Java templated wrapper class that lets you modify object once

Question

I can write my own but I'm guessing this is very standard. Is there a standard java class that does this? I know this has to be a runtime feature because various parts of my code could set it without knowing what each other are doing - I just want this to fail loudly if this happens.

public class MutableOnce<Type T> { private T _t; private boolean _isSet = false; public void set(T t) { if(_isSet) { //raise exception } _t = t; public T get() { return _t; } }

assylias · Accepted Answer · 2012-04-25 14:27:47Z

6

Any reasons why you don't want to simply make the object immutable? It would make the class much easier to use, especially in a concurrent environment (the class in your example is not thread safe).

public class Immutable<T> { private final T _t; public void Immutable(T t) { _t = t; } public T get() { return _t; } //you may want to return a copy of _t if _t is mutable }

edited Apr 25, 2012 at 14:27

answered Apr 25, 2012 at 14:21

assylias

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4 Comments

djechlin Over a year ago

Other requirement is that the boolean I'll be wrapping is false by default, which means it has to exist before it's set - which is why an immutable w/ constructor as setter won't work. I could perhaps make it nullable and if(isNull) treat it as false... looking for a simple design that encapsulates the "setting" of this boolean.

assylias Over a year ago

Why don't you create the object when you need it? Before it is created it does not exist... Not sure what you are trying to achieve with this structure. It would help if you could explain why you need that behavior.

djechlin Over a year ago

That would be the nullable design. Rather than "have" one I will need to create a place to store it and maintain logic to check whether I have one already. Given that the default value is false I can avoid this and just have one from the start that can only be overridden once. Sounds like I just have to make up my mind, the complexity is kind of conserved in either design.

assylias Over a year ago

I am not sure I fully understand, but checking if object==null with an immutable design seems easier to design and maintain. If there are more than one object, you could store them in a collection as you create them and check if the collection is empty for example...

Community · Accepted Answer · 2017-05-23 12:03:18Z

After a quick search:

All the fields must be private and preferably final
Ensure the class cannot be overridden - make the class final, or use static factories and keep constructors private
Fields must be populated from the Constructor/Factory
Don't provide any setters for the fields
Watch out for collections. Use Collections.unmodifiable*. Also, collections should contain only immutable Objects
All the getters must provide immutable objects or use defensive copying
Don't provide any methods that change the internal state of the Object.

Source: Possible ways of making an Object immutable

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