Summing n terms of a Fibonacci Sequence

Code java:

 
 
public class FibonacciSequence extends Sequence implements Sequenceable{
 
    private double secondTerm;
 
    public FibonacciSequence(){
        super(1);
        this.secondTerm = 1;
    }
    /**
     * 
     * @param first first term of fib sequence
     * @param second second term of fib sequence
     */
    public FibonacciSequence(double first, double second){
 
        super(first);
        this.secondTerm = second;
    }
/**
 * 
 * @param n 
 * @return 
 */
    @Override
    public double getNthTerm(int n) {
 
        double term1 = super.getFirstTerm();
        double term2 = this.secondTerm;
        double nTerm = 0;
 
      if(n == 1){
          return term1;
      }
      else if (n == 2){
          return term2;
      }
      else{
          for(int i = 3;i <= n ; i++){
              nTerm = term1 + term2;
              term1 = term2;      
              term2 = nTerm;
 
 
          }
         return nTerm;
      }
 
 
 
     }
 
 
/**
 * 
 * @param n 
 * @return 
 */
    @Override
    public double sumNTerms(int n) {
       throw new UnsupportedOperationException("Not supported yet.");
 
 
 
}
}

I know the Fibonacci sequence is firstTerm + secondTerm = thirdTerm, secondTerm + thirdTerm = fourthTerm and so on. I am confused on how to write the syntax to sum n number of terms.