Abstraction on objects

Abstraction on objects is realized as follows:

• We group similar objects (i.e., objects with the same properties) into classes.
• We establish the properties that are relevant for the objects, and in particular, the operations that they support (note that this requires to perform an abstraction on operations).

The ability to support in a very advanced way the abstraction on objects is the fundamental feature of all object-oriented programming languages (such as Java, C++, C#, etc.). In these languages, such a form of abstraction is supported by the ability to define classes directly at the programming language level.

In Java, the definition of a class is characterized by:

• the name of the class, which identifies the class itself, and hence identifies the type of its instances;
• the instance variables (also called data fields), which allow us to store data inside the objects;
• the (instance) methods (also called operation fields), which can be invoked on the objects of the class to perform operations on them.

Note: The instance variables and the methods represent the properties of the objects of a Java class.

Moreover, through suitable access modifiers, we can specify:

• which fields should be visible to the outside the class, i.e., to the clients of the class - such fields are called public;
• which fields should be hidden to the clients since they are not relevant for them - such fields are called private.