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I'd like to create an animation of a few objects ("Spaceships") on the screen, with a start button. This is what I have so far:

public class SpaceGame extends JPanel implements ActionListener 
 //The list of spaceships that should be painted
 LinkedList<Spaceship> playingList = new LinkedList<Spaceship>(); 
 Timer t = new Timer(5, this);

 @Override
 public void actionPerformed(ActionEvent e) 
 ListIterator<Spaceship> iter = playingList.listIterator();
 while (iter.hasNext()) 
 Spaceship s = iter.next();
 s.moveSpaceship();

 
 repaint();
 

 public void paintComponent(Graphics g) 
 super.paintComponent(g);

 for (Spaceship s : playingList)
 s.drawSpaceship(g);
 t.start();
 

 public static void main(String args) 
 SpaceGame game = new SpaceGame();
 JFrame fr = new JFrame();
 fr.setTitle("SPACE GAME");
 fr.setSize(990,690);
 fr.add(game);

 game.playingList .add(new Spaceship(3, 0, 570));
 game.playingList .add(new Spaceship(1, 250, 570));
 game.playingList .add(new Spaceship(2, 500, 570));


 JButton start = new JButton("START");
 start.addActionListener(game);
 fr.add(start,BorderLayout.SOUTH);

 fr.setVisible(true);
 fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

 

where:

import java.awt.Graphics;

public class Spaceship 
 int initialSpeed;
 int locX, locY; //current location 

 public Spaceship(int initalSpeed, int initX, int initY) 
 this.initialSpeed = initalSpeed;
 locX = initX;
 locY = initY;
 

 public void drawSpaceship(Graphics g) 
 g.setColor(Color.GREEN);
 g.fillOval(locX, locY, 100, 40);
 

 public void moveSpaceship() 
 locY -= initialSpeed;
 

Of cousre the idea is that pressing the button would trigger the animation. The problem is that the animation would start automaticaly without the start button being pressed, and then when it ends, the button has no effect. How can I fix this?

Let's start with the glaringly obvious...

public void paintComponent(Graphics g) 
 super.paintComponent(g);

 for (Spaceship s : playingList)
 s.drawSpaceship(g);
 t.start();

Call t.start inside paintComponent is a very, very, very bad idea. You do not control the paint process (ie when paintComponent gets called), so it might be called at any time for any number of reasons, often in quick succession.

You should have a look at Painting in AWT and Swing for more details about how painting works in Swing

Painting should simply paint the current state of the component and nothing else.

The second problem is the fact that both the Timer and the button are calling the same actionPerformed method, which doesn't really make sense. In fact, in a perfect world, you wouldn't implement ActionListener directly like this and instead make use of Anonymous Classes which would guard against outside classes calling the method directly or indirectly.

So, what's the solution? Add a method to your SpaceGame which can be called to start the animation, something like...

public class SpaceGame extends JPanel implements ActionListener 
 //The list of spaceships that should be painted

 LinkedList<Spaceship> playingList = new LinkedList<Spaceship>();
 Timer t = new Timer(5, this);

 public void start() 
 if (t.isRunning()) 
 return;
 
 t.start();
 

 @Override
 public void actionPerformed(ActionEvent e) 
 ListIterator<Spaceship> iter = playingList.listIterator();
 while (iter.hasNext()) 
 Spaceship s = iter.next();
 s.moveSpaceship();

 
 repaint();
 

 public void paintComponent(Graphics g) 
 super.paintComponent(g);

 for (Spaceship s : playingList) 
 s.drawSpaceship(g);
 
 

 public static void main(String args) 
 EventQueue.invokeLater(new Runnable() 
 @Override
 public void run() 
 SpaceGame game = new SpaceGame();
 JFrame fr = new JFrame();
 fr.setTitle("SPACE GAME");
 // This is unadvisable :/
 fr.setSize(990, 690);
 fr.add(game);

 game.playingList.add(new Spaceship(3, 0, 570));
 game.playingList.add(new Spaceship(1, 250, 570));
 game.playingList.add(new Spaceship(2, 500, 570));

 JButton start = new JButton("START");
 start.addActionListener(new ActionListener() 
 @Override
 public void actionPerformed(java.awt.event.ActionEvent e) 
 game.start();
 
 );
 fr.add(start, BorderLayout.SOUTH);

 fr.setVisible(true);
 fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
 );
 

Then update your main method to call it...

public static void main(String args) 
 EventQueue.invokeLater(new Runnable() 
 @Override
 public void run() 
 SpaceGame game = new SpaceGame();
 JFrame fr = new JFrame();
 fr.setTitle("SPACE GAME");
 // This is unadvisable :/
 fr.setSize(990, 690);
 fr.add(game);

 game.playingList.add(new Spaceship(3, 0, 570));
 game.playingList.add(new Spaceship(1, 250, 570));
 game.playingList.add(new Spaceship(2, 500, 570));

 JButton start = new JButton("START");
 start.addActionListener(new ActionListener() 
 @Override
 public void actionPerformed(java.awt.event.ActionEvent e) 
 game.start();
 
 );
 fr.add(start, BorderLayout.SOUTH);

 fr.setVisible(true);
 fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
 );