First steps

Simple first program
1. choose empty activity
2. we will edit activity_main.xml and MainActivity.java
3. use api level 16
4. Get rid of "hello world" 5. Drag button
6 width = match parent, change background colour
7 changethe id of the button and also the text

8 onclick property, speccify the name of method
9. public void toDo(View v){

                          }

in oncreate
Button buttonblue;
buttonBlue = (Button) findViewById(R.id.button_blueinvisible);
define button in main activity
if ( v.equlas(buttonblue)) then do the stuff
Toast.makeText(getApplicationcontext(), "asdfadsfa", Toast.LENGTH_SHORT).show();

Below is the code from the coursera site

package fr.centralesupelec.galtier.block01;
 
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
 
public class MainActivity extends AppCompatActivity {
 
    // the 2 buttons defined in the activity_main.xml file
    Button buttonBlue, buttonPink;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
 
        // display the GUI defined in the activity_main.xml file
        setContentView(R.layout.activity_main);
 
        // retrieve references to the Views defined in the activity_main.xml
        buttonBlue = (Button) findViewById(R.id.button_blueInvisible);
        buttonPink = (Button) findViewById(R.id.button_pinkPanther);
    }
 
    /**
     * called when a button from activity_main.xml is clicked
     * @param v the View which triggered the method call: should refer to buttonBlue or buttonPink
     */
    public void toDo(View v) {
        if (v.equals(buttonBlue))
            // buttonBlue was clicked
            // turn the button invisible
            v.setVisibility(View.INVISIBLE);
        if (v.equals(buttonPink))
            // buttonPink was clicked
            // display a pop-up message for a short duration
            Toast.makeText(getApplicationContext(),
                    "to do to do to do...",
                    Toast.LENGTH_SHORT).show();
    }
}