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!".