In Java, class relationships are a fundamental aspect of object-oriented programming that define how classes interact with each other. These relationships are crucial for designing robust and maintainable applications. The default relationship in Java is the Is-A relationship, which is inherently present due to the implicit superclass for all classes, java.lang.Object.

The Is-A Relationship

The Is-A relationship, also known as inheritance, is established through the extends keyword. It signifies that a class is a subtype of another class, inheriting its properties and behaviors. This relationship is unidirectional and tightly coupled, meaning changes in the superclass can affect the subclass. For instance, if we have a class Car that extends the class Vehicle, we can say that a Car is a Vehicle. This relationship allows for code reusability and reduces redundancy.

Here's a simple example of the Is-A relationship: