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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Java records make it easy to model a fixed set of values, but they are not automatically deeply immutable. Their component fields are final; objects referenced by those fields can still be mutable. To make a record protect its state, validate and copy mutable inputs in its constructor, and consider whether accessors expose mutable objects.
Are Java records immutable?
Records are shallowly immutable. Oracle’s Java SE 26 Record API describes a record as a “shallowly immutable, transparent carrier for a fixed set of values, called the record components.” The fields corresponding to record components are final, so they cannot be reassigned after construction. But a final reference does not freeze the object it refers to.
For example, in record Person(String name, List<String> roles) {}, neither component field can be reassigned. Yet the caller may still hold the list passed to the constructor and change it. The generated roles() accessor also returns the stored list, so its recipient may be able to change that list. The record’s references are fixed, but its reachable state is not necessarily fixed.
How to protect mutable record components
Copy input collections in the constructor
A compact constructor can replace an incoming collection with a protected copy before the record stores it:
record Person(String name, List<String> roles) {
Person {
roles = List.copyOf(roles);
}
}The record’s generated canonical constructor assigns the resulting parameter values to the component fields. List.copyOf creates an unmodifiable list and rejects null elements. Changes to the caller’s original list will not change the list held by this record, and callers cannot structurally modify the returned list.
This is not deep immutability: if a list element is itself mutable, its state can still change. Protect mutable elements separately when the record’s invariants depend on them. Oracle’s record classes guide and Record API describe explicit constructors and accessors as tools for defensive copying, validation, and normalization.
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Decide whether an accessor needs protection
For a mutable component such as an array or mutable date type, copying only at construction may not be enough. A caller could mutate the stored object through an accessor. Depending on the component and ownership model, return a defensive copy from a custom accessor, or store an immutable representation instead. For an array, for instance, copy both when accepting it and when returning it if callers must not share access to the stored array.
Validate and normalize at construction
Use the compact or canonical constructor to enforce the record’s invariants: reject missing values, check allowed ranges, or normalize a value into its intended form. A canonical constructor is also where a defensive copy can ensure the record stores the representation it promises.
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What the compiler provides
A record header declares its components. Unless you explicitly implement the corresponding members, the compiler supplies a private final field and public accessor for each component, a canonical constructor, and value-oriented implementations of equals, hashCode, and toString. The Record API and Java Language Specification, Java SE 26 Edition define this record behavior.
Records also have a consistency contract: constructing a new instance by passing the results of a record’s accessors to its canonical constructor must produce an instance that compares equal to the original. Custom constructors and accessors should preserve that transparent, value-carrier behavior; normalization should be consistent rather than making reconstruction change the record’s value.
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Why mutable components can cause trouble with equality and hash codes
Record equality and hash-code behavior is based on component values. If a component refers to mutable state, changing that state can alter the record’s value-oriented behavior or break expectations about using the record as a hash-based key. For example, a key stored in a HashMap or HashSet should not have equality- or hash-relevant state changed while it is in the collection.
Before choosing a defensive-copy strategy, consider these questions:
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- Input protection: Can the caller retain and mutate the object passed to the constructor?
- Output protection: Can a caller change stored state through an accessor?
- Element protection: Are objects inside a copied collection mutable?
- Value behavior: Could mutation affect equality or hash-code expectations?
- Invariant: What exact state must remain stable for this record to be valid?
Copying has a cost, so apply it to protect a real invariant or ownership boundary rather than copying every component mechanically.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Records, Java versions, and serialization
Records were previewed in Java SE 14 and became a permanent language feature in Java SE 16, according to Oracle’s Java Language Changes for Java SE 17. Code targeting Java SE 16 or later can use records without enabling preview features.
For serializable records, serialized state is based on the record components, and deserialization invokes the canonical constructor. Constructor validation therefore remains relevant when deserialized instances must satisfy the same invariants as ordinarily constructed ones. Oracle’s Serializable Records article explains this behavior.
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