Why am I getting “Static method cannot be referenced from a nonstatic context: String String.valueOf(Object)”?

The string class's valueOf() method is a static method.

  Static methods need to be called like this: Class.staticMethodName() i.e. String.valueOf()

What you're currently doing is using an instance of a string to try to call a static method, which (as the error indicates) is not allowed.

bad:

selqenddate.valueOf(selqenddate)

good:

String.valueOf(selqenddate)

Of course, you don't need to use String.valueOf() at all here because selquarter is a string, and substringBetween() also returns a string.

Instead, you need to be using a method that takes a String as input, and gives you a Date as output such as Date.parse()

This will probably resolve java nonstatic method cannot be referenced from a static context error.