Static methods in interfaces in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of calling static method in interface

What is the output of the following Java code?

Java

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

public class Test {
    public static void main(String[] args) {
        System.out.println(Calculator.add(5, 7));
    }
}

A 12

B Compilation error: cannot call static method in interface

C Runtime error

D 0

💡 Hint

Static methods in interfaces can be called using the interface name.

Static methods defined in interfaces can be called using the interface name directly. Here, Calculator.add(5, 7) returns 12.

🧠 Conceptual

intermediate

Where can static methods in interfaces be called from?

Which of the following is true about calling static methods defined in a Java interface?

A They can be called using the interface name anywhere in the code.

B They can only be called from within the interface itself.

C They can be called using an instance of a class implementing the interface.

D They cannot be called at all; static methods are not allowed in interfaces.

💡 Hint

Think about how static methods are accessed in classes.

Static methods in interfaces are accessed using the interface name, not through instances or implementing classes.

🔧 Debug

advanced

Identify the error in static method usage

What error will this code produce?

Java

interface Printer {
    static void print() {
        System.out.println("Printing from interface");
    }
}

class Document implements Printer {
    public void print() {
        System.out.println("Printing from class");
    }
}

public class Test {
    public static void main(String[] args) {
        Document doc = new Document();
        doc.print();
        Printer.print();
        doc.Printer.print();
    }
}

A Compilation error at Printer.print();

B Compilation error at doc.Printer.print();

C Runtime error at doc.print();

D

No errors; prints:
Printing from class
Printing from interface
Printing from interface

💡 Hint

Check how static methods in interfaces are accessed.

Static methods in interfaces cannot be called through an instance or implementing class reference. The line doc.Printer.print(); causes a compilation error.

📝 Syntax

advanced

Valid syntax for static method in interface

Which option correctly declares a static method inside a Java interface?

A void static show() { System.out.println("Hello"); }

B static void show();

C static void show() { System.out.println("Hello"); }

D public static void show();

💡 Hint

Static methods in interfaces must have a body.

Static methods in interfaces must be declared with a body. Option C is correct syntax. Options C and D lack method bodies, causing errors. Option C has wrong keyword order.

🚀 Application

expert

Using static methods in interfaces for utility

Given this interface, what will be the output of the main method?

Java

interface Utils {
    static String greet(String name) {
        return "Hello, " + name + "!";
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(Utils.greet("Alice"));
        String message = Utils.greet(null);
        System.out.println(message == null ? "No greeting" : message);
    }
}

A Compilation error

B

Hello, Alice!
No greeting

C

Hello, Alice!
NullPointerException

D

Hello, Alice!
Hello, null!

💡 Hint

What happens when you concatenate a string with null in Java?

Concatenating a string with null results in the string "null". So Utils.greet(null) returns "Hello, null!".

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