Replace Maybe with nullable transaction method.

bramble-api/src/main/java/org/briarproject/bramble/api/Maybe.java

@@ -1,33 +0,0 @@
-package org.briarproject.bramble.api;
-
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-
-import java.util.NoSuchElementException;
-
-import javax.annotation.Nullable;
-import javax.annotation.concurrent.Immutable;
-
-/**
- * Minimal stand-in for `java.util.Optional`. Functionality from `Optional`
- * can be added as needed.
- */
-@Immutable
-@NotNullByDefault
-public class Maybe<T> {
-
-	@Nullable
-	private final T value;
-
-	public Maybe(@Nullable T value) {
-		this.value = value;
-	}
-
-	public boolean isPresent() {
-		return value != null;
-	}
-
-	public T get() {
-		if (value == null) throw new NoSuchElementException();
-		return value;
-	}
-}

bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java

@@ -89,6 +89,14 @@ public interface DatabaseComponent {
 	<R, E extends Exception> R transactionWithResult(boolean readOnly,
 			DbCallable<R, E> task) throws DbException, E;
 
+	/**
+	 * Runs the given task within a transaction and returns the result of the
+	 * task, which may be null.
+	 */
+	@Nullable
+	<R, E extends Exception> R transactionWithNullableResult(boolean readOnly,
+			DbCallable<R, E> task) throws DbException, E;
+
 	/**
 	 * Stores a contact associated with the given local and remote pseudonyms,
 	 * and returns an ID for the contact.

bramble-api/src/main/java/org/briarproject/bramble/api/db/DbCallable.java

@@ -1,6 +1,12 @@
 package org.briarproject.bramble.api.db;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.Nullable;
+
+@NotNullByDefault
 public interface DbCallable<R, E extends Exception> {
 
+	@Nullable
 	R call(Transaction txn) throws DbException, E;
 }

bramble-api/src/main/java/org/briarproject/bramble/api/db/DbRunnable.java

@@ -1,5 +1,8 @@
 package org.briarproject.bramble.api.db;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
 public interface DbRunnable<E extends Exception> {
 
 	void run(Transaction txn) throws DbException, E;