HttpClient What is the difference between setHeader and addHeader?

Question

When using Apache HttpClient version :

<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.5.6</version>
</dependency>

What is the difference between setHeader and addHeader?

    httpPost.addHeader("AuthenticationKey",authenticationKey);
    httpPost.addHeader("Content-Type","application/json");

    httpPost.setHeader("Cache-Control", "no-cache"); // HTTP 1.1
    httpPost.setHeader("Pragma", "no-cache"); // HTTP 1.0
    httpPost.setHeader("X-Requested-With", "XMLHttpRequest"); // mimics a browser REST request

Answer 1

As you can read from documentation:

addHeader(String name, String value

Adds a header to this message. The header will be appended to the end of the list.

setHeader(String name, String value

Overwrites the first header with the same name. The new header will be appended to the end of the list, if no header with the given name can be found.