Java learning – Immutability with Records – Constructors

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Records, in Java, can help to implement immutability due to their inherent properties and restrictions.

  1. Final Fields:

    • Fields defined within a record are implicitly final. This means that once a record instance is created and its fields are assigned values in the constructor, these values cannot be changed.
  2. Auto-generated Methods:

    • Records automatically generates the methods equals(), hashCode(), and toString() based on the record fields. These generated methods use all the record’s fields to determine equality, compute hash codes, and provide string representations.
  3. Compact Constructor:

    • The record constructor assigns values to the fields during object creation. Once these fields are set, they cannot be changed, reinforcing the immutability of the instance.

Here’s an example illustrating how records achieve immutability with a record:

public record Point(int x, int y) {
    // The fields declared in the constructor are final by default

With this Point record, the fields x and y are implicitly final. Once a Point object is instantiated, its x and y values cannot be modified:

public static void main(String...args) {
	Point origin = new Point(0, 0);

	int x = origin.x(); // there is no setX 
	int y = origin.y();

Records provide a concise and safe way to model immutable data types, making code more predictable, easier to reason about, and less prone to unintended side effects or bugs caused by mutable states.

The record itself does not prohibit mutation of the objects it references if those objects are themselves mutable.

public record Line(Point start, Point end) {

If the Point type in the constructor is mutable, the Line will be mutable too, since indirectly it will be possible to change the values of the Point, changing to a different line.

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