LazyGetter, from Cyril Mottier https://cyrilmottier.com/2016/06/20/launch-screens-from-a-tap-to-your-app/

LazyGetter.java

import java.util.Optional;
import java.util.concurrent.atomic.AtomicReference;

public abstract class LazyGetter<T> {

    private final AtomicReference<Optional<T>> mCachedValue = new AtomicReference<>(Optional.<T>empty());

    public final T get() {
        Optional<T> value = mCachedValue.get();
        if (!value.isPresent()) {
            synchronized (mCachedValue) {
                value = mCachedValue.get();
                if (!value.isPresent()) {
                    mCachedValue.set(Optional.ofNullable(initialValue()));
                }
            }
        }
        return mCachedValue.get().get();
    }

    protected abstract T initialValue();

    /**
     * Example
     */
    private static final LazyGetter<String> LAZY_STRING = new LazyGetter<String>() {
        @Override
        protected String initialValue() {
            return "LazyString";
        }
    };
}