# Tutorial - User Guide { #tutorial-user-guide }

This tutorial shows you how to use **FastAPI** with most of its features, step by step.

Each section gradually builds on the previous ones, but it's structured to separate topics, so that you can go directly to any specific one to solve your specific API needs.

It is also built to work as a future reference so you can come back and see exactly what you need.

## Run the code { #run-the-code }

### Separate Program { #separate-program }

To achieve this, you will normally have a **separate program** that would make sure your application is run on startup. And in many cases, it would also make sure other components or applications are also run, for example, a database.

## Decision

Use "NullAway" in JSpecify mode to check for null-related errors in Java code.

Consider improper uses of `null` detected by NullAway a compilation error (not a warning, and not a separate CI job to test).

Do not suppress NullAway errors without a justification.
Prefer rearranging the code to avoid the error, unless there is a provable performance penalty or a significant loss of code readability.

a Contributor if it was added to the Program by such Contributor itself or
anyone acting on such Contributor's behalf. Contributions do not include additions
to the Program which: (i) are separate modules of software distributed in
conjunction with the Program under their own license agreement, and (ii) are
not derivative works of the Program.

   "Contributor" means any person or entity that distributes the Program.

//   - n > 0: at most n subslices; the last subslice will be the unsplit remainder;
//   - n == 0: the result is nil (zero subslices);
//   - n < 0: all subslices.
//
// To split around the first instance of a separator, see [Cut].
func SplitN(s, sep []byte, n int) [][]byte { return genSplit(s, sep, 0, n) }

// SplitAfterN slices s into subslices after each instance of sep and
// returns a slice of those subslices.

    @Nonnull
    Optional<String> option();

    /**
     * Returns the option followed by a string representation of the given path elements.
     * The path elements are separated by an option-specific or platform-specific separator.
     * If the given {@code paths} argument contains no element, then this method returns an empty string.
     *
     * <h4>Examples</h4>

func hashDeterministicString(m map[string]string) uint64 {
	// Seed (random)
	crc := uint64(0xc2b40bbac11a7295)
	// Xor each value to make order independent
	for k, v := range m {
		// Separate key and value with an individual xor with a random number.
		// Add values of each, so they cannot be trivially collided.
		crc ^= (xxh3.HashString(k) ^ 0x4ee3bbaf7ab2506b) + (xxh3.HashString(v) ^ 0x8da4c8da66194257)
	}

	if !filepath.IsAbs(path) {
		return fmt.Errorf("Invalid argument, path (%s) is expected to be absolute", path)
	}
	var crossMounts mountInfos
	for _, mount := range mts {
		// Add a separator to indicate that this is a proper mount-point.
		// This is to avoid a situation where prefix is '/tmp/fsmount'
		// and mount path is /tmp/fs. In such a scenario we need to check for

     of this section 3.

3.2 When the Program is Distributed as Source Code:

  a) it must be made available under this Agreement, or if the
  Program (i) is combined with other material in a separate file or
  files made available under a Secondary License, and (ii) the initial
  Contributor attached to the Source Code the notice described in
  Exhibit A of this Agreement, then the Program may be made available

            data["client_secret"] = form_data.client_secret
        return data
    ```

    Note that for OAuth2 the scope `items:read` is a single scope in an opaque string.
    You could have custom internal logic to separate it by colon characters (`:`) or
    similar, and get the two parts `items` and `read`. Many applications do that to
    group and organize permissions, you could do it as well in your application, just