How to create a variable that can be set only once but isn't final in Java

Question

I want a class that I can create instances of with one variable unset (the id), then initialise this variable later, and have it immutable after initial

Answer 1

I have this class, similar to JDK's AtomicReference, and I use it mostly for legacy code:

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;

import javax.annotation.Nonnull;
import javax.annotation.concurrent.NotThreadSafe;

@NotThreadSafe
public class PermanentReference {

    private T reference;

    public PermanentReference() {
    }

    public void set(final @Nonnull T reference) {
        checkState(this.reference == null, 
            "reference cannot be set more than once");
        this.reference = checkNotNull(reference);
    }

    public @Nonnull T get() {
        checkState(reference != null, "reference must be set before get");
        return reference;
    }
}

I has single responsibilty and check both get and set calls, so it fails early when client code misuse it.