Chapter 5: Variables in Java

This chapter is only about variables: a name that holds a value you can read and change while your program runs. We do not mix in full data-type theory here—that is Chapter 6. We still need enough type words (int, String) to show real examples.

1. Topic title

Variables in Java: declaration, initialization, naming rules, and why they exist

2. What it means

A variable is a named piece of memory the compiler knows about. You pick a type (next chapter goes deep) and a name. The JVM reserves space (or a reference, for objects) and lets you assign values.

Two common patterns:

Declaration then assignment:

int score;
score = 10;

Declaration with initialization:

int score = 10;

The = is assignment: “put this value into this variable.” It is not the same as “equals” in math.

3. Why it is used

Programs work on data: user IDs, prices, counters, flags. Without variables, you would hard-code every value or repeat literals everywhere. Variables let you:

• Compute results and store them.
• Read the same value from many lines of code using one name.
• Change a value as time goes on (a loop counter, a running total).

4. Real-world example

An e-commerce cart might keep:

• itemCount — how many lines are in the cart.
• subtotal — money sum before tax.
• freeShippingApplied — yes/no for a rule.

Each of those is a variable (with types you will formalize in Chapter 6). The checkout logic reads and updates them step by step.

5. Java code example

public class VariableDemo {
    public static void main(String[] args) {
        int items = 3;
        double priceEach = 4.99;
        double subtotal = items * priceEach;
 
        String customerName = "Alex";
 
        System.out.println("Customer: " + customerName);
        System.out.println("Items: " + items);
        System.out.println("Subtotal: " + subtotal);
 
        items = items + 1;
        subtotal = items * priceEach;
        System.out.println("After adding one item, items = " + items);
        System.out.println("New subtotal: " + subtotal);
    }
}

Sample output:

Customer: Alex
Items: 3
Subtotal: 14.97
After adding one item, items = 4
New subtotal: 19.96

6. Explanation of code

• int items = 3; — items is an integer variable, starting at 3.
• double priceEach = 4.99; — double holds decimal numbers; good for money learning (production money often uses BigDecimal—later topic).
• double subtotal = items * priceEach; — expression on the right is evaluated, then stored in subtotal.
• String customerName = "Alex"; — String is a reference type; the variable holds a reference to text in memory.
• items = items + 1; — read old value, add one, write back. Same idea as “increment cart line count.”

7. Common mistakes

Using a variable before assigning it (for locals). If you declare int x; and then System.out.println(x); without assigning, the compiler stops you. Good safety net.

Bad names like a, b, x1 in real code. Fine for a five-line demo; painful in a fifty-line method.

Thinking = means “equals forever.” It means “assign now.” Later you can assign again.

Re-declaring inside the same block:

int score = 1;
int score = 2; // compile error in Java

Use score = 2; instead.

8. Best practices

Use camelCase for variables: itemCount, maxRetries, isActive. That is the Java community default.

Names should read like plain language: totalPrice beats tp.

Declare variables close to first use when possible. Scattering declarations at the top of a huge method makes readers hunt for meaning.

Prefer final when a value should not change (Chapter 37 goes deeper, but you can peek now):

final int maxLoginAttempts = 5;

That signals “do not reassign this by accident.”

9. Small practice task

Write a main program with:

• int books = 2;
• double pricePerBook = 12.50;
• String readerName = your first name as a string literal.

Print a one-line summary like: Reader Maria has 2 books, estimated cost 25.0 (you can build the string with + for now). Then change books to 5 and only re-run—no duplicate literals for the count in the print line (use the variable).

Beginner tip

When the compiler says “variable might not have been initialized,” it is doing you a favor. Assign a sensible default or compute the value before you read it.