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Java Type System Interview Questions

Concise Q&A covering primitives vs. reference types, autoboxing pitfalls, generics, wildcards, and type erasure.

Beginner​

Q: What is autoboxing?
A: Automatic conversion between primitives and their wrapper types (e.g., int ↔ Integer). Watch out for == vs equals() when comparing wrappers.

Intermediate​

Q: Why do generics use type erasure in Java?
A: To preserve backward compatibility with older JVMs and bytecode; generic type information isn't available at runtime, so operations requiring runtime type checks are limited.

Advanced​

Q: Explain PECS (Producer Extends, Consumer Super).
A: Use ? extends T when you only read (produce) T from a structure; use ? super T when you only write (consume) T into a structure.