Declare final variable, but set later

Question

I know this is fairly simple topic, but I really want to wrap my head around it.

This is what I\'m trying to do, but it doesn\'t like the final modifier. Is there an

Answer 1

The following Worm (Write-Once-Read-Many) class could help in this kind of scenario.

We could create a nested Wrapper class, that stores the final variable you need. To initialize this variable, you just should call a constructor of the wrapper object. When you call the method getData(), you will get a reference of the final variable in case it is initialized, otherwise, you will get null.

The methods getData() and setData(T data) are required to be thread-safe. To provide it, we use a volatile modifier for the wrapper object. Reading a volatile variable is synchronized and writing to a volatile variable is synchronized, too. Even though some efforts were made to make this code thread-safe I didn't test it in this respect. Depending on the level of thread safety you may consider to make setter and getter synchronized.

public class Worm {
    private volatile Wrapper wrapper;

    public Worm() {}
    public Worm(T data) throws IllegalAccessError
    {
        setData(data);
    }

    public T getData()
    {
        if (wrapper == null)
            return null;
        return wrapper.data;
    }

    public void setData(T data) throws IllegalAccessError
    {
        if (wrapper != null)
            throw new IllegalAccessError();
        else
            wrapper = this.new Wrapper<>(data);
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Worm other = (Worm) obj;
        return Objects.equals(this.getData(), other.getData());
    }

    @Override
    public int hashCode() {
        return Objects.hashCode(this.getData());
    }

    final private class Wrapper {
        final private T data;

        Wrapper(T data) {
            this.data = data;
        }
    }
}