android方法/ java中未使用的参数

Question

In java, we use method parameters for a number of reasons but I've never used parameters in my methods unless I needed them

When defining a method in android for a button that I want to click, I had my method defined in XML under the Button element

android:onClick="showText" 

then, in my ProjectName.java file I defined the method

public void showText(View view){  

} 

The View parameter is never used by me.. I have no need for it. By asking questions on Stackoverflow I was told that the View in the parameter is the view that was clicked. Button is a subclass of View as is TextView and since that is the case, that answer sort of makes sense. But the question I have is.... is this the rule for just the onClick method? Also, why can I not pass the Button as a parameter? Button is the actual "View" being clicked, so why not Button?

Furthermore, are there other methods similar to this and if so, do they all follow the rule just like the View parameter, where let's say if I had a class called "A" and it had a subclass "B" and that class had a subclass "C", and if I wanted to use the onClick method, I can only pass the Highest Class as the parameter?

To simplify that, are there ever cases where I would pass anything else OTHER THAN "View" as the parameter to these methods?

Thank you.

Answer 1

It depends on the method we're talking about. As the onClick event is generated in the View level it can only pass you a View . However you can easyly cast it to a Button , you just have to check if it is one.

if (view instanceof Button) Button btn = (Button) view;

In other cases you may have other parameters such as a TextWatcher when you add aa text changed listener to an EditText .

Answer 2

The reason it uses View for the parameter type is because the method setOnClickListener is not actually a member of the Button class, but rather a member of the View class (from which button inherits).

Here it is: http://developer.android.com/reference/android/view/View.html#setOnClickListener(android.view.View.OnClickListener)

As such, the parameter type has to be a View, because it is defined at that level.

This is also useful because you might use the same onClick handler on several different Views. Any View can have an onClick event.