How generics work at runtime

Java Intermediate

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Generics in Java provide compile-time type safety and improve code reusability by allowing classes and methods to operate on various types without explicit casting. However, the way generics function at runtime is different from their behavior at compile time. This is due to a process known as type erasure.

At runtime, the generic type information is removed, and generic types are replaced by their upper bounds (or Object if no explicit bound is provided). Understanding this mechanism is crucial for grasping how generics work and what limitations they impose during runtime.

Key Concepts: Type Erasure

Type erasure is the process by which the Java compiler removes all generic type information during compilation. The key points are:

Compile-Time	Runtime (After Erasure)
Generic type parameters are maintained.	Generic type parameters are removed.
Type safety is enforced by the compiler.	The runtime uses non-generic types (usually `Object` or the bounded type).
Allows code like `Box<Integer>` which preserves type information.	A `Box<Integer>` is treated simply as a `Box` at runtime.

Example: Demonstrating Type Erasure

In this example, we create a generic class Box<T> and demonstrate that, at runtime, the type parameter is erased. We use the getClass() method to show that two instances of Box<Integer> and Box<String> have the same runtime class.

Java

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Example Explanation:

Generic Class Declaration:
- The class Box<T> is declared with a type parameter T that represents the type of the content.
Instance Creation:
- Two instances, intBox and strBox, are created with types Integer and String, respectively.
Type Erasure Demonstration:
- The getClass() method is used on both instances. Despite different compile-time types, both instances have the same runtime class because generic type information is erased.
Conclusion:
- This example clearly shows that while generics provide compile-time type safety, at runtime, they are treated as their raw types.

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