Pro Android with Kotlin

xxii
Introduction
final EditText et = findViewById(R.id.edit);
final Button btn = findViewById(R.id.btn);
final TextView text = findViewById(R.id.text);
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
String entered = et.getText().toString();
text.setText("You entered '" + entered +
"' and pressed 'Go'");
}
});
}
}
Here are a few notes about the previous Java code:

The public in front of the class says that it is visible from everywhere.
It cannot be omitted here since otherwise the framework could not use 
the class.

The setContentView() changes something by virtue of the “set,” 
which is such a common construct that you might want to write it 
more concisely as contentView = s.th.instead, even with a variable 
of name "contentView" not actually existing or being private. A couple 
of competitor languages allow for this type of syntax. In Groovy for 
example, you can write contentView = s.th.and the language will 
internally translate it to setContentView().

The final in front of the three declarations is necessary in Java up to 
version 7 because the variables are going to be used in the anonymous 
inner class that comes a little later.

Also, for the setOnClickListener() method, you might want to use 
.onClickListener = s.th. instead. It’s the same for the .setText() a 
little later.

The argument to setOnClick-Listener() is an object of an anonymous 
inner class; it is already an abbreviation of first declaring and then 
instantiating and using it. But you could be even more expressive with 
syntax like btn -> do s.th. or similar, just not in the Java language 
(well, at least not before Java 8).

For the et.getText() method, you could just as well write something 
like et.text, which would express the same thing but is shorter.
A sister project that does the same thing but with Kotlin support is written in the Kotlin 
language as follows:
package de.pspaeth.simplekotlin
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_main.*

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