Chapter 10: if Statement in Java

This chapter covers only the if statement: one condition, one optional block of code that runs when the condition is true. We do not cover else here—that is Chapter 11.

1. Topic title

The if statement: testing a condition and running code only when it passes

2. What it means

Syntax shape:

if (condition) {
    // runs only when condition is true
}

condition must be a boolean expression: something that evaluates to true or false. Examples: age >= 18, balance > 0, name.equals("admin") (for String content—never == for String values).

The body is usually written in curly braces { }. If you omit braces, only the next single statement belongs to the if. That trips people up—see mistakes below.

3. Why it is used

Programs need to react to the world: enough stock, valid password, business hours open. if is the smallest building block for “do this only when the situation matches.” Every larger rule (discount ladders, permissions) eventually sits on top of many if-style checks.

4. Real-world example

A parcel locker checks weight before opening the deposit slot:

• If weight is over the limit, show an error and do not unlock.
• If weight is fine, allow the next step.

The “over limit” path can be a single if that prints a message and returns early from a method (once you learn methods beyond main).

5. Java code example

public class IfOnlyDemo {
    public static void main(String[] args) {
        int temperature = 38;
 
        if (temperature >= 38) {
            System.out.println("Fever advisory: rest and hydrate.");
        }
 
        int stock = 3;
        if (stock < 5) {
            System.out.println("Low stock warning: reorder soon.");
        }
 
        boolean loggedIn = true;
        if (loggedIn) {
            System.out.println("Welcome back.");
        }
 
        int score = 40;
        if (score >= 40) {
            System.out.println("Pass (minimum met).");
        }
    }
}

Output:

Fever advisory: rest and hydrate.
Low stock warning: reorder soon.
Welcome back.
Pass (minimum met).

Each if is independent. All conditions that are true run their bodies.

6. Explanation of code

• temperature >= 38 compares two ints; result is boolean.
• stock < 5 is another comparison.
• if (loggedIn) is allowed because loggedIn is already boolean. Do not write if (loggedIn == true) in real code—it is noisy.
• Multiple ifs in a row each get evaluated. There is no “otherwise” branch in this chapter—if you need “if not, do B,” wait for Chapter 11.

7. Common mistakes

Dropping braces and adding a second line you thought was inside the if:

if (x > 0)
    System.out.println("positive");
    System.out.println("this always runs");

The second println always runs. Always use { } while learning.

Using = instead of == for numbers — in Java, if (x = 5) does not compile for int the way old C habits suggest; still, know the difference for boolean assignments in conditions in other contexts.

Putting a semicolon after if (condition); — empty if, then the next line always runs. Evil bug.

Wrong String check: if (role == "admin") compares references. Use if ("admin".equals(role)) to avoid null issues.

8. Best practices

Always use braces {} for if bodies, even one-liners. Style guides (Google, most teams) agree.

Keep conditions readable — extract complex checks to a boolean variable with a good name: boolean eligibleForDiscount = ....

Prefer early if + return inside methods later in the series to avoid deep nesting—paired with Chapter 13 you will see why.

9. Small practice task

Write a program with int hour = a value between 0 and 23. Use one if (no else) that prints Good evening only when hour >= 18. Run with hour = 20 and confirm output. Change hour to 10 and confirm nothing prints from that if.

Beginner tip

Read if aloud: “If this is true, then do that block.” If you cannot read the condition aloud clearly, simplify the expression first.