I tell my students all the time. IN THE BEGINNING STOP TRYING TO LEARN EVERY LANGUAGE! In the beginning, get a job, learn one language really well and then expand your knowledge of other languages. You will find out that all other languages are 90% the same.
I know. But, it's hard for students to make an informed choice, especially when a specific programming language is taught in school, and it is a requirement to master said language to graduate -- it's part of the curriculum, after all.
That's what I felt with Java. I was good -- like really good -- at it once. Java 7 era. But I had no other option. I spent more time with Java (two semesters - that's a whole year) and only a fraction of a semester -- or one semester at most -- for the others.
Then, I joined the workforce, joined online classes, and found something better. So, it's hard to really choose one language when you are a student.
But, I do agree with your advice to your students. Focus on one thing -- and be a master. I didn't learn that in school. I actually learned that from reading non-fiction books and relevant articles :)
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