What’s a variable?
This is a concept that is a basic building block of programming languages and is not specific to Java nor to Object Oriented Programming. If you are familiar with what a variable is feel free to skip this section.
Variables have both 1) a value and 2) an identifier that we can use to access that value, either to read it or to modify it.
Let’s look at some examples in Java:
int year = 2017;
Here, we are declaring a variable named
year (the identifier) and we are assigning it an initial value of
In Java, all variables need to have a defined type, which indicates the type of data of the value that will be stored. The type precedes the name of the variable, in this case,
int, which is used to store integer numbers. We’ll cover other data types in future sections.
In this example, there are actually 2 separate things happening in the same line:
We are defining the
yearvariable of type
int: This is the
int yearpart of the line.
And we are also assigning it an initial value: The
= 2017part of the line.
Example 2 - Assigning a value of a mismatching type
int year = 2017.5;
We are doing the same as in the previous example, but instead of assigning a value of
2017, we are trying to assign a value of
This code fails to compile and Java would report an error. When declaring the variable, we used the type
int, that is used to store integer numbers, but we are trying to assign a non-integer value to it.
Java would indicate that it cannot convert from 2017.5 to an integer value.
Example 3 - Modifying the variable’s value
int year = 2017; year = 2015;
As mentioned before, the value of variables can be modified. In this case, our
year variable started with a value of
2017 and later in our code, we are changing its value to
Here, the previous value of the
year variable is lost (unless of course we store it in a separate variable).
Notice that in the second line we don’t specify the type of the variable (
int). This is because, the variable has already been declared in the first line. In the second line we are just making use of a variable that already exists.
Example 4 - Declaring the variable twice
int year = 2017; int year = 2015;
To clarify what we mean by the last paragraph, assume we had the code above. In this case, we are telling Java that we are trying to declare two separate variables with the same name
year, both of type
This code wouldn’t compile correctly and would report an error. The error would be in line 2, where Java would indicate that there is a duplicate variable called
Example 5 - Declaring and then assigning a value
int year; year = 2017;
The code above is perfectly valid. We can declare a variable
int year without having to assign it an initial value explicitly.
We’ll cover default values later and explain when Java will assign a default value to variables and when not.
Example 6 - Reading the variable’s value
int year1 = 2017; int year2 = year1;
In this case we are defining two separate
int variables, one called
year1 and one called
The initial value of
year1 is set to
2017, and the initial value of
year2 is set to the value stored in
In this case, in the 2nd line
int year2 = year1, by using the identifier of
year1 Java will read the value of the variable and use that value in the operation, resulting in
2017 being assigned to
year2 as well.
Things to take into account
This is a very quick look into the concept of variables as we’ll be using it in all the course. There is more to cover in terms of variables, but this should be enough for the first chapters.
Java defines different types of variables and the different types of variables have different names. We’ll be introducing them as we go along. You’ll find that their names are used interchangeably in some cases and this can cause confusion. The main combination that is used interchangeably is
fieldwhich we’ll cover when we explain classes.