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; }
}

Answer 1

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
}