How a maven dependecy can be defined without version tag? - maven

Question

I have a pom which works without defining dependency version in pom works fine and another one without dependency version which does not work.
The one which works:
<project>
<parent>
<artifactId>artifact-parent</artifactId>
<groupId>group-parent</groupId>
<version>1.0-SNAPSHOT</version>
</parent>
<dependencies>
<dependency>
<groupId>group-a</groupId>
<artifactId>artifact-a</artifactId>
</dependency>
<dependency>
<groupId>group-a</groupId>
<artifactId>artifact-b</artifactId>
</dependency>
</dependencies>
</project>
This one which does not work:
<project>
<dependencies>
<dependency>
<groupId>group-a</groupId>
<artifactId>artifact-a</artifactId>
</dependency>
<dependency>
<groupId>group-a</groupId>
<artifactId>artifact-b</artifactId>
</dependency>
</dependencies>
</project>
The only thing differ in these two seems to be:
<parent>
<artifactId>artifact-parent</artifactId>
<groupId>group-parent</groupId>
<version>1.0-SNAPSHOT</version>
</parent>
Second one does not work it seems fine to me but my question is why the first one works ?
Quoted from maven pom reference :
This trinity represents the coordinate of a specific project in time,
demarcating it as a dependency of this project.
So my question is how the first one is working?

Answer 1

The main thing to notice here is:
<parent>
<artifactId>artifact-parent</artifactId>
<groupId>group-parent</groupId>
<version>1.0-SNAPSHOT</version>
</parent>
Version of the dependency looks like to be defined in the parent pom. This can be something like this:
<project>
<dependencyManagement>
<dependencies>
<dependency>
<groupId>group-a</groupId>
<artifactId>artifact-a</artifactId>
<version>1.0</version>
</dependency>
<dependency>
<groupId>group-a</groupId>
<artifactId>artifact-b</artifactId>
<version>1.0</version>
</dependency>
</dependencies>
</dependencyManagement>
</project>
Again quoting from the doc :
This is because the minimal set of information for matching a
dependency reference against a dependencyManagement section is
actually {groupId, artifactId, type, classifier}.
Here we did not need to define the {type, classifier} as it is same as default value which is as follows :
<type>jar</type>
<classifier><!-- no value --></classifier>
If this value differ form the default, you need to define it explicitly in both parent pom and child pom.