run:
Conectado OK
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at proxy.SocketConnect.writeLine(SocketConnect.java:51)
    at main.EnviodeDatos.actionPerformed(EnviodeDatos.java:50)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6539)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
    at java.awt.Component.processEvent(Component.java:6304)
    at java.awt.Container.processEvent(Container.java:2239)
    at java.awt.Component.dispatchEventImpl(Component.java:4889)
    at java.awt.Container.dispatchEventImpl(Container.java:2297)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4904)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4535)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4476)
    at java.awt.Container.dispatchEventImpl(Container.java:2283)
    at java.awt.Window.dispatchEventImpl(Window.java:2746)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:760)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:84)
    at java.awt.EventQueue$4.run(EventQueue.java:733)
    at java.awt.EventQueue$4.run(EventQueue.java:731)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:730)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:205)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

package main;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Scanner;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;

import proxy.SocketConnect;
import proxy.SocketInter;
import proxy.SocketInter;
import proxy.SocketConnect;

public class EnviodeDatos extends JFrame implements ActionListener {

    JTextField textmensaje;
    JButton btsend;

    /**
     *
     */
    public EnviodeDatos() {
        textmensaje = new JTextField();
        textmensaje.setBounds(10, 10, 200, 120);
        add(textmensaje);
        btsend = new JButton();
        btsend.setText("SEND");
        btsend.setBounds(10, 135, 120, 20);
        btsend.addActionListener((ActionListener) this);
        add(btsend);
        setLayout(null);
        setSize(400, 400);
        setVisible(true);
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        new EnviodeDatos();
    }

    @Override
    public void actionPerformed(ActionEvent arg0) {
        // TODO Auto-generated method stub
        if (arg0.getSource() == btsend) {

            SocketInter socket = (SocketInter) new SocketConnect("127.0.0.1", 8080);
            socket.writeLine(textmensaje.getText());

            socket.dispose();

        }
    }

}

package proxy;

public interface SocketInter {
    String readLine();
    void  writeLine(String str);
    void  dispose();
}

package proxy;

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

public class SocketConnect implements SocketInter {

    private Socket socket;
    private BufferedReader in;
    private PrintWriter out;

    /**
     * @param socket
     * @param in
     * @param out
     */
    public SocketConnect(String host, int port) {
        try {

            Socket socket = new Socket(host, port);
            in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            out = new PrintWriter(socket.getOutputStream(), true);
            System.out.println("Conectado OK ");
        } catch (IOException e) {
            System.out.println(e.getMessage());

        }
    }

    @Override
    public String readLine() {
        String str = null;
        try {
            str = in.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return str;
    }

    @Override
    public void writeLine(String str) {
        // TODO Auto-generated method stub
        // 4. The wrapper delegates to the target
        try {
            DataOutputStream data = new DataOutputStream(socket.getOutputStream());

            data.writeUTF(str);
            System.out.println("Dato enviado");

        } catch (IOException e) {
            // TODO Auto-generated catch block
            System.out.println("Dato no enviado: " + e.getMessage());
        }

    }

    @Override
    public void dispose() {
        // TODO Auto-generated method stub
        try {
            socket.close();

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}