Chapter 6. Dealing with optional data
This chapter covers
- The null reference, or “the billion-dollar mistake”
- Alternatives to null references
- Developing an Option data type for optional data
- Applying functions to optional values
- Composing optional values
- Option use cases
Representing optional data in computer programs has always been a problem. The concept of optional data is very simple in everyday life. Representing the absence of something when this something is contained in a container is easy—whatever it is, it can be represented by an empty container. An absence of apples can be represented by an empty apple basket. The absence of gasoline in a car can be visualized as an empty gas tank.
Representing the absence of data in computer programs is more difficult. Most data is represented as a reference pointing to it, so the most obvious way to represent the absence of data is to use a pointer to nothing. This is what a null pointer is.
In Java, a variable is a pointer to a value. Variables may be created null (static and instance variables are created null by default), and they may then be changed to point to values. They can even be changed again to point to null if data is removed.