How do Java access modifiers shape package and API boundaries?

Explains private, package-private, protected, and public access through design intent rather than a visibility table alone.

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private restricts a member to its declaring top-level class and nested-class relationship. With no modifier, package-private access keeps it inside the same package. protected also exposes it to subclasses, with Java-specific rules for cross-package access. public exposes it wherever the containing type is accessible. I start with the narrowest access that supports the design, because every public or protected member becomes a contract that callers or subclasses may depend on.

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Practical package design, public API minimization, inheritance exposure, and maintainable boundaries.

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Access control is most useful when it keeps implementation choices local and makes the supported API obvious.
private restricts a member to its declaring top-level class and nested-class relationship. With no modifier, package-private access keeps it inside the same package. protected also exposes it to subclasses, with Java-specific rules for cross-package access. public exposes it wherever the containing type is accessible. I start with the narrowest access that supports the design, because every public or protected member becomes a contract that callers or subclasses may depend on.
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Making everything public for convenience.
Using protected as a general shared-access modifier.
Changing visibility only to make a unit test reach internals.
Assuming package-private means private to one class.

How do Java access modifiers shape package and API boundaries?

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