How to throw RuntimeException ("cannot find symbol")

An Exception is an Object like any other in Java. You need to use the new keyword to create a new Exception before you can throw it.

throw new RuntimeException();

Optionally you could also do the following:

RuntimeException e = new RuntimeException();
throw e;

Both code snippets are equivalent.

Link to the tutorials for completeness.

throw new RuntimeException(msg);

You need the new in there. It's creating an instance and throwing it, not calling a method.

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