所以我已经在Int完成了整个课程，现在我必须将它转换为BigInteger。主要目标是我可以将系数存储为大系数的BigIntegers。我得到代码的空指针错误，但我知道BigInteger是不可变的，需要这种格式。可能只是另一只眼睛或者我可能只是没有正确地做到这一点。
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] = BigInteger.valueOf(1);
deg = 0;
}
/** Creates the linear polynomial of the form P(x) = x + a.
*/
public Polynomial(int a){
coef = new BigInteger[2];
coef[1] = BigInteger.valueOf(1);
coef[0] = BigInteger.valueOf(a);
deg = 1;
}
/** Creates the polynomial P(x) = a * x^b.
*/
public Polynomial(int a, int b) {
coef = new BigInteger[b+1];
coef[b] = BigInteger.valueOf(a);
deg = degree();
}
public Polynomial(BigInteger a, int b) {
coef = new BigInteger[b+1];
coef[b] = 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] != BigInteger.valueOf(0)) d = i;
return d;
}
/** Return the sum of this polynomial and b, i.e., return c = this + b.
*/
public Polynomial plus(Polynomial b) {
Polynomial a = this;
Polynomial c = new Polynomial(0, Math.max(a.deg, b.deg));
for (int i = 0; i <= a.deg; i++) c.coef[i] = c.coef[i].add(a.coef[i]);
for (int i = 0; i <= b.deg; i++) c.coef[i] = c.coef[i].add(b.coef[i]);
c.deg = c.degree();
return c;
}
/** Return the difference of this polynomial and b, i.e., return (this - b).
*/
public Polynomial minus(Polynomial b) {
Polynomial a = this;
Polynomial c = new Polynomial(0, Math.max(a.deg, b.deg));
for (int i = 0; i <= a.deg; i++) c.coef[i] = c.coef[i].add(a.coef[i]);
for (int i = 0; i <= b.deg; i++) c.coef[i] = c.coef[i].subtract(b.coef[i]);
c.deg = c.degree();
return c;
}
/** Return the product of this polynomial and b, i.e., return (this * b).
*/
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 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], 0);
c = term.plus(b.times(c));
}
return c;
}
/** Return true whenever this polynomial and b are identical to one another.
*/
public boolean equals(Polynomial b) {
Polynomial a = this;
if (a.deg != b.deg) return false;
for (int i = a.deg; i >= 0; i--)
if (a.coef[i] != b.coef[i]) return false;
return true;
}
/** Evaluate this polynomial at x, i.e., return this(x).
*/
public int evaluate(int x) {
int p = 0;
for (int i = deg; i >= 0; i--){
coef[i] = coef[i].add(BigInteger.valueOf(x * p));
p = coef[i].intValue();
}
return p;
}
/** Return the derivative of this polynomial.
*/
public Polynomial differentiate() {
if (deg == 0) return new Polynomial(0, 0);
Polynomial deriv = new Polynomial(0, deg - 1);
deriv.deg = deg - 1;
for (int i = 0; i < deg; i++)
deriv.coef[i] = coef[i + 1].multiply(BigInteger.valueOf(i+1));
return deriv;
}
/** Return a textual representationof this polynomial.
*/
public String toString() {
if (deg == 0) return "" + coef[0];
if (deg == 1) return String.valueOf(coef[1]) + "x + " + String.valueOf(coef[0]);
String s = String.valueOf(coef[deg]) + "x^" + deg;
for (int i = deg-1; i > 0; i--) {
if (coef[i].intValue() == 0) continue;
else if (coef[i].intValue() > 0) s = s + " + " + ( coef[i].intValue());
else if (coef[i].intValue() < 0) s = s + " - " + (-coef[i].intValue());
if (i == 1) s = s + "x";
else if (i > 1) s = s + "x^" + i;
}
return s;
}
public static void main(String[] args) {
Polynomial zero = new Polynomial(1, 0);
Polynomial p1 = new Polynomial(4, 3);
Polynomial p2 = new Polynomial(3, 2);
Polynomial p3 = new Polynomial(-1, 0);
Polynomial p4 = new Polynomial(-2, 1);
Polynomial p = p1.plus(p2).plus(p3).plus(p4); // 4x^3 + 3x^2 - 2x - 1
Polynomial q1 = new Polynomial(3, 2);
Polynomial q2 = new Polynomial(5, 0);
Polynomial q = q1.minus(q2); // 3x^2 - 5
Polynomial r = p.plus(q);
Polynomial s = p.times(q);
Polynomial t = p.compose(q);
System.out.println("zero(x) = " + zero);
System.out.println("p(x) = " + p);
System.out.println("q(x) = " + q);
System.out.println("p(x) + q(x) = " + r);
System.out.println("p(x) * q(x) = " + s);
System.out.println("p(q(x)) = " + t);
System.out.println("0 - p(x) = " + zero.minus(p));
System.out.println("p(3) = " + p.evaluate(3));
System.out.println("p'(x) = " + p.differentiate());
System.out.println("p''(x) = " + p.differentiate().differentiate());
Polynomial poly = new Polynomial();
for(int k=0; k<=4; k++){
poly = poly.times(new Polynomial(-k));
}
System.out.println(poly);
}
}
答案 0 :(得分：2)
因此，当您初始化BigInteger数组时，值为null，因为您已指定了一个对象数组（如果它是int[]，则初始值为0）。
从构造函数中可以看出：
public Polynomial(int a, int b) {
coef = new BigInteger[b+1];
coef[b] = BigInteger.valueOf(a);
deg = degree();
}
您只分配了coef[b]，其他值仍为null。
因此，在方法plus(Polynomial b)的循环的第一次迭代中，当您的循环尝试调用c.coef[0]时，NullPointerException为空c.coef[0].add(a.coef[0])。
建议：定义一个方法，将数组中的所有BigInteger值初始化为0，以与int[]的声明一致，并在构造函数中调用。例如：
private static void initializeBigIntegerArray(BigInteger[] bigIntegers) {
for (int i=0; i<bigIntegers.length; i++) {
// So you don't overwrite anything you assign explicitly
if (bigInteger[i] == null) {
bigIntegers[i] = BigInteger.ZERO;
}
}
}
答案 1 :(得分：1)
回想一下，在Java中，对象数组实际上是对象的引用数组。因此，您需要为每个数组元素创建一个BigInteger对象。您未分配的条目不为0，它们为空。
答案 2 :(得分：1)
因此，在plus方法中，您创建此多项式c，其后备数组包含一个零和几个空值。然后你继续尝试对该多项式中的所有系数进行操作，包括所有那些空值。因此，您正在调用尚未创建对象的变量的方法，这就是使您的空指针出现问题的原因。
创建每个多项式时，请确保为后备阵列中的每个条目创建了BigInteger。