Javaサンプル(ボタンクリックと文字列表示)

//Sample01
import java.awt.Button;
import java.awt.FlowLayout;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
public class Sample01 extends Frame implements ActionListener {
  private static final long serialVersionUID = 1L;
  private static final String window_title = "ボタンクリックで文字列表示";
  private static final int window_width = 640;
  private static final int window_height = 400;
  private static final String b0_title = "1999";
  private static final String b1_title = "2000";
  private static final String b2_title = "2001";
  private static final int Position_X = 100;
  private static final int position_Y = 150;
  int flg_button = -1;
  // 主処理
  public static void main(final String ar[]) {
    final Frame f = new Sample01();
    f.setTitle(Sample01.window_title);
    f.setSize(Sample01.window_width, Sample01.window_height);
    f.setVisible(true);
  }
  // 部品セット
  Sample01() {
    this.setLayout(new FlowLayout());
    final Button b0 = new Button(Sample01.b0_title);
    final Button b1 = new Button(Sample01.b1_title);
    final Button b2 = new Button(Sample01.b2_title);
    b0.addActionListener(this);
    b1.addActionListener(this);
    b2.addActionListener(this);
    this.add(b0);
    this.add(b1);
    this.add(b2);
    this.addWindowListener(new WinAdapter());
  }
  // 閉じる
  class WinAdapter extends WindowAdapter {
    @Override
    public void windowClosing(final WindowEvent we) {
      System.exit(0);
    }
  }
  // 描画
  @Override
  public void paint(final Graphics g) {
    final String s0 = "あああああ";
    final String s1 = "いいいいいいいいい";
    final String s2 = "ううう";
    if (this.flg_button > -1) {
      g.clearRect(0, 0, Sample01.window_width - 1,
          Sample01.window_height - 1);
    }
    if (this.flg_button == 0) {
      g.drawString(s0, Sample01.Position_X, Sample01.position_Y);
    }
    if (this.flg_button == 1) {
      g.drawString(s1, Sample01.Position_X, Sample01.position_Y);
    }
    if (this.flg_button == 2) {
      g.drawString(s2, Sample01.Position_X, Sample01.position_Y);
    }
  }
  // イベント
  public void actionPerformed(final ActionEvent ae) {
    if (ae.getActionCommand() == Sample01.b0_title) {
      this.flg_button = 0;
      this.repaint();
    }
    if (ae.getActionCommand() == Sample01.b1_title) {
      this.flg_button = 1;
      this.repaint();
    }
    if (ae.getActionCommand() == Sample01.b2_title) {
      this.flg_button = 2;
      this.repaint();
    }
  }
}