Java 8 Advanced | Optional | Streams | HashMap | Comparable | SQL | Programming

Duration: 20 Minutes

Experience: 3–8 Years

This assessment focuses on advanced Java 8 concepts commonly asked in backend interviews and OCP Java 8 Programmer certification.

Skills Tested:

• Optional
• map() vs flatMap()
• Comparable vs Comparator
• HashMap behavior
• Method References
• Stream Operations
• SQL Joins & Aggregation
• Programming Logic

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Section A – Java OCP Style MCQs (5 Questions)

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Question 1 – Optional

What is the output?

import java.util.Optional;

public class Test {

    public static void main(String[] args) {

        Optional<String> value =
                Optional.ofNullable(null);

        System.out.println(
                value.orElse("Java"));

    }

}

A. null

B. Java

C. Optional.empty

D. NullPointerException

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Question 2 – Stream map()

import java.util.*;

public class Test {

    public static void main(String[] args) {

        List<String> list =
                Arrays.asList("java","spring");

        list.stream()
                .map(String::toUpperCase)
                .forEach(System.out::print);

    }

}

Output?

A.

java spring

B.

JAVASPRING

C.

JAVA SPRING

D. Compilation Error

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Question 3 – Comparable vs Comparator

import java.util.*;

class Employee {

    int id;

    Employee(int id){
        this.id=id;
    }

    public String toString(){
        return String.valueOf(id);
    }

}

public class Test {

    public static void main(String[] args){

        List<Employee> list=
                Arrays.asList(
                        new Employee(3),
                        new Employee(1),
                        new Employee(2));

        Collections.sort(list);

        System.out.println(list);

    }

}

What happens?

A. [1,2,3]

B. [3,1,2]

C. Compilation Error

D. Runtime Exception

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Question 4 – HashMap

Map<String,Integer> map =
        new HashMap<>();

map.put("Java",1);
map.put("Java",2);

System.out.println(map.size());
System.out.println(map.get("Java"));

Output?

A.

2
2

B.

1
2

C.

2
1

D.

Compilation Error

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Question 5 – Method Reference

import java.util.*;

public class Test {

    public static void main(String[] args){

        Arrays.asList("A","B","C")
                .forEach(System.out::print);

    }

}

Output?

A.

ABC

B.

A B C

C.

[A,B,C]

D.

Compilation Error

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Section B – SQL Challenge

Question 6

Tables

EMPLOYEE

EMP_ID	NAME	DEPT_ID
1	John	10
2	Mary	20
3	Steve	10
4	David	30

DEPARTMENT

DEPT_ID	NAME
10	IT
20	HR
30	Finance

Query

SELECT D.NAME,
       COUNT(E.EMP_ID)
FROM DEPARTMENT D
LEFT JOIN EMPLOYEE E
ON D.DEPT_ID=E.DEPT_ID
GROUP BY D.NAME;

Which statement is correct?

A. INNER JOIN output

B. Departments without employees are excluded

C. Every department appears at least once

D. Query throws an error

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Section C – Programming

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Question 7 – First Duplicate Character

Write a Java program to find the first duplicate character.

Example

programming

Output

r

Bonus

Solve using Java Streams.

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Question 8 – Group Strings

Given

List<String> list =
Arrays.asList(
"Java",
"Spring",
"AWS",
"Angular",
"AI");

Using Java 8,

Group strings by their length.

Expected Output

2 -> [AI]

3 -> [AWS]

4 -> [Java]

6 -> [Spring]

7 -> [Angular]

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Question 9 – Programming Logic

Write a Java program to determine whether a String is a palindrome without using reverse().

Example

madam

Output

true

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Answer Key & Explanations

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Question 1

Answer: B

Optional.ofNullable(null) creates an empty Optional.

orElse("Java") supplies the default value.

Output

Java

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Question 2

Answer: B

map() transforms each element.

java
↓
JAVA

spring
↓
SPRING

forEach(System.out::print) prints without spaces.

Output

JAVASPRING

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Question 3

Answer: D

Collections.sort() requires elements to implement Comparable unless a Comparator is supplied.

The code compiles because the raw sort method exists, but at runtime a ClassCastException is thrown when Java attempts to compare Employee objects.

Interview Follow-up:

• How would you fix it?
• Which approach is preferable: Comparable or Comparator?

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Question 4

Answer: B

A HashMap cannot contain duplicate keys.

The second put() replaces the previous value.

Key   Value

Java   2

Size remains 1.

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Question 5

Answer: A

System.out::print is a method reference equivalent to

s -> System.out.print(s)

Output

ABC

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Question 6

Answer: C

LEFT JOIN guarantees that every row from the left table (DEPARTMENT) appears.

Even if a department has zero employees, it will still be returned with a count of 0.

Concepts tested:

• LEFT JOIN
• GROUP BY
• Aggregate Functions

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Question 7 – Sample Solution

Traditional

Set<Character> set = new HashSet<>();

for(char c : input.toCharArray()){

    if(!set.add(c)){

        System.out.println(c);
        break;

    }

}

Complexity

Time

O(n)

Space

O(n)

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Question 8 – Sample Solution

list.stream()

.collect(Collectors.groupingBy(String::length))

.forEach((k,v)->

System.out.println(k+" -> "+v));

Concepts

• groupingBy
• Method Reference
• Collectors

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Question 9 – Sample Solution

boolean palindrome=true;

for(int i=0,j=input.length()-1;
i<j;
i++,j--){

    if(input.charAt(i)!=
       input.charAt(j)){

        palindrome=false;
        break;

    }

}

Complexity

Time

O(n)

Space

O(1)

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Evaluation Guide

Score	Recommendation
9/9	Excellent
7–8	Strong Java 8 Developer
5–6	Average
Below 5	Needs Improvement

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Interviewer’s Notes

This set is designed to identify candidates who understand how Java behaves internally, rather than simply remembering APIs.

Look for candidates who can explain:

• Why Optional avoids NullPointerException.
• The difference between map() and flatMap() (even though only map() is used here).
• Why Collections.sort() requires Comparable or a Comparator.
• Why HashMap.put() replaces values for duplicate keys.
• Why a LEFT JOIN returns all rows from the left table.
• Time and space complexity of their programming solutions.