ICU API proposal: use covariant return in clone() to avoid casts (ICU4J)

[Mihai Niță Ⓤ]

Dear ICU team & users,

I would like to propose the following API for: ICU 78
Please provide feedback by: next Wednesday, 2025-07-09 (more if anything is controversial)

Designated API reviewer: Markus

Ticket: https://unicode-org.atlassian.net/jira/software/c/projects/ICU/issues/ICU-23140

Currently many of our classes implement a public Object clone() method.

This forces anyone using to cast:

BreakIterator b1 = BreakIterator.getWordInstance(locale);

BreakIterator b2 = (BreakIterator) b1.clone();

But starting with Java 5 one can use “covariant return types”.

Meaning that we can return the proper type from clone, eliminating the need for casting.

Old API example:

class Foo {

    @Override

    public Object clone() { ... }

}

// Using it requires cast

Foo foo = (Foo) oldFoo.clone();

Proposed API example:

class Foo implements Cloneable {

    @Override

    public Foo clone() { ... }

}

// Using it does not require cast anymore

Foo foo = oldFoo.clone();

I would like to propose the following changes for ICU 78:

1. Identify ALL ICU4J classes that implement a clone() method returning an Object and change these methods to return the class type. (47 classes, see design doc)
2. Mark all such classes as implementing Cloneable.
3. Remove all existing casts related to clone() calls.

Design document:

https://docs.google.com/document/d/1fypBPKTRuu4LoungIecfDaypk0bPLv_rACDw3MTRBc0

Regards,

Mihai

[Markus Scherer]

sgtm thank you very much!

markus

[Rich Gillam]

Awesome!  I love it!

—Rich

On Jul 2, 2025, at 2:10 PM, Markus Scherer <marku...@gmail.com> wrote:

sgtm thank you very much!

markus

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[Mark Davis Ⓤ]

++1!

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