I have a jsp page which I want to style using Materialize framework, but as soon as I add the href to materialize.min.css in my page (or in the entire project from the beginning), it overrides my existing styles from all the application and I want to be able to call Materialize only when I need it, not to change the whole style because it becomes a mess.
It's also the first time I'm using Materialize, so maybe I'm missing something.
That's how CSS frameworks work, generally. Most of them will normalize and apply a base style to your website, even if you are not using framework specific class names.
I usually work the other way around: I let my framework of choice control all of my UI, and when I want to override something, I use custom CSS styles. To do that, make sure you add a link to your own styles on the tag after the Materialize CSS link.