Problems with java post response

Hi community! let’s see if you can solve my problems.
I’m trying to create a pulse with post http from java and i recieve that error message:
“No query string was present”

I’m sending a String :
String data = “‘mutation’ { ‘create_item’(board_id: xxx, group_id: ‘topics’, item_name: ‘Api created’) {id}}”;
And content type application/graphql

If anyone can help me please!
Thank you!