MockParcel for testing Parcelables implementations with the new Android's Unit testing support (http://tools.android.com/tech-docs/unit-testing-support). Only String and Long read/write implemented here. Add mock methods for other types.

MockParcel.java

import android.os.Parcel;
import java.util.ArrayList;
import java.util.List;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;

import static org.mockito.Matchers.anyInt;
import static org.mockito.Matchers.anyLong;
import static org.mockito.Matchers.anyString;
import static org.mockito.Mockito.doAnswer;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;

public class MockParcel {

    public static Parcel obtain() {
        return new MockParcel().getMockedParcel();
    }

    Parcel mockedParcel;
    int position;
    List<Object> objects;

    public Parcel getMockedParcel() {
        return mockedParcel;
    }

    public MockParcel() {
        mockedParcel = mock(Parcel.class);
        objects = new ArrayList<>();
        setupMock();
    }

    private void setupMock() {
        setupWrites();
        setupReads();
        setupOthers();
    }

    private void setupWrites() {
        Answer<Void> writeValueAnswer = new Answer<Void>() {
            @Override public Void answer(InvocationOnMock invocation) throws Throwable {
                Object parameter = invocation.getArguments()[0];
                objects.add(parameter);
                return null;
            }
        };
        doAnswer(writeValueAnswer).when(mockedParcel).writeLong(anyLong());
        doAnswer(writeValueAnswer).when(mockedParcel).writeString(anyString());
    }

    private void setupReads() {
        when(mockedParcel.readLong()).thenAnswer(new Answer<Long>() {
            @Override public Long answer(InvocationOnMock invocation) throws Throwable {
                return (Long) objects.get(position++);
            }
        });
        when(mockedParcel.readString()).thenAnswer(new Answer<String>() {
            @Override public String answer(InvocationOnMock invocation) throws Throwable {
                return (String) objects.get(position++);
            }
        });
    }

    private void setupOthers() {
        doAnswer(new Answer<Void>() {
            @Override public Void answer(InvocationOnMock invocation) throws Throwable {
                position = ((Integer) invocation.getArguments()[0]);
                return null;
            }
        }).when(mockedParcel).setDataPosition(anyInt());
    }
}

this does not work if Parcelable class has nullable String field

I made some edits to this approach (and also converted it to Kotlin) - you can check it out here: https://gist.github.com/milosmns/7f6448a3602595948449d3bfaff9b005
It's a good approach overall, I've been able to test everything I needed using this.

In Mockito 2, you can switch to the nullable matcher to make this match null strings as well:

doAnswer(answer).when(mParcel).writeString(nullable(String.class));

anyString() used to also match null, but it doesn't anymore: mockito/mockito#185

Also works for nullable Parcelables.