Question

I wonder if there s a way to define a class in such a way that instances of it will never be members of another class (only local variables), or the other way round - only members but never local.

Is there any way in which a class can dictate the scope of it s prospective instances?

Answer 1

I don t think so. But I have no definitive proof.

Answer 2

To limit the scope you d some sort of class annotation or class modifier and the virtual machine needed the functionality to check, whether a class (or any subclass of this restricted class) was assigned to a member or local variable and violated the constraint.

Just imagine, you had a class with the - just invented - onlylocal modifier, indicating that you only allow instances in local variables.

public onlylocal class LocalUseOnlyClass implements Serializable {
  //...
}

and in another class someone just did in a constructor:

private Object member;
public MyOtherClass(Serializable something) {
  this.member = something
}

The Compiler couldn t detect, if you passed an instance of LocalUseOnlyClass to that constructor, so the JVM had to check and throw an exception or an error.

BTW & OT: what s your intention? - maybe there s an alternative to fulfill your underlying requirement.

Answer 3

no. member and local variable can be assigned to each other.

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