### Question:
SystemOutErr implements OutErr { @Override public void setErr(PrintStream err) { System.setErr(err); }  @Override PrintStream out(); @Override PrintStream err(); @Override void setOut(PrintStream out); @Override void setErr(PrintStream err);  }### Answer:
@Test public void sets_the_real_stderr() { PrintStream newStream = new PrintStream(new NullOutputStream()); systemOutErr.setErr(newStream); assertThat(System.err, is(newStream)); }