if Statements

Think of a simple program as a recipe. You follow the steps in order, from first to last, without skipping.

1. Preheat oven to 350°F.
2. Mix flour and sugar.
3. Bake for 30 minutes.

This is called sequential execution. It’s the default behavior of your code. But what if you need to make a choice? What if the recipe said, "If you're making a chocolate cake, add cocoa powder"? That's a decision point. In Java, we create these decision points with if statements.

The One-Way if Statement

A one-way if statement is for when you only need to do something extra under a specific condition. If the condition isn't met, you just move on.

Think about an online store. They might offer free shipping for orders over $75.

The logic is: If the cart total is greater than $75, then set the shipping cost to $0. If it's not, you do... nothing. The shipping cost just stays what it was.

Here’s how that looks in Java:

double cartTotal = 82.50;
double shippingCost = 5.99;

// Check if the condition is true
if (cartTotal > 75.00) {
  // This code ONLY runs if the condition is true
  System.out.println("Congrats! You qualify for free shipping.");
  shippingCost = 0.00;
}

// This line runs regardless, after the if statement is done
System.out.println("Your shipping cost is: $" + shippingCost);

Let's break it down:

1. if: This is the keyword that starts the selection statement.
2. (cartTotal > 75.00): This is the Boolean expression, our condition. It must evaluate to either true or false. Here, since 82.50 is greater than 75.00, it evaluates to true.
3. { ... }: These curly braces define the body of the if statement. Any code inside this block will only execute when the condition is true.
4. Because the condition was true, the code inside the braces runs. It prints the "Congrats!" message and sets shippingCost to 0.00. The final output will be "Your shipping cost is: $0.0".

What if cartTotal was $50.00? The condition 50.00 > 75.00 would be false. The entire block of code inside the curly braces would be skipped. The program would jump right to the final System.out.println, and the output would be "Your shipping cost is: $5.99".

The Two-Way if-else Statement

Sometimes, doing nothing isn't enough. You need to provide an alternative action. This is a two-way choice: if the condition is true, do one thing; otherwise (else), do a different thing.

Let's check if a student, Priya, is old enough to vote. The voting age in the US is 18.

The logic is: If Priya's age is 18 or greater, then print "You are eligible to vote." Else (otherwise), print "You are not yet eligible to vote."

This creates two distinct paths. Priya is either eligible or she isn't. One of these two things must be true.

Here’s the Java code:

int priyasAge = 17;

if (priyasAge >= 18) {
  // This is the "true" path
  System.out.println("You are eligible to vote.");
} else {
  // This is the "false" path
  System.out.println("You are not yet eligible to vote.");
}

// The program continues here after one of the above blocks has run.

Let's trace this:

1. The condition is priyasAge >= 18.
2. Since priyasAge is 17, the expression 17 >= 18 evaluates to false.
3. Because the condition is false, the program skips the if block.
4. It moves to the else keyword and executes the code in the else block.
5. The output will be "You are not yet eligible to vote."

If priyasAge were 25, the condition 25 >= 18 would be true. The if block would execute, printing "You are eligible to vote." The else block would be completely ignored.

This is a critical point: With an if-else statement, exactly one block of code—either the if body or the else body—will always run. Never both, and never neither.

This ability to change the path of execution is fundamental. if and if-else statements are what allow our programs to react differently to different inputs and situations, moving them from simple calculators to dynamic, intelligent applications.

Let's walk through a couple of examples to solidify this. The key is to think like the computer, evaluating the Boolean condition first.

Time to try it on your own. Don't worry about getting it perfect on the first try; focus on setting up the logic.

1. 1
   Even or Odd?

   Write a program that takes an integer variable, let's call it number, and determines if it is even or odd. Print "The number is even." or "The number is odd." accordingly.
2. 2
   Amusement Park Ride

   An amusement park ride in Chicago has a height requirement: you must be at least 48 inches tall to ride. Write a program that checks a heightInInches variable. If the person is tall enough, print "You can ride the roller coaster!" Otherwise, print "Sorry, you are not tall enough to ride."