What's the difference between <?> and <? extends Object> in Java Generics?

What's the difference between <?> and <? extends Object> in Java Generics?

In Java generics, <T> is a type parameter that represents a placeholder for a specific type, and it allows you to create generic classes and methods that can work with different types. Both <T> and <T extends Object> are ways to declare type parameters, but they have different implications:

  1. <T>:

    • When you use just <T> as a type parameter, it represents an unbounded wildcard. This means that the type T can be any reference type, including classes and interfaces.
    • It allows you to use any object type as the actual type argument when you instantiate a generic class or call a generic method.
    • Example: List<T>, where T can be any type like Integer, String, or a custom class.

  2. <T extends Object>:

    • When you use <T extends Object> as a type parameter, it explicitly bounds the type parameter T to be a subtype of Object. In practice, this doesn't add any meaningful constraint because all reference types in Java inherently extend Object.
    • It essentially means the same thing as just <T>.
    • Using <T extends Object> is redundant and is typically not seen in practice.

Here's an example to illustrate the point:

class MyGenericClass<T> { private T value; public MyGenericClass(T value) { this.value = value; } public T getValue() { return value; } } public class Main { public static void main(String[] args) { MyGenericClass<Integer> integerGeneric = new MyGenericClass<>(42); MyGenericClass<String> stringGeneric = new MyGenericClass<>("Hello"); // Both <T> and <T extends Object> are effectively the same in this context MyGenericClass<?> wildcardGeneric = new MyGenericClass<>(new Object()); System.out.println(integerGeneric.getValue()); // 42 System.out.println(stringGeneric.getValue()); // Hello System.out.println(wildcardGeneric.getValue()); // (prints the object) } }

In this example, both <T> and <T extends Object> are used to define the MyGenericClass. They behave the same way because all Java reference types are subtypes of Object. However, it's more common and idiomatic to use <T> to indicate an unbounded wildcard when you want to work with generic types without any specific constraints. Using <T extends Object> is not necessary and is less common in practice.

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