GUI

import java.awt.BorderLayout;
import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.filechooser.FileNameExtensionFilter;
import javax.swing.JLabel;
import javax.swing.SwingConstants;
import javax.swing.JTextArea;
import javax.swing.JButton;
import javax.swing.JFileChooser;

import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.awt.event.ActionEvent;

public class Okno extends JFrame {

	private JPanel contentPane;

	/**
	 * Launch the application.
	 */
	public static void main(String[] args) {
		EventQueue.invokeLater(new Runnable() {
			public void run() {
				try {
					Okno frame = new Okno();
					frame.setVisible(true);
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		});
	}

	/**
	 * Create the frame.
	 */
	public Okno() {
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setBounds(100, 100, 450, 300);
		contentPane = new JPanel();
		contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
		contentPane.setLayout(new BorderLayout(0, 0));
		setContentPane(contentPane);

		JLabel lblNazwaPliku = new JLabel("Nazwa pliku");
		lblNazwaPliku.setHorizontalAlignment(SwingConstants.CENTER);
		contentPane.add(lblNazwaPliku, BorderLayout.NORTH);

		JTextArea textArea = new JTextArea();
		textArea.setEditable(false);
		contentPane.add(textArea, BorderLayout.CENTER);

		JPanel panel = new JPanel();
		contentPane.add(panel, BorderLayout.SOUTH);

		JButton btnOtworz = new JButton("Otworz");
		btnOtworz.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg0) {
				JFileChooser chooser = new JFileChooser();
			    FileNameExtensionFilter filter = new FileNameExtensionFilter("txt");
			    chooser.setFileFilter(filter);
			    int returnVal = chooser.showOpenDialog(Okno.this);
			    if(returnVal == JFileChooser.APPROVE_OPTION) {
			       lblNazwaPliku.setText(chooser.getSelectedFile().getName());
			       try(BufferedReader br = new BufferedReader(new FileReader(chooser.getSelectedFile().getAbsolutePath()))) {
			    	    StringBuilder sb = new StringBuilder();
			    	    String line = br.readLine();

			    	    while (line != null) {
			    	        sb.append(line);
			    	        sb.append(System.lineSeparator());
			    	        line = br.readLine();
			    	    }
			    	    String everything = sb.toString();
			    	    textArea.append(everything);
			    	} catch (FileNotFoundException e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					} catch (IOException e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					}
			    }
			}
		});
		panel.add(btnOtworz);

		JButton btnWyjdz = new JButton("Wyjdz");
		btnWyjdz.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg0) {
				System.exit(0);
			}
		});
		panel.add(btnWyjdz);
	}

}