If/Else If/Else

Sometimes we need more than two branches. Imagine a rocket launching from the ground. Depending on how many seconds are left, we want to do different things

If we have 100 seconds left → start the onboard computers

If we have 60 seconds left → check the connection with the control tower

If we have 20 seconds left → start the secondary engines

If we have 10 seconds left → start the main engines

Otherwise → do nothing special

In Java we use else if: two words, spelled out

public class Main {
    public static void main(String[] args) {
        int seconds = 100;

        if (seconds == 100) {
            System.out.println("Starting all onboard computers");
        } else if (seconds == 60) {
            System.out.println("Checking connection with the control tower");
        } else if (seconds == 20) {
            System.out.println("Starting secondary engines");
        } else if (seconds == 10) {
            System.out.println("Starting the main engines");
        } else {
            System.out.println(seconds + " seconds has no effect");
        }
    }
}

Run this. Then change seconds to 60, 20, 10, 9, 42. See how the output changes

The chain runs top to bottom. At the first condition that is true, Java enters that block, runs it, and jumps out of the whole chain. The remaining branches are never checked. This is important: if seconds == 60, the chain prints "Checking connection..." and then exits

Why not just write a bunch of separate ifs? Like this

public class Main {
    public static void main(String[] args) {
        int seconds = 60;
        if (seconds == 100) { System.out.println("100"); }
        if (seconds == 60) { System.out.println("60"); }
        if (seconds == 20) { System.out.println("20"); }
        else { System.out.println("other"); }
    }
}

The problem: each if is independent. The else at the end belongs only to the last if. So for seconds = 60, the third if fails (60 != 20), and the else kicks in printing "60 seconds has no effect", which is wrong, we already handled 60 above!

Rule of thumb: when we test the same variable for multiple values, we chain with if / else if / else

We can also nest ifs inside one another

public class Main {
    public static void main(String[] args) {
        int seconds = 5;
        boolean errorDetected = false;

        if (seconds < 10) {
            if (errorDetected) {
                System.out.println("Error detected. Canceling the mission");
            } else {
                System.out.println("No error detected. Taking off...");
            }
        }
    }
}

Here, only if we’re in the last 10 seconds, we check the error flag. Nested ifs are fine, but if you nest 5 levels deep, your code becomes unreadable. Try to keep things flat when you can

Mission: Launch Sequence

The countdown timer is running. Depending on how many seconds remain until launch, the system performs a different action:

• at 100 seconds → print Starting all onboard computers
• at 60 seconds → print Checking connection with the control tower
• at 20 seconds → print Starting secondary engines
• at 10 seconds → print Starting the main engines
• in any other case → print Standing by...

Store the number of seconds in an int variable named seconds and write an if / else if / else chain that prints the correct action.

Examples

At 100 seconds:

Starting all onboard computers

At 60 seconds:

Checking connection with the control tower

At 200 seconds (no branch matches, else runs):

Standing by...