code: public @NotNull FileAssert hasSameContentAs(@NotNull File expected) {
      checkNotNull(expected);
      isNotNull();
      assertExists(actual).assertExists(expected);
      try {
        LineDiff[] diffs = comparator.compareContents(actual, expected);
        if (!isNullOrEmpty(diffs)) {
          fail(expected, diffs);
        }
      } catch (IOException e) {
        cannotCompareToExpectedFile(expected, e);
      }
      return this;
    }
docs: Verifies that the content of the actual {@code File} is equal to the content of the given one. Adapted from <a
  href="http://junit-addons.sourceforge.net/junitx/framework/FileAssert.html" target="_blank">FileAssert</a> (from <a
  href="http://sourceforge.net/projects/junit-addons">JUnit-addons</a>.)
  
  @param expected the given {@code File} to compare the actual {@code File} to.
  @return this assertion object.
  @throws NullPointerException if the file to compare to is {@code null}.
  @throws AssertionError       if the the actual {@code File} is {@code null}.
  @throws AssertionError       if the content of the actual {@code File} is not equal to the content of the given one.