Я ценю помощь. Мне удалось закончить изменение всего в этом классе в формате BigInteger, кроме метода compose. Может кто-нибудь помочь мне с этой последней частью относительно того, почему она не работает правильно? Я действительно ценю это, спасибо.

import java.math.BigInteger;

public class Polynomial {
private BigInteger[] coef;  // coefficients
private int deg;     // degree of polynomial (0 for the zero polynomial)



/** Creates the constant polynomial P(x) = 1.
  */
public Polynomial(){
    coef = new BigInteger[1];
    coef[0] = new BigInteger("1");
    deg = 0;
}



/** Creates the linear polynomial of the form P(x) =  x + a.
  */
public Polynomial(int a){
    coef = new BigInteger[2];
    coef[1] = new BigInteger("1");
    coef[0] = new BigInteger(Integer.toString(a));
    deg = 1;
}

/** Creates the polynomial P(x) = a * x^b.
  */
public Polynomial(int a, int b) {
    coef = new BigInteger[b+1];
    for(int i = 0; i < b; i++){
        if(coef[i] == null)
            coef[i] = new BigInteger("0");

    }
    coef[b] = new BigInteger(Integer.toString(a));
    deg = degree();
}


/** Return the degree of this polynomial (0 for the constant polynomial).
  */
public int degree() {
    int d = 0;
    for (int i = 0; i < coef.length; i++)
        if (coef[i] != null) d = i; // check to make sure this works
    return d;
}

/** Return the composite of this polynomial and b, i.e., return this(b(x))  - compute using Horner's method.
  */
public Polynomial compose(Polynomial b) {
    Polynomial a = this;
    Polynomial c = new Polynomial(0, 0);
    for (int i = a.deg; i >= 0; i--) {
        Polynomial term = new Polynomial(a.coef[i].intValue(), 0);
        c = term.plus(b.times(c));
    }
    return c;
}

  public Polynomial times(Polynomial b) {
    Polynomial a = this;
    Polynomial c = new Polynomial(0, a.deg + b.deg);
    for (int i = 0; i <= a.deg; i++)
        for (int j = 0; j <= b.deg; j++)
            c.coef[i+j] = c.coef[i+j].add((a.coef[i].multiply(b.coef[j])));
    c.deg = c.degree();
    return c;
}

/** Return a textual representation of this polynomial.
  */
public String toString() {
    if (deg ==  0) return "" + coef[0];
    if (deg ==  1) return coef[1] + "x + " + coef[0];
    String s = coef[deg] + "x^" + deg;
    for (int i = deg-1; i >= 0; i--) {
        if      (coef[i] == null) continue;
        else if (coef[i].intValue()  > 0) s = s + " + " + ( coef[i]);
        else if (coef[i].intValue()  < 0) s = s + " - " + (coef[i].negate());
        if(coef[i].intValue() != 0)
            if      (i == 1) s = s + "x";
        else if (i >  1) s = s + "x^" + i;
    }
    return s;
}

public static void main(String[] args) {
   Polynomial p = new Polynomial(1,2);
   Polynomial q = new Polynomial(2,3); 
   Polynomial t    = p.compose(q); // incorrect
   System.out.println("p(q(x))     = " + t);  // incorrect


  }

}