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Immutable Class

Cannot be changed (mutated)

Whenever changed - a new instance is created

What makes a class immutable?

  • Instance variable is final (value can't be changed)
  • class = final - So subclass can't be created
  • No setters

How to create an immutable class

  • Create final class with all final data members.
  • Whichever way you decide to initialize the object and add value to these fields make sure to perform a deep cloning for any reference fields.
  • You have to be careful when there's a reference in the members as well. For example, arrays in Java are mutable objects.

💡 Use clone method to perform deep cloning. You can use the same method for any Cloneable object.

final public class ImmutableClass {
  final private int id;
  final private Map<String, Integer> m;
  final private int[] arr;

  public ImmutableClass(int id, HashMap<String, Integer> m, int[] a){
      this.id = id;
      this.m = new HashMap<>();
      this.arr = arr.clone();
      

      for (String key : m.keySet()) {
          this.m.put(key, m.get(key));
      }
  }

    public int getId() {
        return id;
    }

    public int getMapValue(String key){
      return this.m.getOrDefault(key, -1);
    }

    public int[] getArr(){
        return arr.clone()
    }
}

Benefits

The main benefits of immutability come in case of multi-threaded applications, functional programming and high-security systems.

  • Thead safety
  • Predictable behavior
  • Enhanced security: ensures that sensitive data cannot be modified after its created preventing unintended or malicious change.
  • Caching:
    • Immutable objects can be safely shared across different parts of program.
    • Hashcodes for hash-based structures can be precomputed and cached - since they won't change
  • Works well with Functional Programming
    • Immutability is one of the principles of functional programming
    • Immutable objects ensure that functions don't modify anything outside their scope

Use cases

References

Backlinks