What is type erasure in Java Generics

💡 Concept: Type Erasure

Type erasure is the process by which the Java compiler removes all generic type information during compilation, replacing them with raw types or bounds.

📘 Quick Intro

Java Generics provide type safety at compile time but due to type erasure, generic type parameters are removed at runtime. This ensures backward compatibility with older Java versions.

🧠 Analogy

Think of Generics like labels on a folder (e.g., "Invoices<String>"). Type erasure is like removing the label before putting it in a drawer—at runtime, the JVM doesn’t know what type it originally referred to.

🔧 Technical Explanation

At compile-time, generics ensure type safety.
During compilation, type parameters are replaced with their bounds (or Object if unbounded).
Resulting bytecode uses raw types, and actual type info is not retained at runtime.
Because of type erasure, method overloading based solely on generic types is not allowed.
Reflection cannot access actual generic types without explicit annotations or metadata.

🎯 Use Cases

✅ Writing type-safe collections and utilities at compile-time
✅ Ensuring backward compatibility with pre-Java 5 bytecode
✅ Understanding limitations of generics in reflection and method overloading

💻 Example: Type Erasure in Generics


public class Box<T> {
    T value;
    public void set(T val) { value = val; }
    public T get() { return value; }
}

// After compilation, becomes:
public class Box {
    Object value;
    public void set(Object val) { value = val; }
    public Object get() { return value; }
}

❓ Interview Q&A

Q1: What is type erasure?
A: It's the removal of generic type info during compilation.

Q2: Why does Java use type erasure?
A: For backward compatibility with older versions.

Q3: Is generic type info available at runtime?
A: No, it is erased at compile-time.

Q4: What is the bound of an unbounded generic type?
A: Object.

Q5: Can you overload methods with different generic types?
A: No, due to erasure.

Q6: Can reflection access generic types directly?
A: No, not without annotations or tricks.

Q7: What happens to <T> at runtime?
A: It is replaced with its bound (or Object).

Q8: Can you create instances of generic types at runtime?
A: No, because type info is erased.

Q9: What is the impact of type erasure on performance?
A: Minimal, mostly affects type introspection.

Q10: How to retain generic type info at runtime?
A: Use annotations or explicitly pass Class objects.

📝 MCQs

Q1. What is type erasure in Java?

Runtime cast failure
Removal of generic types during compilation
Looping through lists
Type conversion issue

Q2. Why was type erasure introduced?

To simplify syntax
For runtime performance
To maintain backward compatibility
To replace raw types

Q3. What replaces unbounded type parameters?

int
T
Object
Void

Q4. Is generic info available at runtime?

Yes
No
Only in Spring
Only with reflection

Q5. Can you overload methods on generic type?

Yes
No
Only static methods
Only if generic is bounded

Q6. Which concept does type erasure affect?

Loops
Interfaces
Reflection and overloading
Enums

Q7. Which version introduced generics?

Java 7
Java 1.2
Java 5
Java 8

Q8. After erasure, what does List<String> become?

List<Object>
List
ArrayList
Map

Q9. Which is a workaround to get type info?

Use hashCode()
Use new T()
Use Class<T> param
Use clone()

Q10. Does erasure impact compiled bytecode?

No
Yes
Sometimes
Only in methods

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