1 files changed, 76 insertions, 0 deletions
diff --git a/src/main/java/bjc/data/SingleSupplier.java b/src/main/java/bjc/data/SingleSupplier.java
new file mode 100644
index 0000000..bc1fbc4
--- /dev/null
+++ b/src/main/java/bjc/data/SingleSupplier.java
@@ -0,0 +1,76 @@
+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);
+	}
+}