Multiplication in Java

For many situations, the int data type will be adequate for storing integer variables. It can hold values between (roughly) + or - 2 billion. You can multiply two integers as follows:

UBUNTU > cat prog7.java
public class prog7
{
public static void main (String args[])
  {
  // Declare 2 integers:
  int num1 = 1234;
  int num2 = 4321;

  // Multiply them together:
  int num3 = num1 * num2;

  // Display the result:
  System.out.println("num1 = " + num1);
  System.out.println("num2 = " + num2);
  System.out.println("num1 * num2 = " + num3);
  }
}
UBUNTU > javac prog7.java
UBUNTU > java prog7
num1 = 1234
num2 = 4321
num1 * num2 = 5332114
UBUNTU >

... but what happens if the result is too big to store in an int variable?

UBUNTU > cat prog8.java
public class prog8
{
public static void main (String args[])
  {
  // Declare 2 integers:
  int num1 = 123456;
  int num2 = 654321;

  // Multiply them together:
  int num3 = num1 * num2;

  // Display the result:
  System.out.println("num1 = " + num1);
  System.out.println("num2 = " + num2);
  System.out.println("num1 * num2 = " + num3);
  }
}
UBUNTU > javac prog8.java
UBUNTU > java prog8
num1 = 123456
num2 = 654321
num1 * num2 = -824525248
UBUNTU >

According to my calculator, the result should be 80,779,853,376 so some arithmetic overflow must have taken place. As soon as I find out how to handle this, I will do a worked example to demonstrate.

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Java Short Integers

These are 16-bit signed integers. They can store values between -32768 and +32767 both inclusive. If you try to assign numbers outside this range, you get a compilation error: 

UBUNTU > cat prog5.java
public class prog5
{
public static void main (String args[])
  {
  // Declare a short integer:
  short num1;

  // Try to assign some invalid values:
  num1 = -32769;
  num1 = 32768;
  }
}
UBUNTU > javac prog5.java
prog5.java:9: possible loss of precision
found   : int
required: short
  num1 = -32769;
          ^
prog5.java:10: possible loss of precision
found   : int
required: short
  num1 = 32768;
         ^
2 errors
UBUNTU >

...but numbers within the range are accepted:

UBUNTU > cat prog6.java
public class prog6
{
public static void main (String args[])
  {
  // Declare a short integer:
  short num1;

  // Assign the minimum valid value:
  num1 = -32768;

  // Display the value:
  System.out.println("num1 = " + num1);

  // Assign the maximum valid value:
  num1 = 32767;

  // Display the value:
  System.out.println("num1 = " + num1);
  }
}
UBUNTU > javac prog6.java
UBUNTU > java prog6
num1 = -32768
num1 = 32767
UBUNTU >

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How to Define Integer Variables in Java

The program below declares 3 integer variables, num1, num2 and num3. It assigns values to num1 and num2 then adds them together and puts the result in num3: 

UBUNTU >cat prog1.java
public class prog1
{
public static void main (String args[])
  {
  // Declare 3 integer variables:
  int num1, num2, num3;

  // Give them values:
  num1 = 1;
  num2 = 2;
  num3 = num1 + num2;
 
  // Display the result:
  System.out.println
  (num1 + " + " + num2 + " = " + num3);
  }
}
UBUNTU >

I compiled it then ran it and it produced the output below:

UBUNTU >javac prog1.java
UBUNTU >java prog1
1 + 2 = 3
UBUNTU >

Here is another example:

UBUNTU > cat prog25.java
public class prog25
{
public static void main (String args[])
  {
  int a = 10, b = 20, c = 30;
  System.out.println ("a = " + a);
  System.out.println ("b = " + b);
  System.out.println ("c = " + c);
  int x, y, z;
  x = y = z = 40;
  System.out.println ("x = " + x);
  System.out.println ("y = " + y);
  System.out.println ("z = " + z);
  }
}
UBUNTU > javac prog25.java
UBUNTU > java prog25
a = 10
b = 20
c = 30
x = 40
y = 40
z = 40
UBUNTU >