java - do while didn't go to next line even though it should end -

i'm on socket project. when i'm debuging code found problem can't find out why, below code

 // code open socket connection , put inputstream bufferedreader called 'in'   string input;  input= (string)in.readline(); do{      system.out.println(input);  }while(input!=null);  out.close(); in.close(); socket.close(); 

as can see, above code create infinite loop, when send string server side, keeps println string.

if below

 string input;  do{     input= (string)in.readline();     system.out.println(input);  }while(input!=null);  out.close(); in.close(); socket.close(); 

the problem solved , , works great. every time send string server site, print once. didn't exit loop , go next line close socket connection .

so here question. if put within while loop. should loop end , go next line disconnect connection?? can explain why while loop execute once , didn't go next line?

best

if input= (string)in.readline(); in do-while loop, when read in null, print null , exit loop. then, out.close();,in.close();, , socket.close(); execute.