We should annotate out fields with @Merge annotation.

public class TestEntity {
    @Merge private String login;
    @Merge private String password;
}

Then you should call diff methods:

Getting differences of object

DiffUtils.difference(testEntityInstance1, testEntityInstance2)

Or merging objects and getting string of changes

DiffUtils.merge(testEntityInstance1, testEntityInstance2, false)

If you want create your own format of changes you should implement Difference interface.