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Type erasure is a process used by the Java compiler to enforce generic type safety while maintaining backward compatibility with older versions of Java that do not support generics. Generics were introduced in Java 5, but to allow older JVMs to run code written with generics, Java employs type erasure during compilation.
When you write a generic class or method in Java, the compiler uses the specified type parameters for type checking at compile time. However, at runtime, these type parameters are removed or “erased” and replaced with their bound types (usually Object if no bound is specified). This means the JVM doesn’t retain information about generic types, and all the type safety benefits of generics are only available at compile time.
For example:
List<String> list = new ArrayList<>();
list.add(“Hello”);
At compile time, the compiler ensures only String values can be added. But after type erasure, the bytecode becomes:List list = new ArrayList();
list.add(“Hello”);
Here, the generic type String is removed, and the list becomes a raw type. The compiler may insert type casts to ensure type correctness where needed.Type erasure has some important implications:
No Runtime Type Checking: You can’t use instanceof with parameterized types (if (obj instanceof List<String>) is invalid).
No Creation of Generic Arrays: You can’t do new T[] because the runtime doesn’t know what T is.
No Access to Type Parameters: You can’t reflectively access the type arguments (T.class is invalid).
Understanding type erasure is crucial for advanced Java development, especially when designing libraries or frameworks. It explains many generic-related limitations and why runtime type information about generics is unavailable.
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