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package bjc.data;
import java.util.function.Supplier;
/**
* A supplier that can only supply one value.
*
* Attempting to retrieve another value will cause an exception to be thrown.
*
* @author ben
*
* @param <T>
* The supplied type
*/
public class SingleSupplier<T> implements Supplier<T> {
/* The next supplier ID. */
private static long nextID = 0;
/* The supplier to yield from. */
private final Supplier<T> source;
/* Whether this value has been retrieved yet. */
private boolean gotten;
/* The ID of this supplier. */
private final long id;
/*
* The place where the supplier was instantiated.
*
* @NOTE This is both slow to create, and generally bad practice to keep
* exceptions around without throwing them. However, it is very useful to find
* where the first instantiation was.
*/
private Exception instSite;
/**
* Create a new single supplier from an existing value.
*
* @param supp
* The supplier to give a single value from.
*/
public SingleSupplier(final Supplier<T> supp) {
source = supp;
gotten = false;
id = nextID++;
}
@Override
public T get() {
if (gotten == true) {
final String msg = String.format(
"Attempted to retrieve value more than once from single supplier #%d",
id);
final IllegalStateException isex = new IllegalStateException(msg);
isex.initCause(instSite);
throw isex;
}
gotten = true;
try {
throw new IllegalStateException("Previous instantiation here.");
} catch (final IllegalStateException isex) {
instSite = isex;
}
return source.get();
}
@Override
public String toString() {
return String.format("SingleSupplier [source='%s', gotten=%s, id=%s]", source,
gotten, id);
}
}
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