You have a public field.
Make the field private and create access methods for it.
One of the pillars of object-oriented programming is Encapsulation, the ability to conceal object data. Otherwise, all objects would be public and other objects could get and modify the data of your object without any checks and balances! Data is separated from the behaviors associated with this data, modularity of program sections is compromised, and maintenance becomes complicated.
If the data and behavior of a component are closely interrelated and are in the same place in the code, it is much easier for you to maintain and develop this component.
You can also perform complicated operations related to access to object fields.
When Not to Use
In some cases, encapsulation is ill-advised due to performance considerations. These cases are rare but when they happen, this circumstance is very important.
Say that you have a graphical editor that contains objects possessing x- and y-coordinates. These fields are unlikely to change in the future. What's more, the program involves a great many different objects in which these fields are present. So accessing the coordinate fields directly saves significant CPU cycles that would otherwise be taken up by calling access methods.
As an example of this unusual case, there is the Point class in Java. All fields of this class are public.
How to Refactor
Create a getter and setter for the field.
Find all invocations of the field. Replace receipt of the field value with the getter, and replace setting of new field values with the setter.
After all field invocations have been replaced, make the field private.
Encapsulate Field is only the first step in bringing data and the behaviors involving this data closer together. After you create simple methods for access fields, you should recheck the places where these methods are called. It is quite possible that the code in these areas would look more appropriate in the access methods.