Learn Java the Hard Way (Second Edition)

Exercise 6: Mathematical Operations

Now that we know how to declare and initialize variables in Java, we can do some mathematics with those variables.

 1 public class MathOperations {
 2     public static void main( String[] args ) {
 3         int a, b, c, d, e, f, g;
 4         double x, y, z;
 5         String one, two, both;
 6 
 7         a = 10;
 8         b = 27;
 9         System.out.println( "a is " + a + ", b is " + b );
10 
11         c = a + b;
12         System.out.println( "a+b is " + c );
13         d = a - b;
14         System.out.println( "a-b is " + d );
15         e = a+b*3;
16         System.out.println( "a+b*3 is " + e );
17         f = b / 2;
18         System.out.println( "b/2 is " + f );
19         g = b % 10;
20         System.out.println( "b%10 is " + g );
21 
22         x = 1.1;
23         System.out.println( "\nx is " + x );
24         y = x*x;
25         System.out.println( "x*x is " + y );
26         z = b / 2;
27         System.out.println( "b/2 is " + z );
28         System.out.println();
29 
30         one = "dog";
31         two = "house";
32         both = one + two;
33         System.out.println( both );
34     }
35 }

What You Should See

The plus sign (+) will add two integers or two doubles together, or one integer and one floating-point value (in either order). With two Strings (like on line 32) it will concatenate⁵ the two Strings together.

The minus sign (-) will subtract one number from another. Just like addition, it works with two integers, two floating-point values, or one integer and one double (in either order).

An asterisk (*) is used to represent multiplication. You can also see on line 15 that Java knows about the correct order of operations. b is multiplied by 3 giving 81 and then a is added.

A slash (/) is used for division. Notice that when an integer is divided by another integer (like on line 17) the result is also an integer and not floating-point.

The percent sign (%) is used to mean ‘modulus’, which is essentially the remainder left over after dividing. On line 19, b is divided by 10 and the remainder (7) is stored into the variable g.

Modular arithmetic is a fairly simple mathematical operation that just isn’t often taught in public school or even introductory university math curriculum. Wikipedia’s example is good enough: we do modular arithmetic every time we add times on a typical 12-hour clock. If it is 7 o’clock now, what time will it be in eight hours? Well, once we hit 12:00 we “wrap around”, so it will be 3 o’clock. (8+7 = 15, 15-12 = 3)

Put another way, 15 divided by 12 is 1 with a remainder of 3.

Modular arithmetic is used more than you would think in programming, but I won’t be using it too much in the book.

Footnotes: