RFR: 8305457: Implement java.io.IO [v7]

[Pavel Rappo]

On Fri, 10 May 2024 07:23:38 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> Pavel Rappo has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix System.console().readln(null) in jshell
>>   
>>   Without it, jshell hangs on me. Will think of a test.
>
> src/java.base/share/classes/java/io/IO.java line 47:
> 
>> 45: 
>> 46:     private IO() {
>> 47:         throw new Error("no instances");
> 
> Is this necessary?

No, it's not necessary.

My guess is that throwing an unconditional error (usually with a snarky message) from the inside the sole private constructor is a historical idiom to deter deep reflectors with their `setAccessible` and what not. Maybe that idiom is outdated with modules and recent efforts on cranking up the [integrity](https://openjdk.org/jeps/8305968), that I don't know.

FWIW, here's a snippet[^*] from "Item 4: Enforce noninstantiability with a private constructor" you referred to in your earlier comment:

> A default constructor is generated only if a class contains no explicit constructors, so a class can be made noninstantiable by including a private constructor:
>
>        // Noninstantiable utility class
>        public class UtilityClass {
>            // Suppress default constructor for noninstantiability
>            private UtilityClass() {
>                throw new AssertionError();
>            }
>            ...  // Remainder omitted
>        }
>
> Because the explicit constructor is private, it is inaccessible outside of the class. The `AssertionError` isn’t strictly required, but it provides insurance in case the constructor is accidentally invoked from within the class. It guarantees the class will never be instantiated under any circumstances.

[^*]: I created that snippet from the second edition, whose page 19 is conveniently publicly available here: https://www.google.ie/books/edition/Effective_Java/ka2VUBqHiWkC?hl=en&gbpv=1&dq=Item+4:+Enforce+noninstantiability+with+a+private+constructor&pg=PA19&printsec=frontcover

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PR Review Comment: https://git.openjdk.org/jdk/pull/19112#discussion_r1596615677