The simplebutton (src/main/java/com/pi4j/example/components) is a template class, that you can use in your own Java-project. You can take any Button you want to. Like for example this one: Arcade Button The Template Class gives you the option to check the state of the button, and to create simple events if the button is pressed or depressed.
A simple example on how to use the Button-Class from the Hardware-Catalog :
// Initialize the button component
final var button = new SimpleButton(pi4j, 26, Boolean.TRUE);
// Register event handlers to print a message when pressed (onDown) and depressed (onUp)
button.onDown(() -> System.out.println("Pressing the Button"));
button.onUp(() -> System.out.println("Stopped pressing."));
button.whilePressed(10000, () -> System.out.println("Button is pressed."));
// Wait for 15 seconds while handling events before exiting
System.out.println("Press the button to see it in action!");
sleep(15000);
// Unregister all event handlers to exit this application in a clean way
button.onDown(null);
button.onUp(null);
button.whilePressed(0, null);
The class is implemented in the two sample projects Theremin and Potobooth.