Variance in Java

With its powerful object-oriented programming capabilities, Java provides programmers with multiple mechanisms to develop flexible and efficient code. One of the often overlooked but crucial concepts is variance. Understanding variance is crucial to mastering Java, especially when working with generics and collections. This article takes an in-depth look at variance in Java, covering its types (covariance, contravariance, and invariance) and its practical applications.

Understanding variance

Variance refers to the relationship between subtypes of more complex types and the subtypes of their components. Simply put, it determines how the type hierarchy of classes is preserved when these classes are used as type parameters. Variants become particularly important when dealing with generics, providing a framework to ensure type safety while allowing a degree of flexibility in assignments.

Variance can be divided into three main types:

• Covariance - If ClassB is a subclass of ClassA, then Collection can be considered a subclass of Collection.

• Contravariance - If ClassB is a subclass of ClassA, then Collection can be considered a subclass of Collection. p>

• Immutability - Collection and Collection have no subtype relationship, regardless of the relationship between ClassA and ClassB.

Let’s delve deeper into each concept.

Covariance in Java

In Java, covariance is achieved by using wildcards and the extends clause. Let us consider an example -

List<Animal> animals = new ArrayList<>();
<List<super Cat>cats=animals;

In this case, you can add a Cat object or any instance of it to cats, but you cannot read from cats and treat the result as Cat because it may contain any supertype of Cat, including Animal or Object . Therefore, you can write to cats, but you cannot read it in a type-safe manner

Immutability in Java

Immutability is the default behavior in Java and means that there is no subtyping relationship between Collection and Collection, regardless of the relationship between ClassA and ClassB. This may seem limiting, but it's crucial for type safety. In Java, List is not a subtype of List