Why separate the API definition and implementation JARs

Question

Asked by overexchange At 28 December at 07:41

For jstl tags, an API javax.servlet.jsp.jstl-api-1.2.1.jar & implementation javax.servlet.jsp.jstl-1.2.1.jar are provided.

For servlets, an API in servlet-api.jar & implementation jar from tomcat or GlassFish are provided.

For collections, API like java.util.List & corresponding implementations like java.util.ArrayList & java.util.LinkedList are provided.

IDE Netbeans is another example.

Implementation jar includes both the API(mostly interfaces) and its implementation, What are the advantages in providing a solution with an API JAR and their corresponding implementations(a separate JAR), for programmers/developers to use?
For developing enterprise applications using java, Is providing an API the standard approach for stable contract between developers?

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