Wednesday, August 24, 2016

JAVA 5 - Manipulating variables through operators

Last time we looked at declaring variables.  But a variable isn't really much use unless we learn to do something with it - for that we need operators.

Operators allow you to manipulate variables in lots of ways.  They're in truth the powerhouse of your programming language.  There are a whole load, but we're going to focus on just a few.  To avoid this series spiralling, you're going to have to research some yourself - which is okay, as most programmers do just that.  Looking for operators as they need them.

Arithmetic operators

You're probably semi-familiar with these already - they're the operators which allow you to do arithmetic!

As a physicist, most of the early software I wrote was about simulating a problem in maths or physics, so using these was a must.

The operators +, -, / are probably familiar as add, minus, divide.  Looking a little different from normal maths, * is used for multiplication.

In code, you can set out such mathematical statements as you need.

Integer maths

Let's start out by declaring two integers,

int num1 = 5;
int num2 = 3;

Once declared, we don't have to continuously use "int" anymore.  So we can use,

num1 + num2;

This will return 8.  They're added.

num1 - num 2;

This will return 2.  They're subtracted.

num1 / num2;

Might surprise you.  Remember these numbers can only have whole number values.  It returns 1, because 3 goes into 5 only once.  There is no decimal, and there is no rounding up.

To counter this, there is the % operator which is the modulus.  This is "the remainder" if you remember back to maths at high school.

num1 % num2;

Returns 2.  Once you divide 5 by 3, you have 2 left over.

num1 * num2;

Returns 15.  Multiplied.

num1 / 2;

Returns 4.  This is a reminder that you don't always have to use variables - whether using int, double or String.  You can use fixed values (sometimes called literals) if you want.

Double maths

This is the maths of decimal numbers.

double num3 = 5;
double num4 = 3

num3 + num4;

This will return 8.0.  They're added.

num3 - num 4;

This will return 2.0.  They're subtracted.

num3 / num4;

Will return 1.666666667.  Decimal points!

num3 * num4;

Returns 15.  Multiplied

Mixing data types

If you try mixing ints with doubles in an operation, weird things happen.  You can override this by using a cast function.  If you put

  • (int) before a double number, it treats it as an integer
  • (double) before an int number, it treats it as a double

So ...

(int) num3 / num2;

Returns 1.

(double) num1 / num3;

Returns 1.66666667.

Mixed arithmetic

Hopefully you've seen this before somewhere.  But if you have a long calculation such as ...

7 - 2 * 3;

What's the answer?  Computers apply multiplication and division before they apply addition and subtraction.

We tend to process from right to left so would expect an answer of 15.

A computer sees it as...
2 * 3 = 6
7 - 6 = 1

And answers 1.

You can use brackets to set orders.  Things inside brackets are processed before things outside.  So

(7 - 2) * 3;

Is 15.  If this shocks you, you need to look this up.


Arithmetic doesn't seem to make much sense in terms of strings.  However the + operator joins together (sometimes called concatonate) two Strings to make return a longer string made of both parts.

String str1 = "Hello ";
String str2 = "World";
String str3;

str3 = str1 + str2;

Returns "Hello World".

Assignment operators

Another key part of this, is assignment operators.  We've used them before for initialisation - we use the name of a variable, the = sign and a value.  The = operator sets the variable to the value we provided.

So if we've declared

int answer;

Then if we do,

answer = 2 * 3;

It returns 6.

If we do the following,

answer = answer + 2;

It will increase the value of answer by 2 ... so now 8.  But we could also do ...

answer += 2;


answer -= 2;

Will decrease the value of answer by 2.

All these will work for double numbers too.



Is a method used for increasing an integer by 1.  You can use,


To reduce it by one.

See operators in action

I have an example on Github filled with operator actions for you to see at first hand.  Find the code here.

Extension material

Look for other types of basic operators here.

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