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path: "/osa-1/4-muuttujat" # NOTE: not translated

title: "Variables"

<text-box variant='learningObjectives' name='Learning Objectives'>

- You understand the concept of a variable. You know what variable types, names and values are.

- You know how to create and use string, integer, float, and boolean type variables.

We already familiarized ourselves with strings while reading inputs. Let's now move on to learn about other widely-used variable types in Java.

A variable can be thought of as a container in which information of a particular type can be stored. Examples of these different types are text (`String`), whole numbers (`int`), decimal numbers(`double`), and true or false values (`boolean`). A value is placed in a variable using an equals sign (`=`). -->

int months = 12;

In the statement above, a value of 12 is assigned to the integer variable named months. The statement reads: "the variable months gets the value 12".

A variable's value can be joined to a string using the + sign as seen in the following example.

String text = "contains text";

int wholeNumber = 123;

double decimalNumber = 3.141592653;

boolean trueOrFalse = true;

System.out.println("Text variable: " + text);

System.out.println("Integer variable: " + wholeNumber);

System.out.println("Decimal variable: " + decimalNumber);

System.out.println("Boolean: " + trueOrFalse);

Output:

Text variable: contains text

Integer variable: 123

Decimal variable: 3.141592653

Booolean: true

<programming-exercise name="Varying variables" tmcname='osa01-Osa01_11.MuuttuvatMuuttujat'>

The exercise base contains a program that prints the following:

Chicken:

3

Bacon (kg):

5.5

Tractor:

No!

And a summary:

3

5.5

No!

Change the program in the given places so that it outputs the following:

Chucken:

9000

Bacon (kg):

0.1

Tractor:

Zetor

And a summary:

9000

0.1

Variable names are unique - no two variables can have the same name. The program in the following example is erroneous since it attempts to create the variable `pii` twice. The variable is created the first time its introduced.

public class Example {

public static void main(String[] args) {

double pii = 3.14;

double pii = 3.141592653;

System.out.println("The value of pi is: " + pii);

}

}

The variable type is mentioned when the variable is first declared. When a new value is assigned to the variable, its type is no longer mentioned.

A variable exist from the moment it first appears, and its initial value is preserved until another value is assigned to it. A variable's value is changed using an statement that comprises the variable name, an equals sign, and the new value to be assigned. Remember to keep in mind that the variable type is only written during the initial variable declaration.

Output:

Let's look at the preceding program's execution step-by-step. When the variable appears in the program for the first time, the computer is informed of both its type (`int` in this case) and its name (`luku` in this case), and the computer creates a 'named container' for the variable. Then, the value on the right side of the equals sign is copied to this named container.

Whenever a variable is referred to by its name in a program - here the goal is to print the string "The value of the variable is " and the value of `luku` variable, the variable is retrieved from a container that's found by its name.

Whenever a value is assigned to a variable (here `luku = 42`), a check is run to see whether or not a container of the given name already exists. If a container exists, a new value is copied in the place of the old value, and the old value disappears. If a container of the given name is not found, the program execution ends in an error message or it fails to run.

The variable is then referred to again by its name -- the aim is once again to print the string "The value of the variable is " and the value of `luku`. We proceed as normal by retrieving the value of `luku` from a container having its name.

At the end of the program, you'll notice that the original value of the variable has disappeared. A given variable can hold only one value at a time.

| Tyyppi | Esimerkki | Sallitut arvot |

| ------------------------- | ------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |

| Kokonaisuluku, eli `int` | `int luku = 4;` | Kokonaislukumuuttuja voi sisältää kokonaislukuja, joiden arvot ovat välillä -2147483648 ja 2147483647. |

| Liukuluku, eli `double` | `double luku = 4.2;` | Liukulukumuuttuja sisältää desimaalilukuja, joiden suurin mahdollinen arvo on noin 2¹⁰²³ Kun desimaaliluku esitetään liukuluvun avulla, voi luku olla epätarkka; liukuluvulla ei pysty esittämään mitä tahansa desimaalilukua. Taustasyihin palataan kurssilla Tietokoneen toiminta. |

| Merkkijono, eli `String` | `String teksti = "Hei!";` | Merkkijonomuuttuja voi sisältää merkkijonoja. Merkkijonot rajataan hipsuilla. |

| Totuusarvo, eli `boolean` | `boolean tosi = true;` | Totuusarvomuuttuja sisältää joko arvon `true` eli totta tai arvon `false` eli epätotta. |