How to understand robot orientation from quaternion yaw angle

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I have this line of code

geometry_msgs::Quaternion odom_quat = tf::createQuaternionMsgFromYaw(currentth);

Where currentth is the angle in radian to represent the pose of the robot. Then i am making the assignment

odom_trans.transform.rotation = odom_quat;

where i do the propagation. In another node i have a callback as follows

void odometryCallback (const nav_msgs::Odometry::ConstPtr &odom)
{
// ROS_INFO("ODOMETRY RETRIEVED FROM ODOMETRY NODE and orientation is %f",robotCurrentTheta );
  currentX = odom->pose.pose.position.x;
  currentY = odom->pose.pose.position.y;
  robotCurrentTheta= odom->pose.pose.orientation.w; // not sure here
  ROS_INFO("ODOMETRY RETRIEVED FROM ODOMETRY NODE and orientation is %f %f %f %f",
  odom->pose.pose.orientation.x,odom->pose.pose.orientation.y,odom->pose.pose.orientation.z,odom->pose.pose.orientation.w );
}

Now how i can obtain robot's orientation in radian? am i missing something.