Covariance, Invariance, and Contravariance in Java: What's the Difference and Why Should I Care?
- Susan SarandonOriginal
- 2024-11-12 01:38:02268browse
Covariance, Invariance, and Contravariance Explained in Plain English
Introduction:
Understanding covariance, invariance, and contravariance can be challenging, but it's essential for comprehending type transformations and relationships in Java. This article aims to simplify these concepts and provide real-world examples to enhance understanding.
Covariance:
Covariance describes a type transformation where the subtype relation is preserved. If type A is a subtype of type B, then the transformation f(A) will result in f(B) being a subtype of f(A).
Example:
Consider a transformation f(List) = List
Invariance:
Invariance occurs when the subtype relation is not affected by type transformations. If A is a subtype of B, then f(A) will not be a subtype or supertype of f(B).
Example:
Java generics behave in an invariant manner. For example, f(ArrayList) = ArrayList. Since ArrayList and ArrayList are different types, neither is a subtype or supertype of the other.
Contravariance:
Contravariance is the inverse of covariance. The subtype relation is reversed during transformations. If A is a subtype of B, then f(B) will be a subtype of f(A).
Example:
Consider the type transformation f(A[]) = B[]. In Java, arrays are covariant, which means that if A[] is a subtype of B[], then f(A[]) = B[] will also be a subtype of f(B[]) = B[].
Applications:
Covariance, invariance, and contravariance play crucial roles in Java:
- Assignment: Only objects of compatible types can be assigned to each other. The compiler checks if the source type is a subtype of the target type.
- Method Invocation: When invoking a method, the actual parameters must be a subtype of the declared parameter types. The method's return type must be a supertype of the expected return type.
- Overriding: In Java, the parameter and return types of overridden methods must match or covary with the original method's types.
The above is the detailed content of Covariance, Invariance, and Contravariance in Java: What's the Difference and Why Should I Care?. For more information, please follow other related articles on the PHP Chinese website!
Related articles
See more- How to Wrap Text in JOptionPane Dialogs for Better Readability?
- Native Image Quick Reference — GraalVM for JDK - graalvm
- ProcessBuilder vs Runtime.exec(): Why Do They Behave Differently When Executing External Commands?
- How to Effectively Compare XML Documents in Java?
- How to Format Dates with Locale-Specific Format Strings in Java?