Variables & Primitive Data Types — Java Reference, Syntax & Examples

Syntax

type variableName = value;

Examples

Declaring Primitive Variables

Creating variables of each common primitive type.

int age = 21;
double gpa = 3.8;
char grade = 'A';
boolean isStudent = true;
long population = 8000000000L;  // L suffix for long literals
float price = 19.99f;            // f suffix for float literals

System.out.println(age + " " + gpa + " " + grade + " " + isStudent);

Constants with final

Using the 'final' keyword to declare a variable that cannot be reassigned.

final double PI = 3.14159;
final int MAX_USERS = 100;

// PI = 3.14;  // This would cause a compile error

System.out.println("Max users allowed: " + MAX_USERS);

Variable Scope

Understanding how a variable's visibility is limited to the block it's declared in.

public class ScopeExample {
    static int classField = 10;  // accessible throughout the class

    public static void main(String[] args) {
        int localVar = 5;  // only accessible inside main()

        if (localVar > 0) {
            int blockVar = 20;  // only accessible inside this if block
            System.out.println(blockVar);
        }

        System.out.println(classField + localVar);
        // System.out.println(blockVar);  // Error: out of scope here
    }
}

Default Values

Instance and static fields get default values automatically if not explicitly initialized (local variables do not).

public class Defaults {
    int number;        // defaults to 0
    boolean flag;       // defaults to false
    String text;         // defaults to null

    public static void main(String[] args) {
        Defaults d = new Defaults();
        System.out.println(d.number);  // 0
        System.out.println(d.flag);    // false
        System.out.println(d.text);    // null
    }
}

Best practices

Use camelCase for variable names (studentAge, not StudentAge or student_age) to follow Java conventions
Choose the smallest primitive type that safely fits your data - use int for whole numbers unless you specifically need long's larger range
Use 'final' for values that should never change after initialization, both for safety and to communicate intent to other developers
Prefer double over float for decimal values unless memory is a serious constraint - float has noticeably less precision
Always initialize local variables explicitly - unlike fields, Java will not compile code that reads an uninitialized local variable

Runtime & versions

Java Virtual Machine. Feature availability depends on the language and runtime version used by your project; consult the official reference below.

Java Language Specification

Specifications & further reading

Java Language Specification