I am trying to show a git diff between two commits for a file. Basically, I did it as in <https://github.com/centic9/jgit-cookbook/blob/master/src/main/java/org/dstadler/jgit/porcelain/ShowChangedFilesBetweenCommits.java>

You can see my full code under <https://github.com/svenhornberg/JGitDiff>

```
public class RepoDiff {

    public void compare(byte[] fileContentOld, byte[] fileContentNew) {
        try {
            Repository repository = createNewRepository();
            Git git = new Git(repository);

            // create the file
            commitFileContent(fileContentOld, repository, git, true);
            commitFileContent(fileContentNew, repository, git, false);

            // The {tree} will return the underlying tree-id instead of the commit-id itself!
            ObjectId oldHead = repository.resolve("HEAD^^^^{tree}");   //here is my nullpointer
            ObjectId head = repository.resolve("HEAD^{tree}");

            System.out.println("Printing diff between tree: " + oldHead + " and " + head);

            // prepare the two iterators to compute the diff between
            ObjectReader reader = repository.newObjectReader();
            CanonicalTreeParser oldTreeIter = new CanonicalTreeParser();
            oldTreeIter.reset(reader, oldHead);
            CanonicalTreeParser newTreeIter = new CanonicalTreeParser();
            newTreeIter.reset(reader, head);

            // finally get the list of changed files
            List<DiffEntry> diffs= new Git(repository).diff()
                            .setNewTree(newTreeIter)
                            .setOldTree(oldTreeIter)
                            .call();
            for (DiffEntry entry : diffs) {
                System.out.println("Entry: " + entry);
            }
            System.out.println("Done");

        } catch (Exception exception) {
            exception.printStackTrace();
        }
    }

/**
 * Adds and optionally commits fileContent to a repository
 * @param commit True if file should be committed, False if not
 * @throws Exception catch all for testing
 */
private void commitFileContent(byte[] fileContent, Repository repository, Git git, boolean commit) throws Exception {

    File myfile = new File(repository.getDirectory().getParent(), "testfile");
    myfile.createNewFile();

    FileOutputStream fos = new FileOutputStream (myfile); 
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    baos.write(fileContent);
    baos.writeTo(fos);

    // run the add
    git.add().addFilepattern("testfile").call();

    if(commit) {
        // and then commit the changes
        git.commit().setMessage("Added fileContent").call();
    }

    fos.close();
}

/**
 * Helperfunction from
 * https://github.com/centic9/jgit-cookbook
 */
public static Repository createNewRepository() throws IOException {
    // prepare a new folder
    File localPath = File.createTempFile("TestGitRepository", "");
    localPath.delete();

    // create the directory
    Repository repository = FileRepositoryBuilder.create(new File(
            localPath, ".git"));
    repository.create();

    return repository;
    }
}

```

the code results in this message: 

```
Printing diff between tree: null and AnyObjectId[c11a3a58c23b0dd6e541c4bcd553197772626bc6]
java.lang.NullPointerException
at  org.eclipse.jgit.internal.storage.file.UnpackedObjectCache$Table.index(UnpackedObjectCache.java:146)
at org.eclipse.jgit.internal.storage.file.UnpackedObjectCache$Table.contains(UnpackedObjectCache.java:109)
at org.eclipse.jgit.internal.storage.file.UnpackedObjectCache.isUnpacked(UnpackedObjectCache.java:64)
at org.eclipse.jgit.internal.storage.file.ObjectDirectory.openObject(ObjectDirectory.java:367)
at org.eclipse.jgit.internal.storage.file.WindowCursor.open(WindowCursor.java:145)
at org.eclipse.jgit.treewalk.CanonicalTreeParser.reset(CanonicalTreeParser.java:202)
at javadiff.RepoDiff.compare(RepoDiff.java:40)
at javadiff.GitDiff.main(GitDiff.java:30)

```

**the following line must be false, but it is like in the example from jgit-cookbook**

```
 ObjectId oldHead = repository.resolve("HEAD^^^^{tree}");   //here is my nullpointer

```

I tested `HEAD^1{Tree}` but this does not work, it looks like `{tree}` must be in the string for resolving a tree instead of a commit id. Any ideas to get it working?