Why is `robot.keyRelease(KeyEvent.VK_CONTROL)` necessary?

Whe开发者_StackOverflow中文版n using Robot class, what is the meaning of:

robot.keyRelease(KeyEvent.VK_CONTROL);

Shouldn't the code below be sufficient to send the event?

Robot robot = new Robot();
robot.keyPress(KeyEvent.VK_CONTROL);

keyPress will send an event that a key has been pressed down. keyRelease will send the event that the key has been released. If you want to simulate typing, you might want to do something like:

public class SuperRobot extends Robot {
    public void typeKey(int keyCode) {
        keyPress(keyCode);
        delay(20);
        keyRelease(keyCode);
    }
}

public static void main(String[] args) {
    try {
        SuperRobot r = new SuperRobot();
        // Now, let's press Ctrl+A
        r.keyPress(KeyEvent.VK_CONTROL);
        r.typeKey(KeyEvent.VK_A);
        r.keyRelease(KeyEvent.VK_CONTROL);
    } catch (Exception ex) { // Either AWTException or SecurityException
        System.out.println("Oh no!");
    }
}

Note that to type something with a mask, like Ctrl+A, we first press down Ctrl, then simulate pressing and releasing A, then release Ctrl. As a general rule, the robot should more-or-less exactly simulate what you as a user would do.

robot.keyRelease(KeyEvent.VK_CONTROL); 

For Releasing the press effect on the key, If you have pressed a key using robot.keyPress(KeyEvent.VK_CONTROL); Then you should Release it too, otherwise once your java application runs, your keyboard has will continue with virtually CTRL key pressed.