Object can't access all the data from another object, but only what I've specified. How to fix?

I'd use something like this ... you need a 2nd variable to capture the Stock, and a method to get the Stocks (getStocks()):

public void FindMostFrequency()
{
    double temp = 0;
    Stock mostFrequentStock = null;
    for (Stock aStock : getStocks()) {
       var frequency = aStock.frequency
       if (temp < frequency) {
           temp = frequency;
           mostFrequentStock = aStock
       }
    }
    System.out.println("The stock with most frequency is: " + mostFrequentStock);
}

It would also be better to access the variable using a getter instead of directly, ie getFrequency()