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

import javax.swing.*;

public class Test {
	public static void main(String[] args) {
		new MyFrame();
	}
}

class MyFrame extends JFrame {
	MyFrame() {
		setSize(400, 300);
		setTitle("MyFrame");
		setLocationRelativeTo(null);
		makeUI();
		setVisible(true);
	}

	void makeUI() {
		add(new MyPanel());
	}
}

class MyPanel extends JPanel {
	MyPanel(){
		setLayout(new GridLayout(4, 4));
		JButton[] b = new JButton[16];
		MyListener listener = new MyListener(b);

		for(int i=0; i<16; i++){
			b[i] = new JButton(String.valueOf(i));
			b[i].addActionListener(listener);
			add(b[i]);
		}

		b[15].setEnabled(false);
		b[15].setText("");

	}
}

class MyListener implements ActionListener {
	private JButton[] btn;
	private int[] nb;

	public MyListener(JButton[] b) {
		btn = b;
		nb = new int[4];
	}
	@Override
	public void actionPerformed(ActionEvent e) {
		JButton b = (JButton)e.getSource();

		for(int i=0; i<16; i++){
			if(b == btn[i])
				break;
			findNeighbor(i);
		}

		for(int j=0; j<nb.length; j++){
			if(nb[j] >= 0 && !btn[nb[j]].isEnabled()){
				btn[nb[j]].setText(b.getText());
				b.setText("");
				b.setEnabled(false);
				btn[nb[j]].setEnabled(true);
			}
		}
	}

	private void findNeighbor(int id) {
		// up
		nb[0] = id-4;

		// down
		nb[1] = id+4;
		if (nb[1] >= 16)
			nb[1] = -1;

		// left
		nb[2] = id-1;
		if (nb[2] < 0 || nb[2]%4 == 3)
			nb[2] = -1;

		// right
		nb[3] = id+1;
		if (nb[3]%4 == 0)
			nb[3] = -1;
	 }
}