Lazy Value

LazyValue.java

public class LazyValue<T> implements Supplier<T> {

   private volatile T value;
   private final Supplier<T> producer;
   private final Object lock = new Object();
   public LazyValue(Supplier<T> producer) {
      this.producer = producer;
   }

   @Override
   public T get() {
      T localValue = value;
      if (localValue == null) {
         synchronized (lock) {
            localValue = value;
            if (localValue == null) {
               value = localValue = producer.get();
            }
         }
      }
      return localValue;
   }
}