Why String's output value is not updating

Question

    DatagramSocket ds = new DatagramSocket(this.my_listening_port);
    byte[] rcv_data = new byte[1024];
    DatagramPacket dp = new DatagramPacket(rcv_data,rcv_data.length);

    String rcv_data_string[] = new String[no_of_child_node];
    for(int i=0; i<no_of_child_node; i++)
    {
        ds.receive(dp);
        rcv_data_string[i] = new String(dp.getData());
        System.out.println("result of child " + i + ": " + rcv_data_string[i]);
    }

    String query_result = new String();
    query_result += this.self_node_id;
    for(int i=0; i<no_of_child_node ; i++)
    {
        System.out.println("result of child " + i + ": " + rcv_data_string[i]);
        query_result = query_result.concat(rcv_data_string[i]);
    }

    ds.close();     
    System.out.println("Final result of me and child : " + query_result);
    sending_result_to_parent(query_result);

Output :

result of child 0: t2
result of child 1: t5
result of child 2: t3t1
result of child 0: t2
result of child 1: t5
result of child 2: t3t1
Final result of me and child : t4t2

my expected final result : Final result of me and child : t4t2t5t3t1

What is the problem with above code ? So can anyone kindly tell what can be the problem?