Guidelines for Designing Immutable Classes in Java
This article outlines seven essential practices for creating immutable Java classes, including declaring the class as final, using private final fields, omitting setters, performing deep copies, overriding equals() and hashCode(), avoiding this‑escape in constructors, and protecting against reflective modification.
1. Declare the class as final to prevent other classes from extending it.
2. Declare all fields as private and use the final modifier; this blocks direct access and ensures the field values cannot be changed after object creation.
3. Do not provide setter methods; all fields must be initialized in the constructor .
4. If the class contains fields that reference mutable objects, ensure those references are also immutable, typically by creating deep copies of the objects in the constructor and getter methods.
5. Override equals() and hashCode() methods so that equality is based on field values rather than object references, and hash codes are computed from those values.
6. In the constructor, avoid this escape, which can lead to thread‑safety problems.
7. Consider whether fields can be altered via reflection and take appropriate measures to protect them.
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