Variables and Data Types in Java

What a Variable Is (and Why You Need It)

A variable is a named storage location in memory that holds a value your program can use and change. When you want to remember information (like an age, a price, or a name), you store it in variables. In Java, every variable has a type, which tells Java what kind of value it can hold and what operations are allowed.

Declaring and Assigning Variables

Declaration: creating a variable

To declare a variable, you write its type followed by its name. Declaration reserves space and sets the rules for what can be stored.

int age;        // declares an int variable named age (no value yet)

Assignment: giving it a value

To assign a value, use the equals sign (=). This is not “equals in math”; it means “store the value on the right into the variable on the left”.

age = 20;       // assigns 20 to age

Declaration + assignment in one line

int age = 20;

Updating a variable

Variables can change during the program. Updating means assigning a new value to the same variable.

age = 21;       // age is now 21

Naming Rules and Readable Naming Conventions

Java naming rules (what is allowed)

• Names can contain letters, digits, underscore (_), and dollar sign ($).
• Names cannot start with a digit.
• Names cannot be Java keywords (like int, class, public).
• Names are case-sensitive: age and Age are different.

Readable naming conventions (what you should do)

• Use camelCase for variables: firstName, isMember, totalPrice.
• Choose descriptive names: prefer price over p.
• Booleans often read like a yes/no question: isMember, hasDiscount, canVote.
• Avoid confusing single-letter names except for short loops (covered elsewhere).

Core Data Types You’ll Use Often

Primitive types

Primitive types store simple values directly.

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• int: whole numbers, e.g., -3, 0, 42.
• double: decimal (floating-point) numbers, e.g., 3.14, 19.99.
• boolean: true or false.
• char: a single character in single quotes, e.g., 'A', '7', '?'.

String (a commonly used reference type)

String stores text like names and sentences. Unlike primitives, String is a reference type (an object). You write string values in double quotes.

String name = "Mina";

Even though it’s not a primitive, you will use String constantly in beginner programs.

Exercise: Store User-Like Data in Variables

In this exercise, you will create variables that represent typical user information: age, price, membership status, an initial, and a name. Then you will print them and update them.

Step 1: Declare and assign the variables

int age = 28;                 // whole number age in years
double price = 19.99;         // decimal price
boolean isMember = true;      // membership status
char initial = 'M';           // first letter
String name = "Mina";        // full name

Step 2: Print the values

Use System.out.println to display values. You can combine text and variables using + (string concatenation).

System.out.println("Name: " + name);
System.out.println("Initial: " + initial);
System.out.println("Age: " + age);
System.out.println("Price: " + price);
System.out.println("Is member? " + isMember);

Step 3: Update variables (simulate changes)

Now pretend time passes or the user changes something: the user has a birthday, the price changes, and membership is canceled.

age = age + 1;                // or: age++
price = 17.49;                // new price
isMember = false;             // membership changed

Print again to confirm the updates:

System.out.println("--- Updated values ---");
System.out.println("Age: " + age);
System.out.println("Price: " + price);
System.out.println("Is member? " + isMember);

Step 4: Try it yourself (small tasks)

• Change name to another value and print it.
• Set initial to match the new name.
• Increase price by 2.00 using the current value (not by typing a new number).

price = price + 2.00;
System.out.println("New price: " + price);

Focused Comparison: int vs double

What they represent

• int is for whole numbers only. It cannot store decimals.
• double is for numbers with decimals (and can also store whole numbers like 5.0).

Common beginner mistake: losing decimals

If you divide two int values, Java performs integer division and drops the decimal part.

int a = 5;
int b = 2;
int resultInt = a / b;        // 2 (not 2.5)
System.out.println(resultInt);

Using double keeps the decimal part:

double resultDouble = 5.0 / 2; // 2.5
System.out.println(resultDouble);

Even if only one side is a double, Java will do decimal division:

double resultDouble2 = a / 2.0; // 2.5
System.out.println(resultDouble2);

Which should you choose?

• Use int for counts: age in years, number of items, number of students.
• Use double for measurements and prices where decimals matter.

Simple Type Conversion (Beginner Cases)

Type conversion happens when you move a value from one type to another. Some conversions are automatic (safe), and some require an explicit cast (because information might be lost).

Case 1: int to double (automatic)

Java can safely convert an int to a double because a double can represent whole numbers.

int items = 3;
double itemsAsDouble = items;   // 3.0
System.out.println(itemsAsDouble);

Case 2: double to int (requires a cast)

Converting a double to an int can lose the decimal part. Java requires you to explicitly cast to show you understand that.

double rating = 4.8;
int ratingInt = (int) rating;   // 4 (decimal part is removed)
System.out.println(ratingInt);

This does not round; it truncates (cuts off) the decimal part.

Case 3: Avoiding integer division by converting

If you have two int values but want a decimal result, convert at least one to double.

int total = 7;
int people = 2;
double average = (double) total / people; // 3.5
System.out.println(average);

Case 4: char and String basics

A char is one character; a String is text of any length. You can combine them in printing:

char initial = 'M';
String name = "Mina";
System.out.println("Initial: " + initial);
System.out.println("Name: " + name);

Remember: 'M' is a char, but "M" is a String.