Why sometimes dependency not contain version property in pom.xml?

Question

I am a newbie of Maven, currently reading Hadoop source code, and found something interesting in some pom.xml files:

Some of the dependency node do not contain version node at all.

Question: why is it like this?

for instance, this pom.xml.

Answer 1

Because specific version of dependency in parent pom.xml file

https://github.com/apache/hadoop/blob/trunk/pom.xml

Reference: https://maven.apache.org/guides/introduction/introduction-to-the-pom.html