How to change the font on the TextView?

Question

First, the default is not Arial. The default is Droid Sans.

Second, to change to a different built-in font, use android:typeface in layout XML or setTypeface() in Java.

Third, there is no Helvetica font in Android. The built-in choices are Droid Sans (sans), Droid Sans Mono (monospace), and Droid Serif (serif). While you can bundle your own fonts with your application and use them via setTypeface(), bear in mind that font files are big and, in some cases, require licensing agreements (e.g., Helvetica, a Linotype font).

EDIT

The Android design language relies on traditional typographic tools such as scale, space, rhythm, and alignment with an underlying grid. Successful deployment of these tools is essential to help users quickly understand a screen of information. To support such use of typography, Ice Cream Sandwich introduced a new type family named Roboto, created specifically for the requirements of UI and high-resolution screens.

The current TextView framework offers Roboto in thin, light, regular and bold weights, along with an italic style for each weight. The framework also offers the Roboto Condensed variant in regular and bold weights, along with an italic style for each weight.

After ICS, android includes Roboto fonts style, Read more Roboto

EDIT 2

With the advent of Support Library 26, Android now supports custom fonts by default. You can insert new fonts in res/fonts which can be set to TextViews individually either in XML or programmatically. The default font for the whole application can also be changed by defining it styles.xml The android developer documentation has a clear guide on this here

First download the .ttf file of the font you need (arial.ttf). Place it in the assets folder. (Inside assets folder create new folder named fonts and place it inside it.) Use the following code to apply the font to your TextView:

Typeface type = Typeface.createFromAsset(getAssets(),"fonts/arial.ttf"); 
textView.setTypeface(type);

Typeface tf = Typeface.createFromAsset(getAssets(),
        "fonts/DroidSansFallback.ttf");
TextView tv = (TextView) findViewById(R.id.CustomFontText);
tv.setTypeface(tf);

You might want to create static class which will contain all the fonts. That way, you won't create the font multiple times which might impact badly on performance. Just make sure that you create a sub-folder called "fonts" under "assets" folder.

Do something like:

public class CustomFontsLoader {

public static final int FONT_NAME_1 =   0;
public static final int FONT_NAME_2 =   1;
public static final int FONT_NAME_3 =   2;

private static final int NUM_OF_CUSTOM_FONTS = 3;

private static boolean fontsLoaded = false;

private static Typeface[] fonts = new Typeface[3];

private static String[] fontPath = {
    "fonts/FONT_NAME_1.ttf",
    "fonts/FONT_NAME_2.ttf",
    "fonts/FONT_NAME_3.ttf"
};


/**
 * Returns a loaded custom font based on it's identifier. 
 * 
 * @param context - the current context
 * @param fontIdentifier = the identifier of the requested font
 * 
 * @return Typeface object of the requested font.
 */
public static Typeface getTypeface(Context context, int fontIdentifier) {
    if (!fontsLoaded) {
        loadFonts(context);
    }
    return fonts[fontIdentifier];
}


private static void loadFonts(Context context) {
    for (int i = 0; i < NUM_OF_CUSTOM_FONTS; i++) {
        fonts[i] = Typeface.createFromAsset(context.getAssets(), fontPath[i]);
    }
    fontsLoaded = true;

}
}

This way, you can get the font from everywhere in your application.