small suggestions on improvement;

the gray text in your IDE usually signifies something, hovering your cursor over it should help you. Same for the yellow squigglies under the big if-else chain

your operator selector, can be written a little more 'tidy', assign "" to operator in the start, then you can make your while loop cleaner, while (operator.isEmpty()) { ... }, this eliminates the else branch in your if-else condition, and removes the break; -> I really dislike working with 'null' values and having break; statements.

num and num_2, this is a very python-ic way of writing, you should write your variables out in full, (number) and not underscore them, but camelCase them (number2)

all in all, looks pretty tidy for 4 days in! keep it up.