Use transactional database API in Bramble.

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

@@ -0,0 +1,33 @@
+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/Pair.java

@@ -0,0 +1,26 @@
+package org.briarproject.bramble.api;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class Pair<A, B> {
+
+	private final A first;
+	private final B second;
+
+	public Pair(A first, B second) {
+		this.first = first;
+		this.second = second;
+	}
+
+	public A getFirst() {
+		return first;
+	}
+
+	public B getSecond() {
+		return second;
+	}
+}