Develping On Android Hello World

This is a series of post as I will hopefully build a simple wallpaper app.

When creating a new application using Android Studio, you will immediately be given a “Hello World!” boilerplate. But how can you extend it?

But firstly, let’s have a quick look at the different files which build up an Android app.

`res/layouts`

I like to think of layouts as your HTML code. It tells you where everything sits. Within Android studio, there is a simple point and click template which allows you to easily and quickly generate something to your liking.

Point and click layouts

The .xml file will then automatically be generated to reflect any changes you make. So what controls the “Hello World!” text?

This line within the TextView does it:

android:text="@string/hello_world"

`res/values`

This is where your variables are listed for your layouts. See within strings.xml you have the resulting “Hello World!” string here:

<string name="hello_world">Hello world!</string>

But its always more interesting to be able to change these things within code. Lets look at how to do that next.

Code

To be able to change the text within .java code, simply reference the field that you wish to change and the use the method .setText(). We then control when and how the text is to be changed. Does it change:

When you first load up the application?
When you change something in settings?
When you click on something?

There are many aspects which we have to think about when and how we should change text. But for starters, lets just change it onCreate. You could see the onCreate method within your .java file. Simply add:

TextView changeText = (TextView) findViewById(R.id.???);
changeText.setText("Hello Again");

But wait! what is the id which is referenced here?

If we go back to the .xml file within res/layout, we should add:

android:id="@+id/YOUR_ID"

Then when you run the emulator you should be able to see the text updated to “Hello Again”.

Finally

Your files should look something like the ones below:

res/layout:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context=".HelloActivity">

    <TextView
        android:id="@+id/hello_world"
        android:text="@string/hello_world"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</RelativeLayout>

src:

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;


public class HelloActivity extends ActionBarActivity {

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


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.hello, menu);
        
        TextView changeText = (TextView) findViewById(R.id.hello_world);
        changeText.setText("Chapman!");
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

Changing Fields to be Editable

To simply change text to be editable, all we have to do is change the “TextView” to EditText" in the .xml file. Then in our code, instead of declaring a “TextView” variable, we use an “EditText” variable, setting the TextView.BufferType.EDITABLE, i.e. :

    <EditText
        android:id="@+id/hello_world"
        android:text="@string/hello_world"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
    />

and

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_simple_text);
        EditText editText = (EditText) findViewById(R.id.hello_world);
        editText.setText("Hello World...", TextView.BufferType.EDITABLE);
    }