Client

import android.os.Bundle;
import android.os.Handler;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;

public class MainActivity extends AppCompatActivity {
  private TextView mTextViewReplyFromServer;
  private EditText mEditTextSendMessage;
  final Handler mHandler = new Handler();

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    initView();
  }

  private void initView() {
    mEditTextSendMessage = findViewById(R.id.edt_send_message);
    mTextViewReplyFromServer = findViewById(R.id.tv_reply_from_server);
    findViewById(R.id.btn_send).setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        sendMessage(mEditTextSendMessage.getText().toString());
      }
    });
  }

  private void sendMessage(final String message) {
    Thread thread = new Thread(new Runnable() {
      @Override
      public void run() {
        try {
          // 对应好服务端 IP 地址 和 端口
          Socket socket = new Socket("172.27.118.2", 9102);

          // 由输出流得到 Writer 并写信息
          PrintWriter printWriter = new PrintWriter(socket.getOutputStream());
          printWriter.println(message);
          printWriter.flush();

          // 由输入流得到 Reader 并读信息
          BufferedReader bufferedReader = new BufferedReader(
              new InputStreamReader(socket.getInputStream()));
          final String read = bufferedReader.readLine();
          System.out.println("read string from server: " + read);

          updateUI(read);

          // 依次关闭
          printWriter.close();
          socket.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    });

    thread.start();
  }

  private void updateUI(final String data) {
    // Can't create handler inside thread that has not called Looper.prepare()
    mHandler.post(new Runnable() {
      @Override
      public void run() {
        String current = mTextViewReplyFromServer.getText().toString();
        if (data.trim().length() != 0) {
          mTextViewReplyFromServer.setText(current + "\nFrom Server : " + data);
        }
      }
    });
  }
}

Server

import android.os.Handler;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
  private Button mButtonStartReceiving;
  private Button mButtonStopReceiving;
  private TextView mTextViewDataFromClient;
  private boolean isEnd = false;
  final Handler mHandler = new Handler();

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    initView();
  }

  private void initView() {
    mButtonStartReceiving = findViewById(R.id.btn_start_receiving);
    mButtonStopReceiving = findViewById(R.id.btn_stop_receiving);
    mTextViewDataFromClient = findViewById(R.id.tv_data_from_client);
    mButtonStartReceiving.setOnClickListener(this);
    mButtonStopReceiving.setOnClickListener(this);
  }

  private void startServerSocket() {
    Thread thread = new Thread(new Runnable() {
      private String stringData = null;

      @Override
      public void run() {
        try {
          ServerSocket serverSocket = new ServerSocket(9102);

          while (!isEnd) {
            //Server is waiting for client here, if needed
            Socket socket = serverSocket.accept();

            // 由输入流得到 Reader 并读信息
            BufferedReader bufferedReader = new BufferedReader(
                new InputStreamReader(socket.getInputStream()));
            final String read = bufferedReader.readLine();
            System.out.println("read string from client: " + read);

            // 由输出流得到 Writer 并写信息
            PrintWriter printWriter = new PrintWriter(socket.getOutputStream());
            printWriter.println("FROM SERVER - " + read.toUpperCase());
            printWriter.flush();

            try {
              Thread.sleep(1000);
            } catch (InterruptedException e) {
              e.printStackTrace();
            }

            updateUI(read);

            if (read.equalsIgnoreCase("STOP")) {
              isEnd = true;
              printWriter.close();
              socket.close();
              break;
            }

            // 依次关闭
            printWriter.close();
            socket.close();
          }
          serverSocket.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    });
    thread.start();
  }

  private void updateUI(final String data) {
    // Can't create handler inside thread that has not called Looper.prepare()
    mHandler.post(new Runnable() {
      @Override
      public void run() {
        String current = mTextViewDataFromClient.getText().toString();
        if (data.trim().length() != 0) {
          mTextViewDataFromClient.setText(current + "\n" + "From Client : " + data);
        }
      }
    });
  }

  @Override
  public void onClick(View v) {
    switch (v.getId()) {
      case R.id.btn_start_receiving:
        startServerSocket();
        mButtonStartReceiving.setEnabled(false);
        mButtonStopReceiving.setEnabled(true);
        break;
      case R.id.btn_stop_receiving:
        //stopping server socket logic you can add yourself
        mButtonStartReceiving.setEnabled(true);
        mButtonStopReceiving.setEnabled(false);
        break;
      default:
        break;
    }
  }
}