- Beeping (using JComponentWithEvents)
// BeepingDemo.java // Demonstrates the timerFired event in JComponentWithEvents.
// To compile and run this Java file, you must: // 1. Download current JComponentsWithEvents.class file from here: // http://www.kosbie.net/cmu/JComponentWithEvents/ // and place it in the same directory as your Java program. // 2. Include a main method that calls the "launch" method // defined in JComponentWithEvents (see below).
import javax.swing.*; import java.awt.*;
class BeepingDemo extends JComponentWithEvents { // This method is called at regular intervals by the timer. public void timerFired() { beep(); }
public void paint(Graphics2D page) { page.fillRect(50, 50, 50, 50); }// Standard main method: public static void main(String[] args) { launch(500, 400); } } - Animation (with
an instance variable)
// AnimationDemo.java // Demonstrates the timerFired event in JComponentWithEvents, // and using an instance variable to retain data between method calls // (to the event handlers and the paint method).
import javax.swing.*; import java.awt.*;
class AnimationDemo extends JComponentWithEvents {// this instance variable persists between method calls! private int rectX = 50;
public void timerFired() { rectX += 5; }public void paint(Graphics2D page) { page.fillRect(rectX, 50, 50, 50); }// Standard main method: public static void main(String[] args) { launch(500, 400); } } - Smoother Animation (with
setTimerDelay in start method)
// Add this method to your Animation program // This method is called exactly once at the start of the program. public void start() { int delayInMilliseconds = 1; setTimerDelay(delayInMilliseconds); }
// MousePressedDemo.java // Demonstrates the mousePressed event in JComponentWithEvents.
import javax.swing.*; import java.awt.*;
class MousePressedDemo extends JComponentWithEvents {
// these instance variables persists between method calls! private int rectX = 50; private int rectY = 50;
// This method is called each time the mouse is pressed. // The parameters contain the (x,y) location of the mouse press. public void mousePressed(int x, int y) { rectX = x; rectY = y; }
public void paint(Graphics2D page) {
page.fillRect(rectX, rectY, 50, 50);
}
// Standard main method:
public static void main(String[] args) { launch(500, 400); }
}
// KeyPressedDemo.java // Demonstrates the mousePressed event in JComponentWithEvents.
import javax.swing.*; import java.awt.*;
class KeyPressedDemo extends JComponentWithEvents {
// these instance variables persists between method calls! private int rectX = 50; private int rectY = 50;
// This method is called each time a key is pressed. // The parameter contains the key that was pressed. // Note that you can use the constants UP, DOWN, LEFT, and RIGHT // to represent the arrow keys. public void keyPressed(char key) { if (key == RIGHT) rectX += 10; else if (key == 'l') // We'll take 'l' to mean "left" edge of the window rectX = 0; }
public void paint(Graphics2D page) {
page.fillRect(rectX, rectY, 50, 50);
}
// Standard main method:
public static void main(String[] args) { launch(500, 400); }
}
- Pausing
// This demo shows how to pause and unpause an // animation using the 'p' key import javax.swing.*; import java.awt.*; class PausingDemo extends JComponentWithEvents { private boolean isPaused = false; private int counter = 0; public void timerFired() { if (isPaused == true) // if we are paused, do nothing, just return immediately! return; beep(); counter++; } public void keyPressed(char key) { if (key == 'p') // turn on or off pausing with the 'p' key isPaused = !isPaused; } public void paint(Graphics2D page) { page.setFont(new Font("Arial", Font.BOLD, 16)); page.drawString("Pause/unpause beeping with 'p' key", 50, 50); page.drawString("Calls to timerFired: " + counter, 50, 100); } public static void main(String[] args) { launch(400, 150);} } - Simulating
TimerFired Events While Paused
// This demo shows how to simulate timer events // using the 't' key while an animation is paused // (using the 'p' key to pause and unpause). // This is VERY helpful for debugging! // To fully understand this code, compare it to the // previous example. In particular, be sure to understand // why we wrote the onTimerFired helper method. import javax.swing.*; import java.awt.*; class SimulatedTimerEventsDemo extends JComponentWithEvents { private boolean isPaused = false; private int counter = 0; // This helper method is called by timerFired when it is not // paused, and also by keyPressed to simulate a timerEvent. public void onTimerFired() { beep(); counter++; } public void timerFired() { if (isPaused == true) // if we are paused, do nothing, just return immediately! return; onTimerFired(); } public void keyPressed(char key) { if (key == 'p') // turn on or off pausing with the 'p' key isPaused = !isPaused; else if (key == 't') // simulate a timer fired event when user presses 't' onTimerFired(); } public void paint(Graphics2D page) { page.setFont(new Font("Arial", Font.BOLD, 16)); page.drawString("Pause/unpause beeping with 'p' key", 50, 50); page.drawString("Paused = " + isPaused, 50, 100); page.drawString("Calls to timerFired: " + counter, 50, 150); } public static void main(String[] args) { launch(400, 200);} }
- Do Not Set
Instance Variables in Paint Methods!
This Program:import javax.swing.*; import java.awt.*; class MyGraphics extends JComponentWithEvents { private int rectX = 0; public void timerFired() { rectX += 30; } public void paint(Graphics2D page) { // This is meant to "wraparound" the rectangle. // The code is correct, but it's in the wrong place. // It belongs in the timerFired method, not here!!! if (rectX > getWidth()-200) rectX = 0; // this line, in particular, is the problem! page.fillRect(rectX, 50, 200, 50); } public static void main(String[] args) { launch(500, 300);} }Works briefly, then generates this output:
Rules:
* Paint methods only GET instance variables.
* Event handlers (timerFired, keyPressed, mousePressed) can GET OR SET instance variables.
See EventsPractice.html
EventsDemo.html