package pl;

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

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;

import java.awt.GridBagLayout;

import javax.swing.JList;

import java.awt.GridBagConstraints;

import javax.swing.JButton;

import java.awt.Insets;

import javax.swing.GroupLayout;
import javax.swing.GroupLayout.Alignment;
import javax.swing.LayoutStyle.ComponentPlacement;

import bl.Gadget;

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

public class GadgetMasterFrame extends JFrame {

	private JPanel contentPane;
	//private JFrame gadgetDetailFrame;

	/**
	 * Create the frame.
	 */
	public GadgetMasterFrame() {
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setBounds(100, 100, 450, 300);
		contentPane = new JPanel();
		contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
		setContentPane(contentPane);
		
		JButton btnNewButton = new JButton("Gadget editieren");x
		
		JButton btnNewButton_1 = new JButton("Gadget erfassen");
		btnNewButton_1.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				//GadgetDetailFrame window = new GadgetDetailFrame(new Gadget(""));
			}
		});
		
		JPanel panel = new JPanel();
		GroupLayout gl_contentPane = new GroupLayout(contentPane);
		gl_contentPane.setHorizontalGroup(
			gl_contentPane.createParallelGroup(Alignment.TRAILING)
				.addGroup(gl_contentPane.createSequentialGroup()
					.addContainerGap()
					.addGroup(gl_contentPane.createParallelGroup(Alignment.LEADING)
						.addGroup(Alignment.TRAILING, gl_contentPane.createSequentialGroup()
							.addComponent(btnNewButton_1)
							.addPreferredGap(ComponentPlacement.RELATED)
							.addComponent(btnNewButton))
						.addComponent(panel, GroupLayout.DEFAULT_SIZE, 428, Short.MAX_VALUE))
					.addContainerGap())
		);
		gl_contentPane.setVerticalGroup(
			gl_contentPane.createParallelGroup(Alignment.LEADING)
				.addGroup(gl_contentPane.createSequentialGroup()
					.addContainerGap()
					.addGroup(gl_contentPane.createParallelGroup(Alignment.BASELINE)
						.addComponent(btnNewButton)
						.addComponent(btnNewButton_1))
					.addPreferredGap(ComponentPlacement.RELATED)
					.addComponent(panel, GroupLayout.DEFAULT_SIZE, 221, Short.MAX_VALUE)
					.addContainerGap())
		);
		panel.setLayout(new BorderLayout(0, 0));
		
		JList list = new JList();
		panel.add(list, BorderLayout.CENTER);
		contentPane.setLayout(gl_contentPane);
	}
}

package pl;

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

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;

import java.awt.GridBagLayout;

import javax.swing.JList;

import java.awt.GridBagConstraints;

import javax.swing.JButton;

import java.awt.Insets;

import javax.swing.GroupLayout;
import javax.swing.GroupLayout.Alignment;
import javax.swing.LayoutStyle.ComponentPlacement;

import bl.Gadget;

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

public class GadgetMasterFrame extends JFrame {

	private JPanel contentPane;
	//private JFrame gadgetDetailFrame;

	/**
	 * Create the frame.
	 */
	public GadgetMasterFrame() {
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setBounds(100, 100, 450, 300);
		contentPane = new JPanel();
		contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
		setContentPane(contentPane);
		
		JButton btnNewButton = new JButton("Gadget editieren");
		
		JButton btnNewButton_1 = new JButton("Gadget erfassen");
		btnNewButton_1.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				//GadgetDetailFrame window = new GadgetDetailFrame(new Gadget(""));
			}
		});
		
		JPanel panel = new JPanel();
		GroupLayout gl_contentPane = new GroupLayout(contentPane);
		gl_contentPane.setHorizontalGroup(
			gl_contentPane.createParallelGroup(Alignment.TRAILING)
				.addGroup(gl_contentPane.createSequentialGroup()
					.addContainerGap()
					.addGroup(gl_contentPane.createParallelGroup(Alignment.LEADING)
						.addGroup(Alignment.TRAILING, gl_contentPane.createSequentialGroup()
							.addComponent(btnNewButton_1)
							.addPreferredGap(ComponentPlacement.RELATED)
							.addComponent(btnNewButton))
						.addComponent(panel, GroupLayout.DEFAULT_SIZE, 428, Short.MAX_VALUE))
					.addContainerGap())
		);
		gl_contentPane.setVerticalGroup(
			gl_contentPane.createParallelGroup(Alignment.LEADING)
				.addGroup(gl_contentPane.createSequentialGroup()
					.addContainerGap()
					.addGroup(gl_contentPane.createParallelGroup(Alignment.BASELINE)
						.addComponent(btnNewButton)
						.addComponent(btnNewButton_1))
					.addPreferredGap(ComponentPlacement.RELATED)
					.addComponent(panel, GroupLayout.DEFAULT_SIZE, 221, Short.MAX_VALUE)
					.addContainerGap())
		);
		panel.setLayout(new BorderLayout(0, 0));
		
		JList list = new JList();
		panel.add(list, BorderLayout.CENTER);
		contentPane.setLayout(gl_contentPane);
	}
}

Update GadgetMasterFrame.java