A class is immutable if the data it stores cannot be modified once initialized.
Java’s String and number classes (such as Integer, Double, BigInteger), as well as LocalDate, are immutable. Immutable classes provide good building blocks for creating more complex objects.
Guidelines for creating an immutable class
- All fields should be private and final. This keeps internals private and prevents data from changing once the object is created.
- Provide getters but no setters for all fields. Not providing setters is essential for making the class immutable.
- Make the class final.
- Make sure that getters do not return mutable objects.