Why interfaces are used in Java - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an interface named Vehicle.

Java

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

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

AVehicle

BCar

CDrive

DMoveable

Attempts:

3 left

💡 Hint

Common Mistakes

Using a class name instead of an interface name.

Forgetting to use the 'interface' keyword.

The keyword interface is used to declare an interface, and Vehicle is the interface name.

2fill in blank

medium

Complete the code to make class Car implement the Vehicle interface.

Java

public class Car [1] Vehicle {
    public void move() {
        System.out.println("Car is moving");
    }
}

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' for interfaces.

Omitting the 'implements' keyword.

In Java, a class uses the implements keyword to adopt an interface.

3fill in blank

hard

Fix the error in the code by completing the interface method declaration.

Java

public interface Animal {
    [1] sound();
}

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

Avoid

Bpublic

Cint

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' when a return value is expected.

Omitting the return type.

Interface methods must declare a return type. Here, sound() returns a String describing the sound.

4fill in blank

hard

Fill both blanks to create a map that maps words to their lengths only if length is greater than 3.

Java

Map<String, Integer> wordLengths = words.stream()
    .filter(word -> word.length() [1] 3)
    .collect(Collectors.toMap(word -> word, word -> word.[2]()));

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

A>

Blength

C<

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' in the filter.

Using 'size()' instead of 'length()' for strings.

The filter keeps words with length greater than 3 using >. The method length() gets the length of the word.

5fill in blank

hard

Fill all three blanks to create a map of uppercase keys to values only if value is negative.

Java

Map<String, Integer> filteredMap = data.entrySet().stream()
    .filter(entry -> entry.getValue() [1] 0)
    .collect(Collectors.toMap(entry -> entry.getKey().[2](), entry -> entry.[3]()));

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

A<

BgetValue

CgetKey

DtoUpperCase

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'getKey()' instead of 'getValue()' for values.

Using '>' instead of '<' in the filter.

The filter keeps entries with value less than 0 using '<'. The key is converted to uppercase with toUpperCase(). The value is accessed with getValue().

Practice

1. Why do Java programmers use interfaces?
interface defines methods but no implementation. What is the main reason to use them?

easy

A. To ensure different classes share common method names and can work together

B. To store data like variables and constants

C. To create objects directly from the interface

D. To replace classes completely

Solution

1. Step 1: Understand what interfaces define

   Interfaces declare methods without code, so classes must implement those methods.

2. Step 2: Recognize the purpose of interfaces

   They allow different classes to share method names, enabling them to work together flexibly.

3. Final Answer:

   To ensure different classes share common method names and can work together -> Option A

4. Quick Check:

   Interfaces share method names = A [OK]

Hint: Interfaces define method rules for classes to follow [OK]

Common Mistakes:

• Thinking interfaces store data
• Believing interfaces create objects
• Confusing interfaces with classes

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

easy

A. interface Vehicle { void move(); }

B. class Vehicle { void move(); }

C. interface Vehicle() { void move(); }

D. interface Vehicle { void move() {} }

Solution

1. Step 1: Check interface declaration syntax

   Interfaces use the keyword interface followed by name and method signatures without bodies.

2. Step 2: Identify correct method declaration

   Methods in interfaces have no body, so void move(); is correct.

3. Final Answer:

   interface Vehicle { void move(); } -> Option A

4. Quick Check:

   Interface syntax = A [OK]

Hint: Interface methods have no body, just signatures [OK]

Common Mistakes:

• Adding parentheses after interface name
• Defining method bodies inside interface
• Using class keyword instead of interface

3. What will be the output of this Java 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

   Class Dog implements Animal and provides the sound() method printing "Bark".

2. Step 2: Trace method call

   Variable a is Animal type but refers to Dog object, so a.sound() calls Dog's method printing "Bark".

3. Final Answer:

   Bark -> Option C

4. Quick Check:

   Interface method call runs Dog's method = C [OK]

Hint: Interface reference calls implemented method in class [OK]

Common Mistakes:

• Expecting interface method to print 'sound'
• Thinking code won't compile without method body in interface
• Confusing output with method name

4. Find the error in this code snippet:

interface Shape {
  void draw();
}
class Circle implements Shape {
  void draw() { System.out.println("Circle drawn"); }
}

medium

A. Interface cannot have methods

B. No error, code is correct

C. Circle class should be abstract

D. Method draw() must be public in Circle class

Solution

1. Step 1: Check method visibility rules

   Interface methods are implicitly public, so implementing methods must be public too.

2. Step 2: Identify method visibility in Circle

   Method draw() in Circle has default (package-private) visibility, missing public.

3. Final Answer:

   Method draw() must be public in Circle class -> Option D

4. Quick Check:

   Interface methods require public implementation = B [OK]

Hint: Implement interface methods as public always [OK]

Common Mistakes:

• Ignoring method visibility mismatch
• Thinking interface methods can be private
• Assuming no error if method is package-private

5. You want to design a system where multiple unrelated classes like Printer, Scanner, and Camera can all be "Connectable" to a computer. Which approach best uses interfaces to achieve this?

hard

A. Use abstract classes for Printer, Scanner, and Camera instead of interfaces

B. Create a Connectable interface with a connect() method, and have each class implement it

C. Add connect() method directly in each class without interface

D. Make Printer, Scanner, and Camera extend a common class Connectable

Solution

1. Step 1: Identify the need for shared behavior

   All classes need a common method connect() to work with the computer.

2. Step 2: Use interface for unrelated classes

   Interfaces allow unrelated classes to share method names without forcing inheritance.

3. Step 3: Evaluate options

   Create a Connectable interface with a connect() method, and have each class implement it uses interface to define connect(), letting each class implement it as needed.

4. Final Answer:

   Create a Connectable interface with a connect() method, and have each class implement it -> Option B

5. Quick Check:

   Interfaces enable shared methods for unrelated classes = D [OK]

Hint: Use interfaces to share methods across unrelated classes [OK]

Common Mistakes:

• Trying to use class inheritance for unrelated classes
• Duplicating methods without interface
• Confusing abstract classes with interfaces