.

In Java, a class implements an interface using the implements keyword.

3fill in blank

hard

Fix the error in the class by correctly overriding the print method from the Printable interface.

Java

public class Report implements Printable {
    public void [1]() {
        System.out.println("Report printing...");
    }
}

Drag options to blanks, or click blank then click option'

Aprint

BPrint

Cdisplay

Dshow

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'P' in method name.

Using a different method name than declared in the interface.

The method name must exactly match the interface method name, including case sensitivity.

4fill in blank

hard

Fill both blanks to declare an interface and a class that implements it.

Java

public interface [1] {
    void display();
}

public class Screen [2] Displayable {
    public void display() {
        System.out.println("Displaying screen...");
    }
}

Drag options to blanks, or click blank then click option'

ADisplayable

Bextends

Cimplements

DDisplay

Attempts:

3 left

💡 Hint

Common Mistakes

Using extends instead of implements for interfaces.

Mismatching interface names.

The interface is named Displayable and the class uses implements to connect to it.

5fill in blank

hard

Fill all three blanks to create an interface, implement it in a class, and override the method.

Java

public interface [1] {
    String getName();
}

public class User [2] Named {
    @Override
    public String [3]() {
        return "User";
    }
}

Drag options to blanks, or click blank then click option'

ANamed

Bimplements

CgetName

Dextends

Attempts:

3 left

💡 Hint

Common Mistakes

Using extends instead of implements for interfaces.

Incorrect method name or signature.

Mismatching interface name.

The interface is Named, the class User implements it, and overrides the getName method.

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