Variables (Numbers)

In programming we often want to store things so we can use them later. The simplest example: numbers. In Java, before storing something, we have to tell the language what kind of value we want to store. That is called a type

public class Main {
    public static void main(String[] args) {
        int age = 18;
        int x = 1;
        System.out.println(age);
        System.out.println(x);
    }
}

int is the type for whole numbers (1, 2, 100, -20, 0). We tell Java: "I am about to store a whole number, and its name is age"

Java is strict about types: it always wants to know what kind of value you put in a variable. The upside is that Java can catch many mistakes before the program even runs

We can do calculations with numbers

public class Main {
    public static void main(String[] args) {
        int a = 2;
        int b = 6;
        int c = a + b;
        System.out.println(c);
    }
}

With the = sign, Java first computes what is on the right, then stores the result in the variable on the left. So c = a + b stores 8 in c

The classic increment by 1

public class Main {
    public static void main(String[] args) {
        int n = 10;
        n = n + 1;
        System.out.println(n);
    }
}

Java looks at n + 1, sees 10 + 1 = 11, and stores 11 back into n

Java even has a shortcut for this

public class Main {
    public static void main(String[] args) {
        int n = 10;
        n++;
        System.out.println(n);
    }
}

n++ is the same thing as n = n + 1. Very common in Java

For numbers with decimals (like 3.14 or 1.75), int is not enough. We use double

public class Main {
    public static void main(String[] args) {
        double pi = 3.14159;
        int k = 33;
        System.out.println(pi + k);
    }
}

Output

36.14159

The result is a double because we mixed an int with a double

Important: if you try to store a decimal in an int, Java will refuse

public class Main {
    public static void main(String[] args) {
        int x = 3.14;   // error
    }
}

Try it. Read the error. Java protects us from accidentally losing the decimal part

A little surprise that catches everyone. Try this

public class Main {
    public static void main(String[] args) {
        int a = 7;
        int b = 2;
        System.out.println(a / b);
    }
}

You might expect 3.5. But you will see 3. Why? Because when we divide an int by an int, Java gives us back an int, dropping the decimals. To get 3.5, we need at least one double

public class Main {
    public static void main(String[] args) {
        double a = 7;
        int b = 2;
        System.out.println(a / b);
    }
}

Now we see 3.5. Keep this in mind, it is a very common beginner bug

Mission: Officer Badge

Commander Cortez needs a new badge printed. Set the correct values for age and height, then print the full line on the badge.

1. Set age to 57
2. Set height to 1.67
3. The final println already builds the message with +, just make sure the variables hold the right values

Input (already set at the top of your code; change the values to test):

• age: the officer's age (int)
• height: the officer's height in metres (double)

Example

With age = 57 and height = 1.67, your program should print

My name is Cortez, I am 57 years old and 1.67 m tall