Java — If Else

The if statement

An if statement runs code only when a condition is true:

int age = 25;

if (age >= 18) {
    System.out.println("You are an adult.");
}

The condition must evaluate to a boolean — true or false. Unlike some languages, Java won't treat numbers as true/false. You must write an explicit comparison.

The if-else statement

Add an alternative path for when the condition is false:

int temperature = 5;

if (temperature > 30) {
    System.out.println("It's hot outside.");
} else {
    System.out.println("It's not that hot.");
}

One of the two blocks always executes — there's no in-between.

else-if chains

When you have multiple possible outcomes:

int score = 78;

if (score >= 90) {
    System.out.println("Grade: A");
} else if (score >= 80) {
    System.out.println("Grade: B");
} else if (score >= 70) {
    System.out.println("Grade: C");
} else if (score >= 60) {
    System.out.println("Grade: D");
} else {
    System.out.println("Grade: F");
}

Java evaluates conditions top to bottom and runs the first matching block. Once a block runs, the rest are skipped. Order matters — put the most specific conditions first.

Nested conditions

You can place if statements inside other if statements:

boolean hasTicket = true;
int age = 16;

if (hasTicket) {
    if (age >= 18) {
        System.out.println("Welcome to the show.");
    } else {
        System.out.println("You need a guardian.");
    }
} else {
    System.out.println("Please buy a ticket first.");
}

Nesting works, but going more than two levels deep makes code hard to read. Refactor deep nests into separate methods or combine conditions with &&.

Combining conditions

Instead of nesting, use logical operators:

boolean hasTicket = true;
int age = 16;

if (hasTicket && age >= 18) {
    System.out.println("Welcome to the show.");
} else if (hasTicket && age < 18) {
    System.out.println("You need a guardian.");
} else {
    System.out.println("Please buy a ticket first.");
}

This reads flatter and clearer than nested blocks. The ternary operator can compress simple decisions even further.

The ternary operator

int age = 20;
String status = (age >= 18) ? "adult" : "minor";
System.out.println(status);  // adult

Format: condition ? valueIfTrue : valueIfFalse.

It's shorthand for a simple if-else that assigns a value. Use it for short, readable assignments. For complex logic, stick with if-else blocks.

Common mistakes

Forgetting braces

// Works — but dangerous
if (age >= 18)
    System.out.println("adult");
    System.out.println("You can vote.");  // always runs!

Without braces, only the very next statement is conditional. The second println always executes regardless of the condition. Always use braces even for single-line bodies:

if (age >= 18) {
    System.out.println("adult");
}

Comparing strings with ==

String color = "red";

// Wrong — compares object references
if (color == "red") { }

// Correct — compares content
if (color.equals("red")) { }

== checks if both variables point to the same object. .equals() checks if they contain the same text. Always use .equals() for string comparison.

Yoda conditions

// This compiles but is unusual
if (18 <= age) { }

// Preferred — reads naturally
if (age >= 18) { }

Put the variable on the left side. It reads like English and reduces mistakes.

Logical operators in conditions

boolean isStudent = true;
int age = 22;
double gpa = 3.5;

// AND — both conditions must be true
if (isStudent && age >= 21) {
    System.out.println("Eligible for senior student discount.");
}

// OR — at least one must be true
if (age < 13 || age > 65) {
    System.out.println("Discounted ticket price.");
}

// NOT — flips the boolean
if (!isStudent) {
    System.out.println("Full price applies.");
}

Short-circuit evaluation

Java stops evaluating as soon as the result is known:

String name = null;

// Safe — first condition is false, second is never checked
if (name != null && name.length() > 0) {
    System.out.println("Name has content.");
}

If name is null, name != null is false. Java skips name.length() > 0 entirely — preventing a NullPointerException. Always put the null check first.

Switch as alternative

When comparing one variable against many fixed values, switch reads cleaner than if-else chains:

String day = "Monday";

switch (day) {
    case "Monday":
    case "Tuesday":
    case "Wednesday":
    case "Thursday":
    case "Friday":
        System.out.println("Weekday");
        break;
    case "Saturday":
    case "Sunday":
        System.out.println("Weekend");
        break;
    default:
        System.out.println("Invalid day");
}

Without break, execution "falls through" to the next case. This can be intentional (grouping Monday–Friday) or a bug. Modern Java's enhanced switch eliminates this:

String type = switch (day) {
    case "Saturday", "Sunday" -> "Weekend";
    case "Monday", "Tuesday", "Wednesday", "Thursday", "Friday" -> "Weekday";
    default -> "Invalid";
};

Mini Practice

1. Write a program that classifies a number as positive, negative, or zero
2. Create a grade calculator with if-else-if for A, B, C, D, F
3. Check if a year is a leap year (divisible by 4, except centuries unless divisible by 400)
4. Write a program that finds the largest of three numbers using if-else
5. Convert the grade calculator to use the ternary operator for each grade

Next: loops — repeating actions →