# Class HttpHeaders

java.lang.Object
java.net.http.HttpHeaders

public final class HttpHeaders extends Object
A read-only view of a set of HTTP headers.

An `HttpHeaders` is not typically created directly, but rather returned from an `HttpRequest` or an `HttpResponse`. Specific HTTP headers can be set for a request through one of the request builder's `headers` methods.

The methods of this class ( that accept a String header name ), and the `Map` returned by the `map` method, operate without regard to case when retrieving the header value(s).

An HTTP header name may appear more than once in the HTTP protocol. As such, headers are represented as a name and a list of values. Each occurrence of a header value is added verbatim, to the appropriate header name list, without interpreting its value. In particular, `HttpHeaders` does not perform any splitting or joining of comma separated header value strings. The order of elements in a header value list is preserved when `building` a request. For responses, the order of elements in a header value list is the order in which they were received. The `Map` returned by the `map` method, however, does not provide any guarantee with regard to the ordering of its entries.

`HttpHeaders` instances are immutable.

Since:
  11

- ## Method Summary

  Modifier and Type
  Method
  Description
  `List<String>`
  `allValues(String name)`
  Returns an unmodifiable List of all of the header string values of the given named header.
  `final boolean`
  `equals(Object obj)`
  Tests this HTTP headers instance for equality with the given object.
  `Optional<String>`
  `firstValue(String name)`
  Returns an `Optional` containing the first header string value of the given named (and possibly multi-valued) header.
  `OptionalLong`
  `firstValueAsLong(String name)`
  Returns an `OptionalLong` containing the first header string value of the named header field.
  `final int`
  `hashCode()`
  Computes a hash code for this HTTP headers instance.
  `Map<String, List<String>>`
  `map()`
  Returns an unmodifiable multi Map view of this HttpHeaders.
  `static HttpHeaders`
  `of(Map<String, List<String>> headerMap, BiPredicate<String,String> filter)`
  Returns an HTTP headers from the given map.
  `String`
  `toString()`
  Returns this HTTP headers as a string.

  ### Methods declared in class Object

  `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

- ## Method Details

  - ### firstValue

    public Optional<String> firstValue(String name)
    Returns an `Optional` containing the first header string value of the given named (and possibly multi-valued) header. If the header is not present, then the returned `Optional` is empty.
    Parameters:
      `name` - the header name
    Returns:
      an `Optional<String>` containing the first named header string value, if present

  - ### firstValueAsLong

    public OptionalLong firstValueAsLong(String name)
    Returns an `OptionalLong` containing the first header string value of the named header field. If the header is not present, then the Optional is empty. If the header is present but contains a value that does not parse as a `Long` value, then an exception is thrown.
    Parameters:
      `name` - the header name
    Returns:
      an `OptionalLong`
    Throws:
      `NumberFormatException` - if a value is found, but does not parse as a Long

  - ### allValues

    public List<String> allValues(String name)
    Returns an unmodifiable List of all of the header string values of the given named header. Always returns a List, which may be empty if the header is not present.
    Parameters:
      `name` - the header name
    Returns:
      a List of headers string values

  - ### map

    public Map<String, List<String>> map()
    Returns an unmodifiable multi Map view of this HttpHeaders.
    Returns:
      the Map

  - ### equals

    public final boolean equals(Object obj)
    Tests this HTTP headers instance for equality with the given object.

    If the given object is not an `HttpHeaders` then this method returns `false`. Two HTTP headers are equal if each of their corresponding maps are equal.

    This method satisfies the general contract of the `Object.equals` method.

    Overrides:
      `equals` in class `Object`
    Parameters:
      `obj` - the object to which this object is to be compared
    Returns:
      `true` if, and only if, the given object is an `HttpHeaders` that is equal to this HTTP headers
    See Also:
      - `Object.hashCode()`
      - `HashMap`

  - ### hashCode

    public final int hashCode()
    Computes a hash code for this HTTP headers instance.

    The hash code is based upon the components of the HTTP headers `map`, and satisfies the general contract of the `Object.hashCode` method.

    Overrides:
      `hashCode` in class `Object`
    Returns:
      the hash-code value for this HTTP headers
    See Also:
      - `Object.equals(java.lang.Object)`
      - `System.identityHashCode(Object)`

  - ### toString

    public String toString()
    Returns this HTTP headers as a string.
    Overrides:
      `toString` in class `Object`
    Returns:
      a string describing the HTTP headers

  - ### of

    public static HttpHeaders of(Map<String, List<String>> headerMap, BiPredicate<String,String> filter)
    Returns an HTTP headers from the given map. The given map's key represents the header name, and its value the list of string header values for that header name.

    An HTTP header name may appear more than once in the HTTP protocol. Such, multi-valued, headers must be represented by a single entry in the given map, whose entry value is a list that represents the multiple header string values. Leading and trailing whitespaces are removed from all string values retrieved from the given map and its lists before processing. Only headers that, after filtering, contain at least one, possibly empty string, value will be added to the HTTP headers.

    API Note:
      The primary purpose of this method is for testing frameworks. Per-request headers can be set through one of the `HttpRequest` `headers` methods.
    Parameters:
      `headerMap` - the map containing the header names and values
      `filter` - a filter that can be used to inspect each header-name-and-value pair in the given map to determine if it should, or should not, be added to the to the HTTP headers
    Returns:
      an HTTP headers instance containing the given headers
    Throws:
      `NullPointerException` - if any of: `headerMap`, a key or value in the given map, or an entry in the map's value list, or `filter`, is `null`
      `IllegalArgumentException` - if the given `headerMap` contains any two keys that are equal ( without regard to case ); or if the given map contains any key whose length, after trimming whitespaces, is `0`