用Java编写窗体应用实现拉橡皮筋的手法绘图，能绘制直线、椭圆、矩形。通过3个按钮选择绘制的图形类型，通过鼠标按下确定起点，通过拖动进行绘制，释放鼠标完成绘制。

好的，下面是一个简单的Java窗体应用程序，可以实现你所描述的功能：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class DrawingApp extends JFrame implements MouseListener, MouseMotionListener {
    private int startX, startY, endX, endY;
    private boolean drawing;
    private int shapeType;
    private JPanel drawPanel;

    public DrawingApp() {
        super("Drawing App");
        setSize(600, 400);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        drawPanel = new JPanel();
        drawPanel.setBackground(Color.WHITE);
        drawPanel.addMouseListener(this);
        drawPanel.addMouseMotionListener(this);
        add(drawPanel, BorderLayout.CENTER);

        JPanel buttonPanel = new JPanel();
        JButton lineButton = new JButton("Line");
        JButton ovalButton = new JButton("Oval");
        JButton rectButton = new JButton("Rectangle");
        lineButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                shapeType = 0;
            }
        });
        ovalButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                shapeType = 1;
            }
        });
        rectButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                shapeType = 2;
            }
        });
        buttonPanel.add(lineButton);
        buttonPanel.add(ovalButton);
        buttonPanel.add(rectButton);
        add(buttonPanel, BorderLayout.NORTH);

        setVisible(true);
    }

    public void mousePressed(MouseEvent e) {
        startX = e.getX();
        startY = e.getY();
        drawing = true;
    }

    public void mouseDragged(MouseEvent e) {
        endX = e.getX();
        endY = e.getY();
        drawPanel.repaint();
    }

    public void mouseReleased(MouseEvent e) {
        endX = e.getX();
        endY = e.getY();
        drawing = false;
        drawPanel.repaint();
    }

    public void mouseMoved(MouseEvent e) {}
    public void mouseClicked(MouseEvent e) {}
    public void mouseEntered(MouseEvent e) {}
    public void mouseExited(MouseEvent e) {}

    public void paint(Graphics g) {
        super.paint(g);
        if (drawing) {
            g.setColor(Color.BLACK);
            switch (shapeType) {
                case 0:
                    g.drawLine(startX, startY, endX, endY);
                    break;
                case 1:
                    g.drawOval(Math.min(startX, endX), Math.min(startY, endY), Math.abs(endX - startX), Math.abs(endY - startY));
                    break;
                case 2:
                    g.drawRect(Math.min(startX, endX), Math.min(startY, endY), Math.abs(endX - startX), Math.abs(endY - startY));
                    break;
            }
        }
    }

    public static void main(String[] args) {
        new DrawingApp();
    }
}

这个程序创建了一个窗体应用，包含一个画布和三个按钮用于选择要绘制的图形类型。当用户按下鼠标时，记录起始点坐标，并将“drawing”标记设置为true；当用户拖动鼠标时，计算当前图形的大小和位置，并实时绘制到画布上；当用户释放鼠标时，完成图形的绘制，将“drawing”标记设置为false。程序还重写了paint()方法，用于在画布上绘制当前正在绘制的图形。

需要注意的是，这个程序只是一个简单的示例，还有很多细节问题需要处理，比如如何实现拉橡皮筋效果、如何处理多个图形绘制等等，你可以根据自己的需求进行修改和完善。