Question

So I’ve been looking around for android tutorials, help questions, etc.. I keep running into questions or tutorials hard for me to understand.

Here’s my questions:

1. When I create an item in the visual designer, piece of code will be created in the .xml.
   How can I get the ID of that item to use it in the .java file later?

2. How can I add callbacks when let’s say a button gets clicked?

---

Here’s what I have so far:

.java

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.widget.Button;

public class MainActivity extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }

    public void Button_click_callback() // Where to add the callback in the .xml?
    {
        // How to get button ID and change the text of it? 
            //Knowing this will help me A LOT!
    }
}

.xml

<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:layout_marginLeft="79dp"
android:layout_marginTop="32dp"
android:text="Button" />

Answer 1

When I create an item in the visual designer, piece of code will be created in the .xml. How can I get the ID of that item to use it in the .java file later?

Step #1: Ensure that you have assigned an ID for the widget in the designer (in your XML above, you will see this as android:id="@+id/button1)

Step #2: In Java, you can get at the Java object for that widget by calling findViewById(R.id.button1) at some appropriate time (e.g., from an Activity, sometime after you call setContentView()).

How can I add callbacks when let’s say a button gets clicked?

Generally, there is a setter method for this, such as setOnClickListener() that you can call on the Button you retrieved by findViewById().

In the specific case of click events on widgets hosted by activities, there is also an android:onClick attribute you can have in the XML, which supplies the name of a method on your Activity that will get called when the widget is clicked, instead of your having to use the setter.