how to know what a button does in android xml code example

Example 1: android onlcik java

Button button = (Button) findViewById(R.id.button_send);
button.setOnClickListener(new View.OnClickListener() {
    public void onClick(View v) {
        // Do something in response to button click
    }
});
//https://developer.android.com/guide/topics/ui/controls/button

Example 2: button android stdui

Button b = findById();