The HttpRequest.Builder class uses three methods to set additional headers:

• header​(String name, String value) and setHeader​(String name, String value): These are used to add headers one by one, as shown in the following code:

HttpRequest request = HttpRequest.newBuilder()
  .uri(...)
  ...
  .header("key_1", "value_1")
  .header("key_2", "value_2")
  ...
  .build();

HttpRequest request = HttpRequest.newBuilder()
  .uri(...)
  ...
  .setHeader("key_1", "value_1")
  .setHeader("key_2", "value_2")
  ...
  .build();

• headers​(String... headers): This is used to add headers separated by a comma, as shown in the following code:

HttpRequest request = HttpRequest.newBuilder()
  .uri(...)
  ...
  .headers("key_1", "value_1", "key_2",
    "value_2", "key_3", "value_3", ...)
  ...
  .build();

For example, the Content-Type: application/json and Referer: https://reqres.in/ headers can be added to the request that's triggered by the https://reqres.in/api/users/2 URI, as follows:

HttpRequest request = HttpRequest.newBuilder()
  .header("Content-Type", "application/json")
  .header("Referer", "https://reqres.in/")
  .uri(URI.create("https://reqres.in/api/users/2"))
  .build();

You can also do the following:

HttpRequest request = HttpRequest.newBuilder()
  .setHeader("Content-Type", "application/json")
  .setHeader("Referer", "https://reqres.in/")
  .uri(URI.create("https://reqres.in/api/users/2"))
  .build();

Finally, you can do something like this:

HttpRequest request = HttpRequest.newBuilder()
  .headers("Content-Type", "application/json",
    "Referer", "https://reqres.in/")
  .uri(URI.create("https://reqres.in/api/users/2"))
  .build();

Depending on the goal, all three methods can be combined in order to specify the request headers.