Anonymous Classes

Advanced JavaTopic 1 of 66 questions

Q1Anonymous classes: what they are, when to use them, their restrictions, and how they compare to lambdas

An anonymous class is a nameless, one-shot subclass or interface implementation written inline where it is used, with no constructor and no instance static members.

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Q2Can an anonymous class extend a class and implement an interface simultaneously?

An anonymous class takes exactly one supertype, so it can never extend a class and implement an interface at the same time.

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Q3What does `this` refer to inside an anonymous class, and how do you reference the enclosing instance?

In an anonymous class, `this` is the anonymous object; to reach the outer object you write `OuterClass.this`.

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Q4Can an anonymous class access effectively-final local variables, and why the restriction?

An anonymous class captures a copy of local variables, so those locals must be effectively final to keep the copy and the original consistent.

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Q5Hidden classes (JEP 371): what they are, why introduced, how created with `defineHiddenClass`, and how they replace `Unsafe.defineAnonymousClass()`

A hidden class is a runtime-generated class that is invisible to normal class loading, usable only through its creator's reference, and independently unloadable.

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Q6Can a sealed class/interface be local or anonymous: why not?

Sealing needs named, compile-time-known heirs, and local and anonymous classes have no stable name to put on the permits list, so they can never be sealed.

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