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Factorial

n! = n * (n - 1) * … * 2 * 1   | JS | .NET |

// <applet code="FactApp.class" width="300" height="240"></applet>

/**
 * 階乗を求める Java Applet
 * @see https://mind.kittttttan.info/java/factapp.html
 */
import java.applet.Applet;
import javax.swing.JPanel;
import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.JTextArea;
import javax.swing.JScrollPane;

import java.awt.BorderLayout;

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

import java.math.BigInteger;

public class FactApp extends Applet implements ActionListener {
	/**
	 * default serialVersionUID
	 */
	private static final long serialVersionUID = 1L;

	private static final int MAX = 99999;
	private JPanel panel1;
	private JPanel panel2;
	private JButton btn;
	private JTextArea ta;
	private JScrollPane sp;
	private JLabel labelt;
	private JTextField tf;

	/**
	 * Initialize
	 */
	public void init() {
		btn = new JButton("OK");
		btn.addActionListener(this);

		tf = new JTextField("777", 7);

		labelt = new JLabel("Time[ms]");
		ta = new JTextArea(10, 20);
		ta.setLineWrap(true);
		sp = new JScrollPane(ta);

		panel1 = new JPanel();
		panel1.add(tf);
		panel1.add(btn);
		panel1.add(labelt);

		panel2 = new JPanel();
		panel2.add(sp);

		add(panel1, BorderLayout.CENTER);
		add(panel2, BorderLayout.SOUTH);
	}

	@Override
	public void actionPerformed(ActionEvent arg0) {
		int n;

		try {
			n = Integer.parseInt(tf.getText());
		} catch (NumberFormatException err) {
			ta.setText("Input Number 1, 2, ...");
			labelt.setText("Error");
			return;
		}

		if (n > MAX) {
			ta.setText("too BIG");
			labelt.setText("Error");
			return;
		}

		long start = System.currentTimeMillis();

		BigInteger p = fact(n);

		long stop = System.currentTimeMillis();

		ta.setText(p.toString());
		labelt.setText(String.valueOf(stop - start) + "ms");
	}

	public static BigInteger fact(int n) {
		BigInteger b = BigInteger.ONE;

		for (int i = 2; i < n + 1; i++) {
			b = b.multiply(BigInteger.valueOf(i));
		}

		return b;
	}
}