If/Else

In real life we often face decisions: if it's cold, we put on a warm sweater, otherwise a t-shirt is enough

In code, we say

public class Main {
    public static void main(String[] args) {
        int temperature = 10;
        if (temperature < 15) {
            System.out.println("put on a sweater");
        } else {
            System.out.println("a t-shirt is enough");
        }
    }
}

Two things to notice in Java

• The condition goes inside parentheses ( )
• The block of code goes inside braces { }

If the temperature is less than 15, we enter the if block (it's cold, dress warm). Otherwise, we enter the else block. Run the code, change the temperature, see what happens

The comparison operators are exactly the ones you'd expect

• < less than
• <= less than or equal
• > greater than
• >= greater than or equal
• == equal (note the two equals signs)
• != not equal

Be careful about the difference between = and ==. = assigns a value, == compares

public class Main {
    public static void main(String[] args) {
        int x = 4;
        if (x = 4) {       // error
            System.out.println("Boo");
        }
    }
}

This won't compile. Java won't even let you make this mistake. The correct version is

public class Main {
    public static void main(String[] args) {
        int x = 4;
        if (x == 4) {
            System.out.println("Boo");
        }
    }
}

We don't always need an else. Sometimes we just want to do something if a condition is true, and otherwise do nothing

public class Main {
    public static void main(String[] args) {
        boolean isUserOnline = true;
        if (isUserOnline) {
            System.out.println("Welcome back");
        }
    }
}

Notice we wrote if (isUserOnline) without == true. Both work, but the shorter form is what Java programmers typically write

A complete example

public class Main {
    public static void main(String[] args) {
        String username = "Tommy Vercetti";
        boolean isUserOnline = true;

        if (isUserOnline) {
            System.out.println(username + " is playing GTA Vice City");
        } else {
            System.out.println(username + " is offline");
        }
    }
}

Change isUserOnline to false and run again. Read the new output. Programming becomes fun the moment you start playing with the values

Mission: Hull Temperature Alert

The station's hull sensor reports the outside temperature.

Create an int variable named temperature (any value you like). Then write an if / else that checks it:

• if the temperature is less than 0 → print it's freezing outside
• otherwise → print water does not freeze

Example

With a temperature of -5, your program should print

it's freezing outside