Variables and Data Types
Variables are containers for storing data. This lesson covers variables and data types in Java.
What is a Variable
A variable is a storage location in memory that holds data.
Three Elements of a Variable
| Element | Description | Example |
|---|---|---|
| Type | What kind of data to store | int, double, String |
| Name | How to find this variable | age, price, name |
| Value | The data stored in the variable | 25, 9.99, "Alice" |
Declaring Variables
JAVA
// Declare first, assign later
int age;
age = 25;
// Declare and assign
int age = 25;
// Declare multiple variables of the same type
int x = 1, y = 2, z = 3;
Naming Rules
Syntax Rules (Must Follow)
| Rule | Correct | Incorrect |
|---|---|---|
| Can only contain letters, digits, underscore, $ | myVar |
my-var |
| Cannot start with a digit | name1 |
1name |
| Cannot use Java keywords | myClass |
class |
| Case-sensitive | age ≠ Age |
— |
Naming Conventions (Recommended)
| Convention | Example | Description |
|---|---|---|
| camelCase | myVariableName |
Variables and methods |
| Meaningful names | studentAge |
Avoid a, b, temp |
| Constants in UPPERCASE | MAX_VALUE |
Separate with underscores |
Primitive Data Types
Java has 8 primitive data types.
Integer Types
| Type | Size | Range | Example |
|---|---|---|---|
byte |
1 byte | -128 to 127 | byte b = 100; |
short |
2 bytes | -32768 to 32767 | short s = 1000; |
int |
4 bytes | -2.1 billion to 2.1 billion | int i = 100000; |
long |
8 bytes | Much larger range | long l = 100L; |
💡 Use
int by default. Use long (with L suffix) when you need a larger range.
Floating-Point Types
| Type | Size | Precision | Example |
|---|---|---|---|
float |
4 bytes | 6-7 decimal places | float f = 3.14f; |
double |
8 bytes | 15 decimal places | double d = 3.14; |
💡 Use
double by default. Add the f suffix when you need float.
Character Type
JAVA
char c1 = 'A'; // Single character in single quotes
char c2 = 'B'; // Can store any character
char c3 = 65; // Can also use ASCII code values
Boolean Type
JAVA
boolean isTrue = true;
boolean isFalse = false;
💡 Note: In Java, boolean values can only be
true or false. They cannot be represented as 0 or 1.
Reference Data Types
Everything other than the 8 primitive types is a reference type.
Common Reference Types
JAVA
String name = "Alice"; // String
int[] arr = {1, 2, 3}; // Array
Object obj = new Object(); // Object
String
JAVA
// Strings use double quotes
String greeting = "Hello, World!";
// String concatenation
String fullName = "John" + " " + "Doe";
// String length
int len = greeting.length(); // 13
Type Conversion
Automatic Type Conversion (small → large)
JAVA
int i = 100;
long l = i; // int automatically converts to long
double d = i; // int automatically converts to double
Explicit Type Casting (large → small)
JAVA
double d = 3.14;
int i = (int) d; // Explicit cast, result is 3 (decimal truncated)
long l = 100;
int i = (int) l; // Explicit cast
⚠️ Note: Explicit casting may lose precision or cause overflow. Use with caution.
Example: Type Conversion
JAVA
public class TypeConversion {
public static void main(String[] args) {
// Automatic type conversion
int a = 100;
long b = a;
double c = a;
System.out.println("int: " + a); // 100
System.out.println("long: " + b); // 100
System.out.println("double: " + c); // 100.0
// Explicit type casting
double d = 3.99;
int e = (int) d;
System.out.println("double: " + d); // 3.99
System.out.println("int: " + e); // 3 (truncated, not rounded)
}
}
final Constants
Use the final keyword to define constants. Once assigned, their value cannot be changed.
JAVA
final double PI = 3.14159;
final String APP_NAME = "MyApp";
final int MAX_SIZE = 100;
// PI = 3.14; // Error! Constants cannot be modified
💡 Naming convention: Constant names should be all uppercase, separated by underscores.
var Local Variable Type Inference
Java 10 introduced the var keyword, which allows automatic type inference for local variables.
JAVA
// Traditional approach
String name = "Alice";
ArrayList<String> list = new ArrayList<String>();
// Using var
var name = "Alice"; // Automatically inferred as String
var list = new ArrayList<String>(); // Automatically inferred as ArrayList<String>
⚠️ Note:
var can only be used for local variables, not for class member variables or method parameters.
❓ Frequently Asked Questions
Q What's the difference between int and Integer?
A int is a primitive type, Integer is a wrapper class (reference type). Integer can be null, int cannot.
Q How do I choose between float and double?
A Generally use double for higher precision. Only use float when you need to save memory or process large amounts of floating-point data.
Q Is String a primitive type?
A No. String is a reference type, but Java has special support for it, allowing direct assignment with string literals.
📖 Summary
- Variables are containers for storing data with three elements: type, name, and value
- Java has 8 primitive types: byte/short/int/long/float/double/char/boolean
- Use int and double by default; use long and float when larger range is needed
- Type conversion is divided into automatic (small → large) and explicit (large → small)
- final defines constants whose values cannot be changed
- var enables automatic type inference (Java 10+)
📝 Exercises
- Variable practice: Define variables to store your name, age, height, and weight, then print them
- Type conversion: Convert the double value 3.14 to int and observe the result; convert int value 100 to double and observe the result
- Constant practice: Define a constant PI for the mathematical constant π, and calculate the area of a circle with radius 5
Next Lesson
In the next lesson, we'll learn about Operators — understanding various operations in Java.