add info about how to return default values from android.jar bug: 30757670 am: abfa7e2c

returned result against the expected result.</p>

<h3 id="mocking-dependencies">Mock Android dependencies</h3>

<p>

By default, the <a href="{@docRoot}tools/building/plugin-for-gradle.html">

Android Plug-in for Gradle</a> executes your local unit tests against a modified

version of the {@code android.jar} library, which does not contain any actual code. Instead, method

calls to Android classes from your unit test throw an exception.

</p>

<p>By default, the <a href=

"{@docRoot}tools/building/plugin-for-gradle.html">Android Plug-in for

Gradle</a> executes your local unit tests against a modified version of the

{@code android.jar} library, which does not contain any actual code. Instead,

method calls to Android classes from your unit test throw an exception. This is

to make sure you test only your code and do not depend on any

particular behavior of the Android platform (that you have not explicitly

mocked).</p>

<p>

You can use a mocking framework to stub out external dependencies in your code, to easily test that

your component interacts with a dependency in an expected way. By substituting Android dependencies

class="external-link">sample code</a>.

</p>

<p>If the exceptions thrown by Android APIs in the

<code>android.jar</code> are problematic for your tests, you can change the behavior so that methods

instead return either null or zero by adding the following configuration in your project's

top-level <code>build.gradle</code> file:</p>

<pre>

android {

  ...

  testOptions {

    unitTests.returnDefaultValues = true

  }

}

</pre>

<p class="caution"><strong>Caution:</strong>

Setting the <code>returnDefaultValues</code> property to <code>true</code>

should be done with care. The null/zero return values can introduce

regressions in your tests, which are hard to debug and might allow failing tests

to pass. Only use it as a last resort.</p>

<h2 id="run">Run Local Unit Tests</h2>