How would you design a genuinely immutable Java class?

Builds an immutable Java type through valid construction, final state, defensive copying, controlled exposure, and safe sharing.

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I would make the class's observable state fixed after construction. Fields are private and normally final, construction validates the complete state, and there are no mutating methods. If a constructor receives a mutable list, array, date-like value, or other mutable object, the class makes a defensive copy. Accessors return immutable values, safe snapshots, or unmodifiable copies rather than the internal mutable object. The class is final, or otherwise designed so a subclass cannot add mutable behavior that breaks the contract.

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Whole-object immutability, nested mutability, construction rules, collections, and practical framework boundaries.

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Immutability is an object-level contract; private final fields are necessary in many designs but are not sufficient when referenced values remain mutable.
I would make the class's observable state fixed after construction. Fields are private and normally final, construction validates the complete state, and there are no mutating methods. If a constructor receives a mutable list, array, date-like value, or other mutable object, the class makes a defensive copy. Accessors return immutable values, safe snapshots, or unmodifiable copies rather than the internal mutable object. The class is final, or otherwise designed so a subclass cannot add mutable behavior that breaks the contract.
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For an immutable OrderSummary, I might use List.copyOf(items) in the constructor and return that same immutable list. An unmodifiable view is not enough if another owner can still mutate the backing list. With a builder, the immutable object copies builder state during so later builder changes cannot affect the result. Validation and normalization happen before fields become observable. Methods such as can return a new instance rather than changing the current one.

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Declaring fields final but retaining a caller-owned mutable collection.
Returning an unmodifiable view over a list someone else can mutate.
Adding setters for framework convenience.
Calling a class immutable while nested objects remain externally mutable.

How would you design a genuinely immutable Java class?

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