Simulate File in Java

I've found that Mockito and Powermock are a good combination for this. Actually there's a blog post with a sample, where the File-class's constructor is mocked for testing purposes. Here's also a small example I threw together:

public class ClassToTest
{
    public void openFile(String fileName)
    {
        File f = new File(fileName);
        if(!f.exists())
        {
            throw new RuntimeException("File not found!");
        }
    }
}

Testing with Mockito + Powermock:

@RunWith(PowerMockRunner.class)
@PrepareForTest(ClassToTest.class)
public class FileTest
{
    @Test
    public void testFile() throws Exception
    {
        //Set up a mocked File-object
        File mockedFile = Mockito.mock(File.class);
        Mockito.when(mockedFile.exists()).thenReturn(true);

        //Trap constructor calls to return the mocked File-object
        PowerMockito.whenNew(File.class).withParameterTypes(String.class).withArguments(Matchers.anyString()).thenReturn(mockedFile);

        //Do the test
        ClassToTest classToTest = new ClassToTest();
        classToTest.openFile("testfile.txt");

        //Verify that the File was created and the exists-method of the mock was called
        PowerMockito.verifyNew(File.class).withArguments("testfile.txt");
        Mockito.verify(mockedFile).exists();
    }
}

If you use JUnit, there is the TemporaryFolder. Files are deleted after the test. Example given on the page:

public static class HasTempFolder {
  @Rule
  public TemporaryFolder folder= new TemporaryFolder();

  @Test
  public void testUsingTempFolder() throws IOException {
      File createdFile= folder.newFile("myfile.txt");
      File createdFolder= folder.newFolder("subfolder");
      // ...
     }
 }

However, I have also used it for testing my Android class read/write capabilities like:

    [...]
    pw = new PrintWriter(folder.getAbsolutePath() + '/' + filename);
    pw.println(data);

How about using mocked stream classes, which override the real ones (like BufferredReader, File) completely (meaning all methods or all methods you use)? The data can be saved as an array of bytes, for example, in some singleton, if they are to be used between different test classes.