Java - repetition in variable declarations

Question

I'm not sure what this is even called, so it has been very difficult to search for an answer.

I am currently learning Java and Android development and have noticed a pattern where the type of object or variable being declared is repeated multiple times and I don't understand why.

For example:

    EditText editText = (EditText) findViewById(R.id.edit_message);

Why does it have to be declared a type of EditText both before and after the assignment operator '='? Why isn't once enough? Why is this necessary?

I come from a ruby background and am having trouble wrapping my head around this.

It's called a cast

awksp
– awksp

2014-05-26 17:17:42 +00:00
Commented May 26, 2014 at 17:17 — awksp
– awksp, Commented May 26, 2014 at 17:17
java is indeed very verbose when it comes to typing.

njzk2
– njzk2

2014-05-26 17:19:29 +00:00
Commented May 26, 2014 at 17:19 — njzk2
– njzk2, Commented May 26, 2014 at 17:19

AmmarCSE · Accepted Answer · 2014-05-26 17:41:55Z

3

This is a result of something called Type Casting

EditText inherits from type View.

Since you want a variable of type EditText, but findViewById returns an object of type View, you have to cast the result of findViewById to EditText

To illustrate:

View uncasted = findViewById(R.id.edit_message);
EditText casted = (EditText)uncasted;
                  //casting happening on right side above

edited May 26, 2014 at 17:41

answered May 26, 2014 at 17:20

AmmarCSE

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GameDroids · Accepted Answer · 2014-05-26 17:27:22Z

The findViewById(...) is a method which will return a View Object (in your case a view called edit_message). Java however sees that you want an EditText Object on the one side

 EditText editText = ...

and that a View is returned on the other side

... = findViewById(R.id.edit_message);

Java can't be sure that EditText is compatible with View (But you know that EditText is compatible with View because EditText is a child of the class View).

so to convince Java that your "edit_message" is actually an EditText Object, you have to cast it explicitly by adding (EditText) before the returning method findViewById(R.id.edit_message);

Afterwards your variable editText is a full blown EditText Object even when it came from a View.

Raghunandan · Accepted Answer · 2014-05-26 17:28:23Z

1

If you look at the source code.

1883    public View findViewById(int id) {
1884        return getWindow().findViewById(id);
1885    }
1886

The return type is View. Since its a EditText object you cast it to EditText.

 EditText editText = (EditText) findViewById(R.id.edit_message);
 // this (EditText) is explicit casting

edited May 26, 2014 at 17:28

answered May 26, 2014 at 17:17

Raghunandan

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