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?

1. Local Variable – Inside method
2. Instance Variable – Inside class, outside method
3. 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?

1. Single-line → //
2. Multi-line → /* */
3. 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, _, $

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