How can I run Hadoop run with a Java class?

Question

I am following the book Hadoop: the definitive Guide.

I am confused on example 3-1.

There is a Java source file, URLCat.java. I use javac to co

Answer 1

We can access HDFS through the hdfs api. My understanding of it is that you can use the hdfs api to contact a hadoop cluster running the dfs and fetch data from it.

Why do we need to invoke the command as hadoop jar URLCat.jar

why not just java URLCat

Why does the client necessarily need to install hadoop and then contact the hadoop cluster?