Java Fundamentals Interview Questions & Answers

Java Syntax & Structure

Q1. What is the basic structure of a Java program?

A Java program must contain a class and a main() method (entry point).

Example:

class Demo {
    public static void main(String[] args) {
        System.out.println("Hello Java");
    }
}

Q2. Why must Java code be inside a class?

Because Java is object-oriented, and everything in Java belongs to a class.

Q3. What is the entry point of a Java program?

public static void main(String[] args)

Q4. Why does main() method have String[] args?

It stores command-line arguments.

Variables

Q5. What is a variable?

A variable is a name given to a memory location where data is stored.

Q6. What are the types of variables in Java?

Local Variable – Inside method
Instance Variable – Inside class, outside method
Static Variable – Declared using static keyword

Example:

class Test {
    int age = 25;       // instance variable
    static String name = "Java";  // static variable

    void show() {
        int x = 10;     // local variable
        System.out.println(x);
    }
}

Q7. Why should variables be initialized?

Because Java does not automatically initialize local variables.

Data Types, Type Casting & Type Promotion

Q8. What are data types in Java?

Two categories:

Primitive (8 types)

byte, short, int, long
float, double
boolean
char

Non-Primitive

String
Arrays
Classes
Interfaces

Q9. What is type casting?

Converting one data type to another.

Two types:

1. Implicit / Widening (smaller → larger)

int a = 10;
double b = a;   // auto conversion

2. Explicit / Narrowing (larger → smaller)

double x = 10.5;
int y = (int) x;   // explicit cast

Q10. What is type promotion in Java?

In expressions, smaller types automatically convert to larger types.

Example:

byte a = 10;
byte b = 20;
int c = a + b;  // byte promoted to int

Operators (Arithmetic, Relational, Logical, Bitwise, Ternary)

Q11. What are arithmetic operators?

Perform mathematical calculations like addition, subtraction, multiplication, division, modulus, increment, and decrement.

+ - * / % ++ --

Example:

int a = 10, b = 3;
System.out.println(a % b);   // 1

Q12. What are relational operators?

Compare two values and return true or false. (== != > < >= <=)

Example:

System.out.println(10 > 5);  // true

Q13. What are logical operators?

Combine boolean expressions. && || !

Example:

boolean x = true, y = false;
System.out.println(x && y); // false

Q14. What are bitwise operators?

Perform operations on individual bits of integers. & | ^ ~ << >> >>>

Example:

int a = 5;   // 0101
int b = 3;   // 0011
System.out.println(a & b);  // 1

Q15. What is the ternary operator?

A shorthand if-else operator that returns one of two values based on a condition.

condition ? value_if_true : value_if_false

Example:

int age = 20;
String result = (age >= 18) ? "Adult" : "Minor";

Comments & Code Formatting

Q16. What are types of comments in Java?

Single-line → //
Multi-line → /* */
Documentation comment → /** */

Example:

// Single-line
/* Multi-line */
/** Documentation comment */

Q17. Why are comments used?

To explain logic and improve code readability.

Q18. What are Java code formatting best practices?

Use meaningful variable names
Proper indentation
Follow CamelCase
Add comments where needed
Keep methods short

Input / Output using Scanner and BufferedReader

Q19. How to take input using Scanner?

import java.util.Scanner;

class InputExample {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int age = sc.nextInt();
        System.out.println("Age: " + age);
    }
}

Q20. What are disadvantages of Scanner?

Slower than BufferedReader
Uses more memory

Q21. How to take input using BufferedReader?

import java.io.*;

class InputExample2 {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String name = br.readLine();
        System.out.println("Name: " + name);
    }
}

Q22. Which is faster – Scanner or BufferedReader?

BufferedReader is faster because it reads data in larger chunks.

Java Keywords & Identifiers

Q23. What are keywords?

Reserved words in Java (cannot be used as variable names).
There are 67 keywords in Java (including new ones).

Examples:

class, int, static, public, return, if, else, extends, final

Q24. What are identifiers?

Names given to:

Variables
Methods
Classes
Objects
Packages

Example:

int age;        // age is identifier
class Student{} // Student is identifier

Q25. What are rules for identifiers?

Cannot start with digit
Cannot use keywords
Case-sensitive
Allowed: letters, digits, _, $

Valid: myVar, _count, $value
Invalid: 1total, class, my-var