You can have a look at JGit unit tests for finding a lot of examples.
That includes org.eclipse.jgit.api.StatusCommandTest
Git git = new Git(db);
writeTrashFile("a", "content of a");
writeTrashFile("b", "content of b");
writeTrashFile("c", "content of c");
git.add().addFilepattern("a").addFilepattern("b").call();
Status stat = git.status().call(); // git status right there
assertEquals(set("a", "b"), stat.getAdded());