CODEWITHSUNDEEP
javaobjectdesign-patternsimmutabilityaccess-modifiers
CBit
CBit

Reputation: 63

Given a mutable class, how to make immutable a specific object of this class?

I got THIS class which is obviously mutable for every instance I create, but I want to know if there´s some kind of wrapper (or something) to make just ONE specific object of THIS class immutable. e.g Collections.unmodifiableList(beanList).

class Animal {
    private String name;
    private String commentary;

    public Animal(String nombre, String comentario) {
        this.name = nombre;
        this.commentary = comentario;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Animal animal = (Animal) o;
        return Objects.equals(name, animal.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name);
    }

    public String getName() {
        return name;
    }

    public String getCommentary() {
        return commentary;
    }

    public void setCommentary(String commentary) {
        this.commentary = commentary;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Upvotes: 2

Views: 112

Answers (1)

Nikolas
Nikolas

Reputation: 44368

The only way I am aware of is to instantiate it and override the methods that are able to modify the particular instance:

Animal animal = new Animal("name", "commentary") {

    @Override
    public void setCommentary(String commentary) {
        throw new UnsupportedOperationException("The Animal is immutable");
    }

    @Override
    public void setName(String name) {
        throw new UnsupportedOperationException("The Animal is immutable");
    }
};

This also satisfied the condition that only one specific instance of the class has a special behavior.

If you need more of them, create a wrapping class that acts as a decorator (isn't exactly). Do not forget to make the class as final otherwise you would be able to override its method in the way I described above and its immutability might break.

Animal animal = new ImmutableAnimal(new Animal("name", "commentary"));

final class ImmutableAnimal extends Animal {

    public ImmutableAnimal(Animal animal) {
        super(animal.getName(), animal.getCommentary());
    }

    @Override
    public void setCommentary(String commentary) {
        throw new UnsupportedOperationException("The Animal is immutable");
    }

    @Override
    public void setName(String name) {
        throw new UnsupportedOperationException("The Animal is immutable");
    }
}

Upvotes: 6

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