Imagine we’re writing a program that enrolls students in courses.

• If a student has completed the prerequisites, then they can enroll in a course.
• Else, they need to take the prerequisite courses.

They can’t take Physics II without finishing Physics I.

We represent this kind of decision-making in our program using conditional or control flow statements. Before this point, our code runs line-by-line from the top down, but conditional statements allow us to be selective in which portions will run.

Conditional statements check a boolean condition and run a block of code depending on the condition. Curly braces mark the scope of a conditional block similar to a method or class.

Here’s a complete conditional statement:

if (true) {

  System.out.println("Hello World!");

}

If the condition is true, then the block is run. So Hello World! is printed.

But suppose the condition is different:

if (false) {

  System.out.println("Hello World!");

}

If the condition is false, then the block does not run.

This code is also called if-then statements: “If (condition) is true, then do something”.