Add parameterised return type.

briar-api/src/main/java/org/briarproject/briar/api/blog/BlogInvitationRequest.java

@@ -21,7 +21,7 @@ public class BlogInvitationRequest extends InvitationRequest<Blog> {
 	}
 
 	@Override
-	public void accept(PrivateMessageVisitor v) {
-		v.visitBlogInvitatioRequest(this);
+	public <T> T accept(PrivateMessageVisitor<T> v) {
+		return v.visitBlogInvitationRequest(this);
 	}
 }

briar-api/src/main/java/org/briarproject/briar/api/blog/BlogInvitationResponse.java

@@ -18,7 +18,7 @@ public class BlogInvitationResponse extends InvitationResponse {
 	}
 
 	@Override
-	public void accept(PrivateMessageVisitor v) {
-		v.visitBlogInvitationResponse(this);
+	public <T> T accept(PrivateMessageVisitor<T> v) {
+		return v.visitBlogInvitationResponse(this);
 	}
 }

briar-api/src/main/java/org/briarproject/briar/api/forum/ForumInvitationRequest.java

@@ -23,7 +23,7 @@ public class ForumInvitationRequest extends InvitationRequest<Forum> {
 	}
 
 	@Override
-	public void accept(PrivateMessageVisitor v) {
-		v.visitForumInvitationRequest(this);
+	public <T> T accept(PrivateMessageVisitor<T> v) {
+		return v.visitForumInvitationRequest(this);
 	}
 }

briar-api/src/main/java/org/briarproject/briar/api/forum/ForumInvitationResponse.java

@@ -21,7 +21,7 @@ public class ForumInvitationResponse extends InvitationResponse {
 	}
 
 	@Override
-	public void accept(PrivateMessageVisitor v) {
-		v.visitForumInvitationResponse(this);
+	public <T> T accept(PrivateMessageVisitor<T> v) {
+		return v.visitForumInvitationResponse(this);
 	}
 }

briar-api/src/main/java/org/briarproject/briar/api/introduction/IntroductionRequest.java

@@ -31,7 +31,7 @@ public class IntroductionRequest extends PrivateRequest<Author> {
 	}
 
 	@Override
-	public void accept(PrivateMessageVisitor v) {
-		v.visitIntroductionRequest(this);
+	public <T> T accept(PrivateMessageVisitor<T> v) {
+		return v.visitIntroductionRequest(this);
 	}
 }

briar-api/src/main/java/org/briarproject/briar/api/introduction/IntroductionResponse.java

@@ -37,7 +37,7 @@ public class IntroductionResponse extends PrivateResponse {
 	}
 
 	@Override
-	public void accept(PrivateMessageVisitor v) {
-		v.visitIntroductionResponse(this);
+	public <T> T accept(PrivateMessageVisitor<T> v) {
+		return v.visitIntroductionResponse(this);
 	}
 }

briar-api/src/main/java/org/briarproject/briar/api/messaging/PrivateMessageHeader.java

@@ -54,7 +54,7 @@ public class PrivateMessageHeader {
 		return read;
 	}
 
-	public void accept(PrivateMessageVisitor v) {
-		v.visitPrivateMessageHeader(this);
+	public <T> T accept(PrivateMessageVisitor<T> v) {
+		return v.visitPrivateMessageHeader(this);
 	}
 }

briar-api/src/main/java/org/briarproject/briar/api/messaging/PrivateMessageVisitor.java

 package org.briarproject.briar.api.messaging;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.briar.api.blog.BlogInvitationRequest;
 import org.briarproject.briar.api.blog.BlogInvitationResponse;
 import org.briarproject.briar.api.forum.ForumInvitationRequest;
@@ -9,23 +10,24 @@ import org.briarproject.briar.api.introduction.IntroductionResponse;
 import org.briarproject.briar.api.privategroup.invitation.GroupInvitationRequest;
 import org.briarproject.briar.api.privategroup.invitation.GroupInvitationResponse;
 
-public interface PrivateMessageVisitor {
+@NotNullByDefault
+public interface PrivateMessageVisitor<T> {
 
-	void visitPrivateMessageHeader(PrivateMessageHeader h);
+	T visitPrivateMessageHeader(PrivateMessageHeader h);
 
-	void visitBlogInvitatioRequest(BlogInvitationRequest r);
+	T visitBlogInvitationRequest(BlogInvitationRequest r);
 
-	void visitBlogInvitationResponse(BlogInvitationResponse r);
+	T visitBlogInvitationResponse(BlogInvitationResponse r);
 
-	void visitForumInvitationRequest(ForumInvitationRequest r);
+	T visitForumInvitationRequest(ForumInvitationRequest r);
 
-	void visitForumInvitationResponse(ForumInvitationResponse r);
+	T visitForumInvitationResponse(ForumInvitationResponse r);
 
-	void visitGroupInvitationRequest(GroupInvitationRequest r);
+	T visitGroupInvitationRequest(GroupInvitationRequest r);
 
-	void visitGroupInvitationResponse(GroupInvitationResponse r);
+	T visitGroupInvitationResponse(GroupInvitationResponse r);
 
-	void visitIntroductionRequest(IntroductionRequest r);
+	T visitIntroductionRequest(IntroductionRequest r);
 
-	void visitIntroductionResponse(IntroductionResponse r);
+	T visitIntroductionResponse(IntroductionResponse r);
 }

briar-api/src/main/java/org/briarproject/briar/api/privategroup/invitation/GroupInvitationRequest.java

@@ -24,7 +24,7 @@ public class GroupInvitationRequest extends InvitationRequest<PrivateGroup> {
 	}
 
 	@Override
-	public void accept(PrivateMessageVisitor v) {
-		v.visitGroupInvitationRequest(this);
+	public <T> T accept(PrivateMessageVisitor<T> v) {
+		return v.visitGroupInvitationRequest(this);
 	}
 }

briar-api/src/main/java/org/briarproject/briar/api/privategroup/invitation/GroupInvitationResponse.java

@@ -21,7 +21,7 @@ public class GroupInvitationResponse extends InvitationResponse {
 	}
 
 	@Override
-	public void accept(PrivateMessageVisitor v) {
-		v.visitGroupInvitationResponse(this);
+	public <T> T accept(PrivateMessageVisitor<T> v) {
+		return v.visitGroupInvitationResponse(this);
 	}
 }