How to get the actual coordinate for all of the joint?

edit

is it possible that i want the transform of one link only, in order to get their specific coordinate?

To get the coordinates of a joint you need a fixed frame (such as world or map) otherwise any joint could have any coordinates depending the frame you are reading the coordinates from.

rosrun tf tf_echo /world /joint

This command can give you the coordinates of a specific joint in the world (can be map or any other frame that is fixed in your world). You will be able to read those coordinates in the terminal but given what you said I assume you actually want to get those coordinates in your code to process them.

You will achieve that using tf by checking the transformation between your reference frame and your desired joint. Here's an example using geometry_msgs::PoseStamped (to get x, y, z) to constantly get the coordinates of a joint :

//Required includes
#include <tf2_geometry_msgs/tf2_geometry_msgs.h>
#include <tf2_ros/transform_listener.h>

tf2_ros::Buffer tf_; 
geometry_msgs::PoseStamped joint_global_frame_pose_stamped;
geometry_msgs::PoseStamped joint_pose_stamped;
//Just an initialization
tf2::toMsg(tf2::Transform::getIdentity(), joint_global_frame_pose_stamped.pose);
tf2::toMsg(tf2::Transform::getIdentity(), joint_pose_stamped.pose);
ros::Rate rate(10.0);
while(ros::ok())
{
    //The frame you want to get the pose 
    joint_pose_stamped.header.frame_id = "joint";
    joint_pose_stamped.header.stamp = ros::Time();
    //Catch exceptions
    try
    {
        //3rd param = The global frame
        tf_.transform(joint_pose_stamped, joint_global_frame_pose_stamped, "map");  
    }
    catch (tf2::LookupException& ex)
    {
        ROS_ERROR_THROTTLE(1.0, "No Transform available Error looking up joint pose: %s\n", ex.what());
    }
    catch (tf2::ConnectivityException& ex)
    {
        ROS_ERROR_THROTTLE(1.0, "Connectivity Error looking up joint pose: %s\n", ex.what());
    }
    catch (tf2::ExtrapolationException& ex)
    {
        ROS_ERROR_THROTTLE(1.0, "Extrapolation Error looking up joint pose: %s\n", ex.what());
    }

    ROS_INFO("The joint position is { %.2f; %.2f; %.2f}", 
            joint_global_frame_pose_stamped.pose.postion.x,
            joint_global_frame_pose_stamped.pose.postion.y,
            joint_global_frame_pose_stamped.pose.postion.z);
}