In this program, you'll learn to check whether a given number is positive or negative. This is done by using a if else statement in Java.

```
public class PositiveNegative {
public static void main(String[] args) {
double number = 12.3;
// true if number is less than 0
if (number < 0.0)
System.out.println(number + " is a negative number.");
// true if number is greater than 0
else if ( number > 0.0)
System.out.println(number + " is a positive number.");
// if both test expression is evaluated to false
else
System.out.println(number + " is 0.");
}
}
```

When you run the program, the output will be:

12.3 is a positive number.

If you change the value of `number` to a negative number (say -12.3), the output will be:

-12.3 is a negative number.

In the above program, it is quite clear how variable `number` is checked to be positive or negative, by comparing it to 0.

If you're not sure, here is the breakdown:

- If a number is greater than zero, it is a positive number.
- If a number is less than zero, it is a negative number.
- If a number equals to zero, it is zero.