PrimeFactorization.java

import java.math.BigInteger;
import java.util.ArrayList;
import java.util.List;

class PrimeFactorization {
    private final String dividend;

    PrimeFactorization(String dividend) {
        this.dividend = dividend;
    }

    List<BigInteger> computePrimeFactor() {
        BigInteger dividend = new BigInteger(this.dividend);
        BigInteger divisor = BigInteger.valueOf(2L);

        // dividendがdivisor以下の場合に例外を投げる
        if (dividend.compareTo(divisor) == -1) {
            throw new IllegalStateException();
        }

        // 除数を2から1ずつ増やしていき、素因数分解をする
        List<BigInteger> list = new ArrayList<>();
        while (dividend.compareTo(BigInteger.ONE) != 0) {
            while (dividend.remainder(divisor).equals(BigInteger.ZERO)) {
                dividend = dividend.divide(divisor);
                list.add(divisor);
            }
            divisor = divisor.add(BigInteger.ONE);
        }

        return list;
    }
}