CODEWITHSUNDEEP
javaandroidmethodsparameters
Space Ghost
Space Ghost

Reputation: 765

Why does an action for a view (Button in this case) have to take a View as a parameter?

I'm a newbie with android development, but not really with Java.

What I don't understand is why a method I define in the Button must to take View as a paremeter.

For example..

in XML 

<Button
        android:id="@+id/button1"
        //etc..
        //etc..
        android:onClick="displayText" />

As can be seen, this is the onClick method. When I'm writing Java code the method displayText will be defined like so

public void displayText(View view){
        TextView myTextView = (TextView)findViewById(R.id.textView1);
        myTextView.setVisibility(View.VISIBLE);
    }

When everything works fine, the text I have originally hidden, will be displayed via a simple click of a button.

When I remove the parameters View view from this method, the app crashes when I try to click the button.

I fail to understand why.

I'm not necessarily doing anything with the view in the parameter. In non android development I could use the view that is in the parameter and do things with it.. but I don't see that I'm doing anything WITH this parameter.. so why is it required in this method?

Upvotes: 0

Views: 90

Answers (3)

Space Ghost
Space Ghost

Reputation: 765

I have searched Stackoverflow but never found a question that suited what I was looking for so I made this question.

The two answers given here we're a little too short for me to wrap my head around a method having parameters but never using them.

Then I looked at the "Related" section on the side and found a question better worded than mine and answers that were longer and satisfied my question and answered it in more detail.

Thank's to all those that responded here. If anyone else finds them selves in the same problem, here is the question/answer that helped me understand it.

Why do android methods accept View objects, but the objects are unused?

Upvotes: 0

frozenkoi
frozenkoi

Reputation: 3248

Potentially, the same method could be used to handle several views' click event. The View view parameter would let you know which one is the source of the event. I.e. which button was clicked.

The reason it is required it's because it has to match the interface. When trying to match the function name in the XML to the functions available, it can't be found.

Upvotes: 2

wvdz
wvdz

Reputation: 16651

A Button is a subclass of a View. So when methods take a View, it means they can be get a Button as input, but also other subclasses of View.

The View is given in the parameter, so you can modify the View that was clicked.

BTW, I didn't know you could implement click listeners like this. I normally do it like this:

button.setOnClickListener(new OnClickListener(){
    @Override
    public void onClick(View view){
        // handle
    }
});

Upvotes: 1

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