Variables in Java

What is a variable

A variable is a named box in memory that holds a value. In Java, every variable has a fixed type that you state when you declare it — Java is statically typed. This means you always know what kind of value a variable can hold, which makes programs safer.

Declaring variables

int    age   = 18;        // whole number
double price = 99.5;      // decimal number
char   grade = 'A';       // single character
boolean pass = true;      // true / false
String name  = "Aman";    // text

System.out.println(name + " is " + age);

The pattern is: type, name, =, value, then ;. Unlike loosely typed languages, you must always write the type in Java.

Common types

int, double, char and boolean are primitive types. String is a class for text, written with a capital S.

Naming rules

• Start with a letter, underscore or dollar sign — not a number.
• After that, letters, numbers and underscores are allowed.
• No spaces and no Java keywords (like int or class).
• Case-sensitive: age and Age are different.
• By convention, use camelCase: totalMarks, not total_marks.

Local vs instance variables

public class Student {
    String name;              // instance variable (per object)

    void show() {
        int marks = 90;       // local variable (inside method)
        System.out.println(name + " " + marks);
    }
}

An instance variable (like name) belongs to each object and lasts as long as it. A local variable (like marks) exists only inside its method and disappears when the method ends.

Constants with final

final int MAX_USERS = 100;
// MAX_USERS = 200;   // ERROR: cannot change a final

Use final to make a variable a constant that cannot change after it is set. By convention, constant names are written in UPPERCASE, like MAX_USERS.

Common mistakes

• Forgetting the type. Java needs it: int age = 18;, not age = 18;.
• Wrong type for the value. Putting 99.5 into an int loses the decimals or errors.
• Case slips. age and Age are different variables.
• Using a variable before assigning it. Local variables must be given a value before use.

Summary

• A variable stores a value and, in Java, has a fixed type.
• Declare with type, name, value: int age = 18;.
• Common types: int, double, char, boolean, String.
• Local variables live in methods; instance variables belong to objects.
• Use final for constants that cannot change.

Next, explore all types in Java data types, or work with numbers using Java operators.