Interface declaration in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an interface named Vehicle.

Java

public [1] Vehicle {
    void start();
}

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

Ainterface

Bclass

Cenum

Dabstract

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'class' instead of 'interface' to declare an interface.

Using 'abstract' keyword alone without 'interface'.

The keyword interface is used to declare an interface in Java.

2fill in blank

medium

Complete the code to declare a method named stop in the interface.

Java

public interface Machine {
    [1] stop();
}

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

Avoid

Bint

Cstatic

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'int' as return type when no value is returned.

Using 'static' or 'final' keywords in method declaration inside interface.

Interface methods are abstract by default and usually have no return value if they perform an action, so void is appropriate here.

3fill in blank

hard

Complete the code to explicitly declare an abstract method named reset in the interface.

Java

public interface Device {
    public [1] void reset();
}

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

Aprivate

Babstract

Cfinal

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' keyword which is not allowed in interface methods.

Using 'private' access modifier which is invalid in interfaces.

Interface methods are implicitly abstract, so adding abstract is correct and allowed.

4fill in blank

hard

Fill both blanks to declare an interface named Calculator with a method calculate returning an int.

Java

public [1] Calculator {
    [2] calculate();
}

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

Ainterface

Bvoid

Cint

Dclass

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'class' instead of 'interface' for the first blank.

Using 'void' as return type when an int is expected.

The keyword interface declares the interface, and the method calculate returns an int.

5fill in blank

hard

Fill all three blanks to declare an interface named Printer with a method print that takes a String parameter and returns nothing.

Java

public [1] Printer {
    [2] print([3] message);
}

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

Ainterface

Bvoid

CString

Dclass

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'class' instead of 'interface' for the first blank.

Using 'int' or other types instead of 'void' for the return type.

Using incorrect parameter types instead of 'String'.

Use interface to declare the interface, void as the return type for no return value, and String as the parameter type.

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