Add utility method for null checks.

bramble-api/src/main/java/org/briarproject/bramble/api/nullsafety/NullSafety.java

@@ -6,10 +6,20 @@ import javax.annotation.Nullable;
 public class NullSafety {
 
 	/**
-	 * Stand-in for `Objects.requireNonNull()`.
+	 * Stand-in for {@code Objects.requireNonNull()}.
 	 */
 	public static <T> T requireNonNull(@Nullable T t) {
 		if (t == null) throw new NullPointerException();
 		return t;
 	}
+
+	/**
+	 * Checks that exactly one of the arguments is null.
+	 *
+	 * @throws AssertionError If both or neither of the arguments are null
+	 */
+	public static void requireExactlyOneNull(@Nullable Object a,
+			@Nullable Object b) {
+		if ((a == null) == (b == null)) throw new AssertionError();
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/transport/StreamContext.java

@@ -9,6 +9,8 @@ import org.briarproject.bramble.api.plugin.TransportId;
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireExactlyOneNull;
+
 @Immutable
 @NotNullByDefault
 public class StreamContext {
@@ -26,8 +28,7 @@ public class StreamContext {
 			@Nullable PendingContactId pendingContactId,
 			TransportId transportId, SecretKey tagKey, SecretKey headerKey,
 			long streamNumber, boolean handshakeMode) {
-		if ((contactId == null) == (pendingContactId == null))
+		requireExactlyOneNull(contactId, pendingContactId);
-			throw new IllegalArgumentException();
 		this.contactId = contactId;
 		this.pendingContactId = pendingContactId;
 		this.transportId = transportId;

bramble-api/src/main/java/org/briarproject/bramble/api/transport/TransportKeySet.java

@@ -7,6 +7,8 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireExactlyOneNull;
+
 /**
  * A set of keys for communicating with a given contact or pending contact
  * over a given transport.
@@ -24,8 +26,7 @@ public class TransportKeySet {
 
 	public TransportKeySet(KeySetId keySetId, @Nullable ContactId contactId,
 			@Nullable PendingContactId pendingContactId, TransportKeys keys) {
-		if ((contactId == null) == (pendingContactId == null))
+		requireExactlyOneNull(contactId, pendingContactId);
-			throw new IllegalArgumentException();
 		this.keySetId = keySetId;
 		this.contactId = contactId;
 		this.pendingContactId = pendingContactId;

bramble-core/src/main/java/org/briarproject/bramble/transport/MutableTransportKeySet.java

@@ -8,6 +8,8 @@ import org.briarproject.bramble.api.transport.KeySetId;
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.NotThreadSafe;
 
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireExactlyOneNull;
+
 @NotThreadSafe
 @NotNullByDefault
 class MutableTransportKeySet {
@@ -22,8 +24,7 @@ class MutableTransportKeySet {
 	MutableTransportKeySet(KeySetId keySetId, @Nullable ContactId contactId,
 			@Nullable PendingContactId pendingContactId,
 			MutableTransportKeys keys) {
-		if ((contactId == null) == (pendingContactId == null))
+		requireExactlyOneNull(contactId, pendingContactId);
-			throw new IllegalArgumentException();
 		this.keySetId = keySetId;
 		this.contactId = contactId;
 		this.pendingContactId = pendingContactId;

bramble-core/src/main/java/org/briarproject/bramble/transport/TransportKeyManagerImpl.java

@@ -36,6 +36,7 @@ import javax.annotation.concurrent.ThreadSafe;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireExactlyOneNull;
 import static org.briarproject.bramble.api.transport.TransportConstants.MAX_CLOCK_DIFFERENCE;
 import static org.briarproject.bramble.api.transport.TransportConstants.PROTOCOL_VERSION;
 import static org.briarproject.bramble.api.transport.TransportConstants.TAG_LENGTH;
@@ -136,6 +137,7 @@ class TransportKeyManagerImpl implements TransportKeyManager {
 	private void addKeys(KeySetId keySetId, @Nullable ContactId contactId,
 			@Nullable PendingContactId pendingContactId,
 			MutableTransportKeys keys) {
+		requireExactlyOneNull(contactId, pendingContactId);
 		MutableTransportKeySet ks = new MutableTransportKeySet(keySetId,
 				contactId, pendingContactId, keys);
 		this.keys.put(keySetId, ks);
@@ -184,6 +186,7 @@ class TransportKeyManagerImpl implements TransportKeyManager {
 	@Nullable
 	private MutableTransportKeySet getOutgoingKeySet(@Nullable ContactId c,
 			@Nullable PendingContactId p) {
+		requireExactlyOneNull(c, p);
 		if (c == null) return pendingContactOutContexts.get(p);
 		else return contactOutContexts.get(c);
 	}
@@ -475,6 +478,7 @@ class TransportKeyManagerImpl implements TransportKeyManager {
 				@Nullable PendingContactId pendingContactId,
 				MutableIncomingKeys inKeys, long streamNumber,
 				boolean handshakeMode) {
+			requireExactlyOneNull(contactId, pendingContactId);
 			this.keySetId = keySetId;
 			this.contactId = contactId;
 			this.pendingContactId = pendingContactId;