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Java Interface types and Comparator

Here are 10 essential multiple-choice questions on Java Interface types and Comparator, covering key concepts.

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Question 1

Which of the following statements about functional interfaces is true?

A functional interface can have multiple abstract methods
Functional interfaces are only available from Java 11 onwards
A functional interface can have multiple default and static methods
Runnable is not a functional interface

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Question 2

What will be the output of the following code?

Java

@FunctionalInterface
interface Geeks {
    void display();
    default void show() { 
        System.out.println("Default method"); 
        
    }
}
public class Test {
    public static void main(String[] args) {
        Geeks obj = () -> System.out.println("Lambda Executed");
        obj.display();
    }
}

Default method
Lambda Executed
Compilation Error
Runtime Error

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Question 3

How is a nested interface defined in Java?

An interface declared inside another interface or class
An interface that extends another interface
An interface that has private methods
An interface with only default methods

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Question 4

Which of the following is NOT true about nested interfaces?

A nested interface inside a class must be declared static
A nested interface can be accessed using OuterClass.InnerInterface
A nested interface can have constructors
A class implementing a nested interface must implement all its methods

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Question 5

Which of the following is a correct example of a Marker Interface?

interface Serializable { void serialize(); }
interface Cloneable {}
interface Marker { void mark(); }
interface Runnable { void run(); }

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Question 6

What is the primary use of a marker interface in Java?

To define constants
To enforce a specific method implementation
To provide metadata for runtime processing
To allow multiple inheritance

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Question 7

What is true about Java’s Comparator interface?

It can compare only primitive types
It is used for natural ordering of objects
It defines the compare() method for custom sorting
It can only be used with TreeSet

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Question 8

What will be the output of the following code?

Java

import java.util.*;
class Student {
    int age;
    Student(int age) { this.age = age; }
}
class AgeComparator implements Comparator<Student> {
    public int compare(Student s1, Student s2) {
        return s1.age - s2.age;
    }
}
public class Test {
    public static void main(String[] args) {
        List<Student> list = Arrays.asList(new Student(20), new Student(18));
        Collections.sort(list, new AgeComparator());
        System.out.println(list.get(0).age);
    }
}

20
18
Compilation Error
Runtime Error

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Question 9

How is Comparator different from Comparable?

Comparator is implemented in the same class as the object being compared
Comparable provides multiple sorting sequences
Comparator allows sorting objects without modifying their class
Comparable is a class, whereas Comparator is an interface

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Question 10

What is the benefit of using a lambda expression with a Comparator?

It makes sorting slower
It eliminates the need for method references
It reduces boilerplate code
It is only supported from Java 17 onwards

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