When a class inherits from a single class, as in the case of BankAccount inheriting from Asset, it is called single-level inheritance or simply single inheritance. In single inheritance, we create a class that inherits the properties of another single class. We may create multiple classes that inherit from a single class. This was stated earlier when we said that bank account, security, and real estate are types of assets. Thus, all these can be represented as classes inheriting from the Asset class, as shown here:
Note that the newly defined classes Security and RealEstate both extend the Asset class and define the fields unique to each. Just like the BankAccount class, both the Security and RealEstate classes will have access to the base class fields and methods.
When a class inherits from a single class, known as the parent class, as in the case of the BankAccount, Security, and RealEstate classes inheriting from Asset, it is called single inheritance. If the class itself does not inherit from any other class, we call it a top-level class.
We will now extend our single-level class hierarchy to multiple levels. A BankAccount inherits from Asset. A bank account can be one of two types: savings or checking. Therefore, we can say the following:
· A bank account “is an” asset.
· A savings account “is a” type of bank account.
· A checking account “is a” type of bank account.
Thus, we could add two more classes, called SavingsAccount and CheckingAccount, to our class hierarchy to represent these additional real-life entities. This is shown in the UML diagram here:
The savings account draws monthly interest, but the bank does not pay any interest on the checking account. Thus, the SavingsAccount class defines a field called interestRate, whereas the CheckingAccount does not. Likewise, CheckingAccount has a unique field or attribute called overdraftLimit, which is missing in the SavingsAccount class. To get a better grasp of these inheritance hierarchies.
In Java, every class inherits from the Object class. Thus, Object is a top-level class.Also, one of the most important thing to be discussed about inheritance is that, we may be wondering whether it is possible to inherit from multiple classes. C++ allows us to inherit from multiple classes. However, Java does not support multiple inheritance. This means a Java class cannot simultaneously inherit the characteristics of two or more Java classes. Java has interfaces, which provide a sort of multiple inheritance.