1 files changed, 103 insertions, 0 deletions
diff --git a/src/main/java/bjc/data/Contexts.java b/src/main/java/bjc/data/Contexts.java
new file mode 100644
index 0000000..b028ad1
--- /dev/null
+++ b/src/main/java/bjc/data/Contexts.java
@@ -0,0 +1,103 @@
+package bjc.data;
+
+import java.util.*;
+
+/**
+ * Utility methods for dealing with contexts.
+ * 
+ * @author Ben Culkin
+ *
+ */
+public class Contexts {
+	/**
+	 * The null context, which always throws an exception.
+	 */
+	public static final Context NULL = new NullContextImpl();
+
+	private Contexts() {
+		throw new UnsupportedOperationException();
+	}
+
+	/**
+	 * Create a new context with no parent.
+	 * 
+	 * @return A context with no parent.
+	 */
+	public static Context create() {
+		return new ContextImpl(NULL);
+	}
+
+	/**
+	 * Create a context with the specified parent.
+	 * 
+	 * @param parent The parent of this context.
+	 * 
+	 * @return A context with the given context as its parent.
+	 */
+	public static Context create(Context parent) {
+		return new ContextImpl(parent);
+	}
+
+	private static class NullContextImpl implements Context {
+		@Override
+		public Context getParent() {
+			return this;
+		}
+
+		@Override
+		public void register(String name, Object o) {
+			throw new UnsupportedOperationException();
+		}
+
+		@Override
+		public Object get(String name) {
+			throw new NoSuchElementException();
+		}
+		
+		@Override
+		public <T> T get(Class<T> contract) {
+			throw new NoSuchElementException();
+		}
+	}
+
+	private static class ContextImpl implements Context {
+
+		private final Context parent;
+
+		private final Map<String, Object> objects;
+
+		public ContextImpl(Context parent) {
+			this.parent = parent;
+			this.objects = new HashMap<>();
+		}
+
+		@Override
+		public void register(String name, Object o) {
+			objects.put(name, o);
+		}
+
+		@Override
+		public Object get(String name) {
+			if (objects.containsKey(name)) {
+				return objects.get(name);
+			}
+			return parent.get(name);
+		}
+
+		@SuppressWarnings("unchecked")
+		@Override
+		public <T> T get(Class<T> contract) {
+			for (Object o : objects.values()) {
+				if (contract.isInstance(o)) {
+					return (T) o;
+				}
+			}
+			return parent.get(contract);
+		}
+
+		@Override
+		public Context getParent() {
+			return parent;
+		}
+	}
+}