Modified Fibonacci Implementation

Question

Purpose

This question comes from a HackerRank problem called Fibonacci Modified. I think I have coded a valid solution, but would like feedback about my approach, implementation, and of course, any errors / edge cases that I may have missed.

Task

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Implementation

public class FibonacciModifiedImpl implements FibonacciModified {
 @Override
 public BigInteger calculateNextTerm(final BigInteger currentTerm, final BigInteger lastTerm) {
 if (-1 == lastTerm.compareTo(BigInteger.ZERO) ) {
 throw new IllegalArgumentException("last term must be non-negative");
 }
 if (-1 == currentTerm.compareTo(lastTerm)) {
 throw new IllegalArgumentException("current term must be greater than or equal to last term");
 }
 final BigInteger squaredTerm = new BigDecimal(Math.pow(currentTerm.doubleValue(), 2)).toBigInteger();
 return squaredTerm.add(lastTerm);
 }
 @Override
 public BigInteger calculateNthTerm(final BigInteger firstTerm, final BigInteger secondTerm, final int n) {
 if (-1 == firstTerm.compareTo(BigInteger.ZERO) ) {
 throw new IllegalArgumentException("first term must be non-negative");
 }
 if (-1 == secondTerm.compareTo(firstTerm)) {
 throw new IllegalArgumentException("second term must be greater than or equal to last term");
 }
 if (n < 1) {
 throw new IllegalArgumentException("number of terms must be greater than 0");
 }
 switch (n) {
 case 1: {
 return firstTerm;
 }
 case 2: {
 return secondTerm;
 }
 default: {
 int counter = 2;
 BigInteger currentTerm = secondTerm;
 BigInteger lastTerm = firstTerm;
 while (counter < n) {
 BigInteger nextTerm = calculateNextTerm(currentTerm, lastTerm);
 lastTerm = currentTerm;
 currentTerm = nextTerm;
 counter++;
 }
 return currentTerm;
 }
 }
 }
}

Answer 1

Generally looks good. Except

 final BigInteger squaredTerm = new BigDecimal(Math.pow(currentTerm.doubleValue(), 2)).toBigInteger();

There is absolutely no reason to cast Math in. The computations are supposed to be exact, which Math is notoriously lacking. I recommend

 final BigInteger squaredTerm = currentTerm.multiply(currentTerm);