首先下载所需字体的.ttf文件(arial.ttf)。把它放在资产文件夹中。(在assets文件夹内创建名为fonts的新文件夹，并将其放置在其中。)使用以下代码应用字体到你的TextView:

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

也许可以简单一点:

public class Fonts {
  public static HashSet<String,Typeface> fonts = new HashSet<>();

  public static Typeface get(Context context, String file) {
    if (! fonts.contains(file)) {
      synchronized (this) {
        Typeface typeface = Typeface.createFromAsset(context.getAssets(), name);
        fonts.put(name, typeface);
      }
    }
    return fonts.get(file);
  }
}

// Usage
Typeface myFont = Fonts.get("arial.ttf");

(请注意，此代码未经测试，但通常这种方法应该工作良好。)

最佳实践是使用Android支持库26.0.0或更高版本。

步骤1:添加字体文件

在res文件夹中创建新的字体资源字典 添加字体文件(.ttf， .orf)

例如，当字体文件为helvetica_neue.ttf时，将生成R.font.helvetica_neue

步骤2:创建字体族

在字体文件夹中添加新的资源文件 在元素中包含每个字体文件、样式和权重属性。

例如:

<?xml version="1.0" encoding="utf-8"?>
<font-family xmlns:android="http://schemas.android.com/apk/res/android">
    <font
        android:fontStyle="normal"
        android:fontWeight="400"
        android:font="@font/helvetica_neue" />
</font-family>

第三步:使用它

在xml布局中:

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:fontFamily="@font/my_font"/>

或添加字体样式:

<style name="customfontstyle" parent="@android:style/TextAppearance.Small">
    <item name="android:fontFamily">@font/lobster</item>
</style>

更多的例子你可以参考文档:

使用字体

我终于找到了一个非常简单的解决方法。

use these Support libraries in app level gradle, compile 'com.android.support:appcompat-v7:26.0.2' compile 'com.android.support:support-v4:26.0.2' then create a directory named "font" inside the res folder put fonts(ttf) files in that font directory, keep in mind the naming conventions [e.g.name should not contain any special character, any uppercase character and any space or tab] After that, reference that font from xml like this <Button android:id="@+id/btn_choose_employee" android:layout_width="140dp" android:layout_height="40dp" android:layout_centerInParent="true" android:background="@drawable/rounded_red_btn" android:onClick="btnEmployeeClickedAction" android:text="@string/searching_jobs" android:textAllCaps="false" android:textColor="@color/white" android:fontFamily="@font/times_new_roman_test" />

在本例中，times_new_roman_test是该字体目录中的字体ttf文件

首先，默认不是Arial。默认为Droid Sans。

其次，要更改为不同的内置字体，请在布局XML中使用android:typeface或在Java中使用setTypeface()。

第三，Android中没有Helvetica字体。内置选项有Droid Sans (Sans)、Droid Sans Mono (monospace)和Droid Serif (Serif)。虽然您可以将自己的字体与应用程序捆绑在一起，并通过setTypeface()使用它们，但请记住，字体文件很大，在某些情况下需要许可协议(例如Helvetica，一种Linotype字体)。

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.

ICS之后，android包含了Roboto字体样式， 阅读更多Roboto

编辑2

随着支持库26的出现，Android现在支持自定义字体 违约。您可以在res/fonts中插入新的字体，这些字体可以单独以XML或编程方式设置为TextViews。整个应用程序的默认字体也可以通过定义它styles.xml来改变，android开发者文档对此有明确的指导

import java.lang.ref.WeakReference;
import java.util.HashMap;

import android.content.Context;
import android.graphics.Typeface;

public class FontsManager {

    private static FontsManager instance;

    private static HashMap<String, WeakReference<Typeface>> typefaces = new HashMap<String, WeakReference<Typeface>>();

    private static Context context;

    private FontsManager(final Context ctx) {
        if (context == null) {
            context = ctx;
        }
    }

    public static FontsManager getInstance(final Context appContext) {
        if (instance == null) {
            instance = new FontsManager(appContext);
        }
        return instance;
    }

    public static FontsManager getInstance() {
        if (instance == null) {
            throw new RuntimeException(
                    "Call getInstance(Context context) at least once to init the singleton properly");
        }
        return instance;
    }

    public Typeface getFont(final String assetName) {
        final WeakReference<Typeface> tfReference = typefaces.get(assetName);
        if (tfReference == null || tfReference.get() == null) {
            final Typeface tf = Typeface.createFromAsset(context.getResources().getAssets(),
                    assetName);
            typefaces.put(assetName, new WeakReference<Typeface>(tf));
            return tf;
        }
        return tfReference.get();
    }

}

通过这种方式，您可以创建一个继承自TextView的视图，并在其构造函数上调用setTypeface。