GWT: Checking if event handler exists

I'm using a key up handler to add and remove event handlers depending on the string value of a text box. I wouldn't want to add or remove an event handler on every key up. 开发者_如何学PythonHow do I first check if a handler already exists?

HandlerRegistration firstHandler = null;
HandlerRegistration secondHandler = null;

public void onKeyUp(KeyUpEvent event) {
    if (countSpaceChar(textBox.getText()) == 0) {
        // code to check if MyFirstHandler is already attached?
        firstHandler = textBox.addKeyUpHandler(new MyFirstHandler(this)); 
    } if (countSpaceChar(textBox.getText()) == 1) {
        firstHandler.removeHandler();
        // code to check if MySecondHandler is already attached?
        secondHandler = textBox.addKeyUpHandler(new MySecondHandler(this));  
    } 

}

if (firstHandler != null) will do the job, and when you remove handler, null it registration:

firstHandler.removeHandler();
firstHandler = null;