4. Variables and assignment
4.1 Variables and data types
A variable is one of the most important concepts in computer programming. A variable should be imagined as a box in which you can store information. The information stored in a variable always has a type. These types include text (String), whole numbers (int), decimal numbers (double), and truth values (boolean). A value can be assigned to a variable using the equals sign (=).
int months = 12;
In the statement above, we assign the value 12 to the variable named months whose data type is integer (int). The statement is read as "the variable months is assigned the value 12".
The value of the variable can be appended to a string with the plus + sign as shown in the following example.
String text = "includes text";
int wholeNumber = 123;
double decimalNumber = 3.141592653;
boolean isTrue = true;
System.out.println("The variable's type is text. Its value is " + text);
System.out.println("The variable's type is integer. Its value is " + wholeNumber);
System.out.println("The variable's type is decimal number. Its value is " + decimalNumber);
System.out.println("The variable's type is truth value. Its value is " + isTrue);
Printing:
The variable's type is text. Its value is includes text
The variable's type is integer. Its value is 123
The variable's type is decimal number. Its value is 3.141592653
The variable's type is truth value. Its value is true
A variable holds its value until it is assigned a new one. Note that the variable type is written only when the variable is first declared in the program. After that we can use the variable by its name.
int wholeNumber = 123;
System.out.println("The variable's type is integer. Its value is " + wholeNumber);
wholeNumber = 42;
System.out.println("The variable's type is integer. Its value is " + wholeNumber);
The output is:
The variable's type is integer. Its value is 123
The variable's type is integer. Its value is 42
4.2 Variable data types are immutable
When a variable is declared with a data type, it cannot be changed later. For example, a text variable cannot be changed into an integer variable and it cannot be assigned integer values.
String text = "yabbadabbadoo!";
text = 42; // Does not work! :(
Integer values can be assigned to decimal number variables, because whole numbers are also decimal numbers.
double decimalNumber = 0.42;
decimalNumber = 1; // Works! :)
Exercise 4: Varying variables
The exercise file initially contains a program which prints:
Chickens:
3
Bacon (kg):
5.5
A tractor:
There is none!
In a nutshell:
3
5.5
There is none!
Change the program in the specified places so that it will print:
Chickens:
9000
Bacon (kg):
0.1
A tractor:
Zetor
In a nutshell:
9000
0.1
Zetor
4.3 Allowed and descriptive variable names
There are certain limitations on the naming of our variables. Even though umlauts, for example, can be used, it is better to avoid them, because problems might arise with
character encoding. For example, it is recommended to use A instead of Ä.
Variable names must not contain certain special characters like exclamation marks (!). Space characters cannot be used, either, as it is used to separate commands into multiple parts. It is a good idea to replace the space character using a
camelCase notation.
Note: The first character is always written in lower case when using the camel case notation.
int camelCaseVariable = 7;
Variable names can contain numbers as long it does not start with one. Variable names cannot be composed solely of numbers, either.
int 7variable = 4; // Not allowed!
int variable7 = 4; // A valid, but not descriptive variable name
Variable names that have been defined before cannot be used. Command names such as System.out.print cannot be used, either.
int camelCase = 2;
int camelCase = 5; // Not allowed, the variable camelCase is already defined!
It is strongly recommended to name variables so that their purpose can be understood without comments and without thinking. Variable names used in this course must be descriptive.
4.3.1 Valid variable names
- lastDay = 20
- firstYear = 1952
- name = "Matti"
4.3.2 Invalid variable names
- last day of the month = 20
- 1day = 1952
- watchout! = 1910
- 1920 = 1