How do I write a if statement to return the difference in an array?

For my method double lastPriceChange() if there are two or more prices in the array, it will return the difference between the last two (previous - last), if there are fewer items in the array then it should just return ZERO.

I am stuck on how I should start my if statements

 //Fields
   double lastPChange; 
   double[] dailyPrices;
   int numberOfDaysWithData;
   
 public void addTrade(double price)
 {
   dailyPrices[numberOfDaysWithData] = price;
   numberOfDaysWithData = numberOfDaysWithData + 1;
   this.numberOfDaysWithData++;
   lastPChange = (numberOfDaysWithData - 2);
 }
 
 public int numTrades()
 {
  return numberOfDaysWithData;
 }
 
 public double lastPriceChange()
 {
   return ;
 }