Android Parcel.obtain() returns null?

Question

i am just developing an Android Application (API 15) and got the following Problem, when trying to write a unit test:

I use android.os.Parcel for saving a class (e.g. if the screen is turned around) and send it to another Activity. If i try to unit test the Parcel Method like this (i got this from the internet):

@Test
public void testParcel() {
    Comment test = new Comment();
    test.setId(testNr0Id);
    test.setComment(testNr0String);


    // Obtain a Parcel object and write the parcelable object to it:
    Parcel parcel = Parcel.obtain();
    test.writeToParcel(parcel, 0);

    // After you're done with writing, you need to reset the parcel for reading:
    parcel.setDataPosition(0);

    // Reconstruct object from parcel and asserts:
    Comment createdFromParcel = Comment.CREATOR.createFromParcel(parcel);
    assertEquals(test, createdFromParcel);
}

then i get a NullPointerException, because the Parcel returned from Parcel.obtain() is null. The JavaDoc is just saying that this Method "returns an parcel Object from the pool". Now my question is: Which pool? And why is this Parcel-Object null? Any suggestions how to make this test run?

Thanks for your help

Michael

Answer 1

I've had the same issue. However after I moved the test from test directory to androidTest directory everything started working fine. I think this is due to the fact that Parcel class is system one, so it needs to be instrumented as well.

Answer 2

To avoid going to Robolectric or instrumentation altogether, e.g. you want to use JUnit only with no devices attached, you can use Mockito to mock everything inside of the Parcel.

I have a simple implementation and I use it across many of my projects:

// Mockito is required for this to work
class ParcelFake {

  companion object {
    @JvmStatic fun obtain(): Parcel {
      return ParcelFake().mock
    }
  }

  private var position = 0
  private var store = mutableListOf<Any>()
  private var mock = mock<Parcel>()

  init {
    setupWrites()
    setupReads()
    setupOthers()
  }

  // uncomment when needed for the first time
  private fun setupWrites() {
    val answer = { i: InvocationOnMock ->
      with(store) {
        add(i.arguments[0])
        get(lastIndex)
      }
    }
    whenever(mock.writeByte(anyByte())).thenAnswer(answer)
    whenever(mock.writeInt(anyInt())).thenAnswer(answer)
    whenever(mock.writeString(anyString())).thenAnswer(answer)
    // whenever(mock.writeLong(anyLong())).thenAnswer(answer)
    // whenever(mock.writeFloat(anyFloat())).thenAnswer(answer)
    // whenever(mock.writeDouble(anyDouble())).thenAnswer(answer)
  }

  // uncomment when needed for the first time
  private fun setupReads() {
    val answer = { _: InvocationOnMock -> store[position++] }
    whenever(mock.readByte()).thenAnswer(answer)
    whenever(mock.readInt()).thenAnswer(answer)
    whenever(mock.readString()).thenAnswer(answer)
    // whenever(mock.readLong()).thenAnswer(answer)
    // whenever(mock.readFloat()).thenAnswer(answer)
    // whenever(mock.readDouble()).thenAnswer(answer)
  }

  private fun setupOthers() {
    val answer = { i: InvocationOnMock ->
      position = i.arguments[0] as Int
      null
    }
    whenever(mock.setDataPosition(anyInt()))
      .thenAnswer(answer)
  }
}

The basic idea is to have a backing storage (i.e. a list) and push everything into it - but you do need to mock a Parcel internally using Mockito to get this to work. I commented out some data types, but I assume you'll get the idea.

This is a result of reading through docs and trying out various approaches. Any edits are welcome.