Understanding Java Variables and Data Types: A Complete Overview

Notes on Variables and Data Types in Java

Introduction

• Programs contain instructions and data.

• A variable is a way to access and store data.

• Data can be of different types such as integer, string, or floating point.

Data Types

• Int: Integer type.

• String: Sequence of characters.

• Float: Floating point number.

Importance of Data Types

• Specify the range of data.

• Determine the memory allocation for the variable.

• In Java, data types must be specified before using a variable (statically typed).

Statically Typed vs Dynamically Typed

• Statically Typed: Languages like Java, C, and C++ require variable declaration before use.

• Dynamically Typed: Languages like Python determine the data type at runtime.

Variable Naming

• Allowed characters: letters (a-z, A-Z), digits (0-9), underscore (_), and dollar sign ($).

• Rules:

  • Cannot begin with a digit.

  • Cannot use keywords (e.g., else, do, for, while).

• Conventions:

  • Camel Case: First word in lowercase, subsequent words start with an uppercase letter.

    • Example: nameOfPlayer, ageOfPlayer, currentYear.

  • Constants: Uppercase letters separated by underscores.

    • Example: MAX_AGE, MAX_PLAYERS.

Types of Variables

• Primitive Types: Basic types like int, char, float, double.

• Non-Primitive Types: Objects of classes (e.g., String, Array).

Primitive Data Types

1. Boolean

   • Values: true, false.

   • Example variables: isValid, isMarried, flag.

2. Byte

   • Size: 8 bits.

   • Range: -128 to 127.

   • Example: Marks of a student.

3. Short

   • Size: 16 bits.

   • Range: -32,768 to 32,767.

4. Int

   • Size: 32 bits.

   • Range: -2^31 to 2^31-1.

   • Example: General integer values.

5. Long

   • Size: 64 bits.

   • Range: -2^63 to 2^63-1.

   • Example: Very large integer values.

6. Float

   • Size: 32 bits.

   • Used for decimal values.

   • Example: weight, price.

7. Double

   • Size: 64 bits.

   • Used for precise decimal values.

8. Char

   • Size: 16 bits.

   • Range: 0 to 2^16-1.

   • Used for storing characters.

   • Example: gender, grade.

public class Main {
    public static void main(String[] args) {
        boolean isValid = true;
        byte marks = 90;
        float pi = 3.14f;
        float div = 15.0f / 4.0f;
        long views = 10000000000L;
        char gender = 'M';
    }
}

Example Program

Interview Questions

1. What are primitive data types in Java?

   • Primitive data types are the most basic data types built into the language, such as int, char, float, double, boolean, byte, short, and long.

2. How do statically typed languages differ from dynamically typed languages?

   • Statically typed languages require the data type of a variable to be declared before use, while dynamically typed languages determine the data type at runtime.

3. What is the significance of memory allocation in variable declaration in Java?

   • Memory allocation determines how much space is allocated for the variable's data, which affects performance and resource usage.

4. Explain camel case naming convention with an example.

   • Camel case involves starting with a lowercase letter for the first word and capitalizing the first letter of each subsequent word. Example: currentYear.

5. What are the rules for naming variables in Java?

   • Variable names can contain letters, digits, underscores, and dollar signs but cannot start with a digit or use keywords.

6. Give an example of a constant variable declaration in Java.

   • Example: final int MAX_AGE = 200;. Constants are written in uppercase letters separated by underscores.

7. What is the range of values for a byte data type in Java?

   • The range is from -128 to 127.

8. Why are char data types 16 bits in Java?

   • Java uses 16-bit characters to support Unicode, allowing representation of characters from various languages.