JAVA Variables for beginners

JAVA variables are elements which store data of different types (numbers, strings, arrays,…).

We may use JAVA variables to store the results of arithmetic operations, the data given by the user and any kind of data used by our programs.

The name “variable” comes from the fact that the value held by a variable changes during the execution of a program in opposition of constant.

The value held by a constant stills the same during the execution of a program.

Computers don’t understand the JAVA programming language but it understands the binary language which is a succession of 0s and 1s.

We are writing programs in JAVA and they are transformed in byte code (a kind of binary code) when they are compiled.

If you are familiar with music files and videos (which are binary files), you may know the units of measure of a binary file.

We measure the size of a binary file composed by 0s and 1s using these units of measure.

JAVA variables types

Each variable has a data type which indicates the size of that variable. Here is a table which describes the size of each data type.

Each variable has a data type which indicates the size of that variable. Here is a table which describes the size of each data type.

The declaration like the other JAVA instructions finishes by a semi-colon “;”.

After declaring a variable, we should initialize it by giving it a value. In fact, the value given to a variable must match the data type of that variable.

When we give a value to a variable, we say that we assigned a value to that variable.

For example, if the type of the variable is Integer, we should initialize it with an integer.

In JAVA, the variables belong to two types:

·       The JAVA variables of simple type (primitive type): integers, floats, characters, Booleans

·       The JAVA variables of complex type (non primitive: objects): we deal with this type of JAVA variables when we study the object oriented programming in JAVA.

So, you should put the provided code inside the main method that we have created in the JAVA development tool  tutorial and display the content of the variable using the instruction System.out.print(variableName) like this example:

public static void main(String[] args){
 int i;
i = 5;
System.out.print(i);
}

JAVA variables of numerical types 

The type byte (8 bits: a bit can have a value which can be either 0 or 1) can contain integers between -128 (- 2⁷) and +127 (+2⁷ - 1)

byte temperature;
temperature = 64;

The type short (2 bytes) can contain integers between -32768 (- 2¹⁵) and +32767 (2¹⁵ - 1)

short maxSpeed;
maxSpeed = 32000;

The type int (4 bytes) can contain integers between – 2147483648 (- 2³¹) and + 2147483647 (+2³¹ -1)

int temperature;
temperature = 15600000; 

The type long (8 bytes) can contain integers between - 2⁶³ and +2⁶³ -1

long companyIncome;
companyIncome = 9460700000000000L;

Remark: you should add the L at the end of the number to inform the JVM that the number is a long.

Otherwise, the JVM will consider it as a variable of type int.

The type float (4 bytes) is used to store the numbers with a floating point:

float pi;
pi = 3.141592653f;

or

float nombre;
nombre = 2.0f;

Remark: you notice the f appended to the number in the first example and you know why we added it.

You may also notice that the comma is replaced by a dot in JAVA.

In the second example, we added a “.0” to the number even if the number is a round number.

The type double (8 bytes) is a number with a floating point like the float but it can hold a bigger number.

double division;
division = 0.333333333333333333333333333333333333333333334d;

Remark: we should use the suffix “d” to inform the JVM that the number is a double.

JAVA variables holding a character

The type char contains only one character surrounded by apostrophes “’”

char character;
character = 'A';

JAVA variables holding a boolean

The variable of type boolean can be either true or false (written without quotation marks) 

boolean question;
question = true;

JAVA variables holding a text

The String data type manages text data (strings). It is a complex data type (the name of a class which belongs to the core of JAVA).

Remark: The name of the data type begins by a capital letter and in the initialization we use the double quotation marks.

Also, bare in mind that the primitive data types begin by a lower case letter: int, char, float, double,…

//First declaration and initialization method
String text;
text = "My first text";
// Second declaration and initialization method
String str = new String();
str = " My second text";
// Third declaration and initialization method
String txt = " My third text ";
// Fourth declaration and initialization method
String t = new String("My fourth text");

The String class‘s name begins by a capital letter because we respect the naming rules.

You should try to respect the naming rules which are the following:

1)     The class names should begin with a capital letter like our String class

2)     The variable names should begin by a lower case letter

3)     If the variable name is composed by many words, we should begin the intermediary words by a capital letter

4)     We should avoid accentuation

Here are some examples of class and variable names:

//class naming rules
public class Kid{}
public class Number{}
public class HealthAndFitness{}
//variables naming rules
String name;
String godOfWar;
int number;
int moreImportantNumber;

We can make the declaration and the initialization at the same time like this:

int number = 32;
float pi = 3.1416f;
char charac = 'z';
String text = new String("hello");

And if you have many JAVA variables of the same type, you can declare and initialize them in the same line.

int number1 = 2, number 2 = 3, number 3 = 0;

Here we declared and initialized three JAVA variables (number1, number2 and number3) of the same type (int) in the same line.