Why runtime exception is unchecked exception?

Question

Generally if any class extends Exception , it becomes checked exception. Runtime exception also extends Exception. Then how is it unchecked e

Answer 1

Yes. Any Throwable is a checked exception, except for Error, RuntimeException, and (direct or indirect) subclasses thereof.

However, these are checked by the compiler, not by the JVM; checked exceptions are a compile-time feature, not a run-time feature. (Update: And I now see that you've edited your question to specify "compiler" rather than "JVM". ☺)

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To elaborate a bit further . . . it's not as though there were any sort of "checked-exception" interface. The logic is simply hard-coded: "any exception class is a checked exception unless it's a subtype of RuntimeException or Error".