Java Data Type Interview Questions and Answers

Java Data Types

Chapter Exercises Video Tutorial Discussion

Scenario 1: Choosing the Right Type

You are building a student grading system. You need to store the number of students in a class, the average grade (which can be a decimal), and whether a student has passed or failed.

Interview Question

Which primitive data types would you choose for each of these variables, and why ?

Answer:

Number of students: int (because it’s a whole number)

Average grade: double (to store decimal values)

Pass/fail: boolean (to represent true/false)

Scenario 2: Memory Matters

You are developing an application for a device with very limited memory. You need to store small whole numbers, such as the age of children in a kindergarten class (ages 3-7).

Interview Question

Which primitive data type is most memory-efficient for this use case, and why ?

Answer:

byte is the most memory-efficient primitive type for small whole numbers, as it uses only 8 bits and can store values from -128 to 127.

Scenario 3: Type Conversion

You have two variables

int totalMarks = 450;  
double percentage = totalMarks / 5;

But when you print percentage, you get 90.0 as expected. Now, you change totalMarks to 451 and print percentage again, but the result is 90.0 instead of 90.2.

Interview Question

Why does this happen, and how would you fix it ?

Answer:

This happens because totalMarks / 5 performs integer division before assigning the result to the double variable. To fix it, make at least one operand a double:

double percentage = totalMarks / 5.0;

Scenario 4: Reference Vs. Primitive

You are writing a function that updates a student’s grade. You notice that when you pass an int variable to the function, the original value does not change, but when you pass an array, the original array changes.

Interview Question

Why does this happen in Java ?

Answer:

Primitive data types (like int) are passed by value, so changes inside the function don’t affect the original variable. Arrays are reference types, so the function receives a reference to the original array and can modify its contents.

Scenario 5: Wrapper Classes in Collections

You want to store a list of student roll numbers in a List. You try to use List<int>, but it gives a compilation error.

Interview Question

Why cannot you use int in a List, and what is the solution ?

Answer:

Java Collections only work with objects, not primitive types. You should use the wrapper class Integer, so declare it as List<Integer>