Java Notes – Variables and Data Types

(int, double, boolean, char, String)

1. What Is a Variable?

A variable is a named location in memory that stores data while a program runs. You declare a variable by stating the data type and the variable name.

int age;
double price;
String name;

A variable must be declared before it is used.

2. What Is a Data Type?

A data type tells Java what kind of data the variable will hold and how much memory it needs. Java is strongly typed, meaning the type cannot change after the variable is declared.

There are two major categories:

A. Primitive Data Types

• Built-in types that store simple values directly in memory.
• They are not objects.

B. Reference Data Types

• These store the memory address of a complex object, not the object itself.
• Example: String

3. Key Data Types You Must Know

int (integer)

• Stores whole numbers (no decimals).
• Range: about −2 billion to +2 billion.
• Most commonly used numeric type.

int students = 30;
int score = 95;

double

• Stores decimal numbers (floating-point values).
• Used for measurements, averages, money (but not perfect for exact money).

double temperature = 98.6;
double gpa = 3.75;

boolean

• Stores true or false.
• Used for logical decisions and conditions.

boolean isFinished = false;
boolean passed = true;

char (character)

• Stores ONE single character.
• Uses single quotes ' '.
• Can store letters, digits, punctuation, or special symbols.

char grade = 'A';
char symbol = '#';

String

• Stores text (multiple characters).
• NOT a primitive type → it is a class / object.
• Uses double quotes " ".

String name = "Mr. Cusack";
String school = "Klein Collins High School";

4. Declaring and Initializing Variables

Declaration only

int age;

Initialization

age = 17;

Declaration + Initialization (common way)

int age = 17;
double price = 4.99;
boolean isReady = true;
char letter = 'C';
String city = "Houston";

5. Naming Variables

Rules:

• Must start with a letter, _, or $.
• Cannot start with a number.
• No spaces.
• Case-sensitive.

Conventions:

• Use camelCase.
• Use meaningful names.

int studentCount;
boolean isValid;
String firstName;

6. Changing Variable Values

Variables can be updated after declaration (except final variables).

int score = 80;
score = 90;   // updated

7. Common Errors

8. Summary Table