Static methods in interfaces in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a static method in an interface.

Java

public interface Calculator {
    static int [1](int a, int b) {
        return a + b;
    }
}

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

Aadd

Bsum

Ccalculate

Dcompute

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-descriptive method name.

Forgetting to declare the method as static.

The method name add is used as the static method name in the interface.

2fill in blank

medium

Complete the code to call the static method from the interface.

Java

int result = Calculator.[1](5, 3);

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

Acompute

Badd

Csum

Dcalculate

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call the method on an instance.

Using a wrong method name.

The static method add is called using the interface name.

3fill in blank

hard

Fix the error in the static method declaration inside the interface.

Java

public interface Printer {
    [1] void print(String message) {
        System.out.println(message);
    }
}

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

Adefault

Bprivate

Cstatic

Dpublic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'default' instead of 'static'.

Omitting the 'static' keyword.

The method must be declared static to be a static method in an interface.

4fill in blank

hard

Fill both blanks to define and call a static method in an interface.

Java

public interface Utils {
    static String [1](String s) {
        return s.toUpperCase();
    }
}

String result = Utils.[2]("hello");

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

AtoUpperCase

Bconvert

Cuppercase

DmakeUpper

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names in declaration and call.

Using non-descriptive method names.

The static method name toUpperCase is used both in declaration and call.

5fill in blank

hard

Fill all three blanks to create a static method in an interface that returns the square of a number and call it.

Java

public interface MathOps {
    static int [1](int n) {
        return n [2] n;
    }
}

int squared = MathOps.[3](4);

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

Asquare

B*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '*' for multiplication.

Using different method names in declaration and call.

The method square multiplies the input by n using the '*' operator and is called as MathOps.square(4).

Practice

1. What is the correct way to call a static method calculate() defined inside an interface MathOps?

easy

A. MathOps.calculate()

B. new MathOps().calculate()

C. calculate()

D. MathOps obj = new MathOps(); obj.calculate()

Solution

1. Step 1: Understand static method call in interfaces

   Static methods in interfaces are called using the interface name, not instances.

2. Step 2: Analyze the options

   Only MathOps.calculate() correctly calls the static method. Creating instances or calling directly is invalid.

3. Final Answer:

   MathOps.calculate() -> Option A

4. Quick Check:

   Static method call = InterfaceName.method() [OK]

Hint: Call static interface methods with InterfaceName.method() [OK]

Common Mistakes:

• Trying to call static method on an instance
• Calling static method without interface name
• Trying to instantiate an interface

2. Which of the following is the correct syntax to declare a static method printMessage inside an interface Logger?

easy

A. static void printMessage();

B. static void printMessage() { System.out.println("Hello"); }

C. public static void printMessage();

D. void static printMessage() { System.out.println("Hello"); }

Solution

1. Step 1: Recall static method syntax in interfaces

   Static methods must have a body and use static keyword before return type.

2. Step 2: Check each option

   Only static void printMessage() { System.out.println("Hello"); } correctly declares and defines the static method. Declarations without a body or with static after the return type are invalid.

3. Final Answer:

   static void printMessage() { System.out.println("Hello"); } -> Option B

4. Quick Check:

   Static method = static + return type + name + () + body [OK]

Hint: Static methods in interfaces need a body and static keyword first [OK]

Common Mistakes:

• Omitting method body in static method
• Placing static keyword after return type
• Declaring static methods without body

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

interface Helper {
    static String greet() {
        return "Hi!";
    }
}
public class Test {
    public static void main(String[] args) {
        System.out.println(Helper.greet());
    }
}

medium

A. Runtime error

B. Compile-time error

C. null

D. Hi!

Solution

1. Step 1: Understand static method call in interface

   The static method greet() is called correctly using Helper.greet().

2. Step 2: Predict output

   The method returns "Hi!" which is printed by System.out.println.

3. Final Answer:

   Hi! -> Option D

4. Quick Check:

   Static method returns "Hi!" printed [OK]

Hint: Static interface methods run when called by InterfaceName.method() [OK]

Common Mistakes:

• Trying to call static method on instance
• Expecting compile error due to interface method
• Confusing static with default methods

4. Identify the error in the following code snippet:

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

public class Demo {
    public static void main(String[] args) {
        Calculator calc = new Calculator();
        System.out.println(calc.add(5, 3));
    }
}

medium

A. Missing return statement in add()

B. Static method add() cannot be called on instance

C. Cannot instantiate interface Calculator

D. No error, code runs fine

Solution

1. Step 1: Check interface instantiation

   Interfaces cannot be instantiated directly using new.

2. Step 2: Analyze method call

   Static methods must be called using interface name, not instance. But the main error is instantiating interface.

3. Final Answer:

   Cannot instantiate interface Calculator -> Option C

4. Quick Check:

   Interfaces cannot be instantiated [OK]

Hint: Interfaces cannot be created with new keyword [OK]

Common Mistakes:

• Trying to instantiate interface
• Calling static method on instance
• Ignoring compile errors on interface instantiation

5. Given the interface Utils with a static method isEven(int n) that returns true if n is even, how can you use this method inside a class NumberChecker to filter even numbers from a list List<Integer> nums using streams?

hard

A. nums.stream().filter(Utils::isEven).toList();

B. nums.stream().filter(n -> Utils.isEven()).toList();

C. nums.stream().filter(n -> isEven(n)).toList();

D. nums.stream().filter(n -> Utils.isEven(n)).collect();

Solution

1. Step 1: Understand method reference syntax

   Static methods can be referenced as InterfaceName::methodName in streams.

2. Step 2: Analyze options for correct syntax

   nums.stream().filter(Utils::isEven).toList(); uses method reference correctly. The lambda n -> Utils.isEven() misses argument n, n -> isEven(n) lacks interface qualification, and .collect() requires a collector.

3. Final Answer:

   nums.stream().filter(Utils::isEven).toList(); -> Option A

4. Quick Check:

   Use InterfaceName::staticMethod for stream filters [OK]

Hint: Use InterfaceName::methodName for static method references in streams [OK]

Common Mistakes:

• Calling static method without argument in lambda
• Using instance method syntax for static methods
• Wrong terminal operation like collect() without collector