Moving objects and timers

“I want the object to randomly pop up from the bottom of the screen reach a certain ascent and then fall back down”

See the runnable example below. What I do is pass a radomDelayedStart to the Shape. Every tick of the timer, the randomDelayedStart decreases til it reaches 0, that’s when the flag to be drawn in raised. Most of the logic is in the Shape class methods, which are called in the Timers Actionlistener. Everything is done in one Timer. For the ascent, I just used a hard coded 50, but you can also pass a random ascent to the Shape. Let me know if you have any questions. I tried to made the code as clear as possible.

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class RandomShape extends JPanel {

    private static final int D_HEIGHT = 500;
    private static final int D_WIDTH = 400;
    private static final int INCREMENT = 8;
    private List<Shape> shapes;
    private List<Color> colors;
    private Timer timer = null;

    public RandomShape() {
        colors = createColorList();
        shapes = createShapeList();

        timer = new Timer(30, new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                for (Shape shape : shapes) {
                    shape.move();
                    shape.decreaseDelay();
                    repaint();
                }
            }
        });
        JButton start = new JButton("Start");
        start.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e) {
                timer.start();
            }
        });
        JButton reset = new JButton("Reset");
        reset.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e) {
                shapes = createShapeList();
                timer.restart();
            }
        });

        JPanel panel = new JPanel();
        panel.add(start);
        panel.add(reset);
        setLayout(new BorderLayout());
        add(panel, BorderLayout.PAGE_START);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        for (Shape shape : shapes) {
            shape.drawShape(g);
        }
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(D_WIDTH, D_HEIGHT);
    }

    private List<Color> createColorList() {
        List<Color> list = new ArrayList<>();
        list.add(Color.BLUE);
        list.add(Color.GREEN);
        list.add(Color.ORANGE);
        list.add(Color.MAGENTA);
        list.add(Color.CYAN);
        list.add(Color.PINK);
        return list;
    }

    private List<Shape> createShapeList() {
        List<Shape> list = new ArrayList<>();
        Random random = new Random();
        for (int i = 0; i < 20; i++) {
            int randXLoc = random.nextInt(D_WIDTH);
            int randomDelayedStart = random.nextInt(100);
            int colorIndex = random.nextInt(colors.size());
            Color color = colors.get(colorIndex);
            list.add(new Shape(randXLoc, randomDelayedStart, color));
        }

        return list;
    }

    class Shape {

        int randXLoc;
        int y = D_HEIGHT;
        int randomDelayedStart;
        boolean draw = false;
        boolean down = false;
        Color color;

        public Shape(int randXLoc, int randomDelayedStart, Color color) {
            this.randXLoc = randXLoc;
            this.randomDelayedStart = randomDelayedStart;
            this.color = color;
        }

        public void drawShape(Graphics g) {
            if (draw) {
                g.setColor(color);
                g.fillOval(randXLoc, y, 30, 30);
            }
        }

        public void move() {
            if (draw) {
                if (y <= 50) {
                    down = true;
                }

                if (down) {
                    y += INCREMENT;
                } else {
                    y -= INCREMENT;
                }
            }
        }

        public void decreaseDelay() {
            if (randomDelayedStart <= 0) {
                draw = true;
            } else {
                randomDelayedStart -= 1;
            }
        }

    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new JFrame();
                frame.add(new RandomShape());
                frame.pack();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
}