Puzzle15

import java.awt.GridLayout;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.SwingUtilities;

public class Puzzle15 {
  private JFrame f;
  private JButton[] a;
  private final List<String> xs = new ArrayList<>(Arrays
      .asList(new String[] {
          "1", "2", "3", "4",
          "5", "6", "7", "8",
          "9", "10", "11", "12",
          "13", "14", "15", " "
      }));
  private final int[][] dir = {
      { 1, 4 }, { -1, 1, 4 }, { -1, 1, 4 }, { -1, 4 },
      { -4, 1, 4 }, { -4, -1, 1, 4 }, { -4, -1, 1, 4 }, { -4, -1, 4 },
      { -4, 1, 4 }, { -4, -1, 1, 4 }, { -4, -1, 1, 4 }, { -4, -1, 4 },
      { -4, 1 }, { -4, -1, 1 }, { -4, -1, 1 }, { -4, -1 }
  };

  private void swapButtonText(JButton a, JButton b) {
    String t = a.getText();
    a.setText(b.getText());
    b.setText(t);
  }

  private boolean finished() {
    List<String> ys = new ArrayList<>(Arrays.stream(a).map(JButton::getText).toList());
    // System.out.println(ys);
    return xs.equals(ys);
  }

  private void clickedAction(int i) {
    for (int j : dir[i]) {
      if (a[i + j].getText().equals(" ")) {
        swapButtonText(a[i], a[i + j]);
        break;
      }
    }
    if (finished()) {
      JOptionPane.showMessageDialog(f, "Finish");
    }
  }

  public Puzzle15(String[] numStr) {
    a = new JButton[16];
    f = new JFrame("My 15 puzzle");
    f.setSize(250, 250);
    f.getContentPane().setLayout(new GridLayout(4, 4));
    for (int i = 0; i < 16; i++) {
      final int index = i;
      a[index] = new JButton(numStr[index]);
      f.getContentPane().add(a[index]);
      a[index].addActionListener(e -> clickedAction(index));
    }
    f.setVisible(true);
  }

  public static void main(final String[] args) {
    SwingUtilities.invokeLater(() -> new Puzzle15(new String[] {
        "1", "2", "3", "4",
        "5", "6", "7", "8",
        "9", "10", "15", "11",
        "13", "14", " ", "12"
    }));
  }
}