Java GWT:检查事件处理程序是否存在

根据文本框的字符串值，我使用一个向上键处理程序来添加和删除事件处理程序。我不想在每个键上添加或删除事件处理程序。如何首先检查处理程序是否已经存在

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));  
    } 

}

如果（firstHandler！=null）

将执行此任务，当您删除处理程序时，将其注册为null：

firstHandler.removeHandler();
firstHandler = null;