public void paint(Graphics g){
g.drawLine(......
....
//along with all of my other draw commands
}

boolean drawHello = true;
boolean drawWorld = false;

protected void paintComponent(Graphics g) {
    super.paintCompoent(g);
    if (drawHello)
             g.drawString("Hello", 50, 50);

    if (drawWorld)
             g.drawString("World", 10, 10);
}

public void actionPerformed(ActionEvent e) {
    drawWorld = true;
    repaint();
}

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 javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class AddSquares {

    private int R = 0;
    private int G = 0;
    private int B = 0;
    private int xLoc = 0;
    private int yLoc = 0;

    List<Square> squares = new ArrayList<>();
    private JButton addSquare = new JButton("Add Square");
    private RectsPanel panel = new RectsPanel();

    public AddSquares() {

        addSquare.addActionListener(new ActionListener(){
            @Override
            public void actionPerformed(ActionEvent e) {
                Color color = new Color(R, G, B);
                squares.add(new Square(xLoc, yLoc, color));
                panel.repaint();
                R += 10;
                G += 20;
                B += 30;
                xLoc += 20;
                yLoc += 20;

            }
        });

        JFrame frame = new JFrame("Draw Squares");
        frame.add(panel, BorderLayout.CENTER);
        frame.add(addSquare, BorderLayout.SOUTH);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }

    private class RectsPanel extends JPanel {

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            for (Square square : squares) {
                square.drawSquare(g);
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(250, 250);
        }
    }

    private class Square {
        int x = 0;
        int y = 0;
        Color color;

        public Square(int x, int y, Color color) {
            this.x = x;
            this.y = y;
            this.color = color;
        }

        public void drawSquare(Graphics g) {
            g.setColor(color);
            g.fillRect(x, y, 75 ,75);
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                AddSquares addSquares = new AddSquares();
            }
        });
    }
}

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

class PaintAnyTime {
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                new PaintAnyTime();
            }
        });
    }

    final BufferedImage image = (
        new BufferedImage(500, 500, BufferedImage.TYPE_INT_ARGB)
    );

    final JFrame frame = new JFrame();

    final JLabel label = new JLabel(new ImageIcon(image));

    final MouseAdapter drawer = new MouseAdapter() {
        Graphics2D g2D;

        @Override
        public void mousePressed(MouseEvent me) {
            g2D = image.createGraphics();
            g2D.setColor(Color.BLACK);
        }

        @Override
        public void mouseDragged(MouseEvent me) {
            g2D.fillRect(me.getX(), me.getY(), 3, 3);
            label.repaint();
        }

        @Override
        public void mouseReleased(MouseEvent me) {
            g2D.dispose();
            g2D = null;
        }
    };

    PaintAnyTime() {
        label.setPreferredSize(
            new Dimension(image.getWidth(), image.getHeight())
        );

        label.addMouseListener(drawer);
        label.addMouseMotionListener(drawer);

        frame.add(label);

        frame.pack();
        frame.setResizable(false);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}