SecondFrame.java

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;

public class SecondFrame extends JFrame implements ActionListener {

    private JTextField jtfSecond1 = null;
    private JButton jbSecond1 = null;

    private StringBuffer sb = null;
    private String result = null;

    private FirstFrame jframeFirst = null;

    public SecondFrame() throws ClassNotFoundException, IOException {

        setTitle("Второе окно");
        setBounds(800, 100, 600, 200);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);

        GridLayout gl = new GridLayout(2, 1);
        setLayout(gl);

        jtfSecond1 = new JTextField();

        jbSecond1 = new JButton();
        jbSecond1.setText("Отправить текст");

        add(jtfSecond1);
        add(jbSecond1);

        jbSecond1.addActionListener(this);

    }

    @Override
    public void actionPerformed(ActionEvent e) {
        try {
            ffM();
            stM(jtfSecond1, jframeFirst.jtfFirst1);
        } catch (ClassNotFoundException | IOException e2) {
            e2.printStackTrace();
        }
    }

    private void ffM() throws IOException, ClassNotFoundException {

        FileInputStream fis = new FileInputStream("text.ser");
        ObjectInputStream oin = new ObjectInputStream(fis);
        jframeFirst = (FirstFrame) oin.readObject();
        oin.close();
    }

    private void stM(JTextField jtfm1, JTextField jtfm2) throws IOException {
        sb = new StringBuffer();
        result = new String();

        for (int i = 0; i < 10; i++) {
            result = sb.append("i#" + i + "=" + jtfm1.getText() + "; ")
                    .toString();
            jtfm2.setText(result);
        }

        FileOutputStream fos = new FileOutputStream("text.ser");
        ObjectOutputStream oos = new ObjectOutputStream(fos);

        oos.writeObject(jframeFirst);
        oos.flush();
        oos.close();
        }
    }