What Does the Question Mark Mean in Java Generics?

Understanding the Enigma of Generics' Question Mark in Java

In Java, the question mark in generics' type parameters denotes a crucial concept known as parameterization. Let's delve into the meaning behind this elusive symbol.

The ? extends operator represents a wildcard that stands for "a class or interface that extends HasWord." This means that any object that is either an instance of HasWord or extends it can be assigned to a type parameter of this kind.

For example, the code snippet:

List<? extends HasWord> wordList = toke.tokenize();

indicates that the wordList can hold a list of objects that either belong to the HasWord class or one of its subclasses. This versatility allows for greater flexibility in code design.

It's important to note that the ? extends operator is often used in scenarios where a method expects an input of a specific type or a subtype of that type. This allows the method to handle a broader range of input objects.

Conversely, the ? super operator denotes a wildcard that represents a superclass of HasWord. This means that any object that is a superclass of HasWord can be assigned to a type parameter of this kind.

The use of wildcards helps strike a balance between type safety and code flexibility. It enables programmers to define generic methods or classes that can operate on a wide range of object types without sacrificing type checking.

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