Variables

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:

Here, we are declaring a variable named year (the identifier) and we are assigning it an initial value of 2017.

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 doing the same as in the previous example, but instead of assigning a value of 2017, we are trying to assign a value of 2017.5.

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.

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 2015.

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.

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 int.

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 year.

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.

In this case we are defining two separate int variables, one called year1 and one called year2.

The initial value of year1 is set to 2017, and the initial value of year2 is set to the value stored in year1.

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.