当我使用Polynomial构造函数创建Monomial数组时，有时也是如此 我得到null对象而不是Monomial对象，有时我得到实际的Monomial。
我不明白为什么
我调试代码，在Polynomial构造函数中我构造了特定的对象，但是当我返回main方法时，有时我得到Monomial的特定数组，有时我在数组中得到一个null。
public class TestPolynomials
{
public static void main(String[] args)
{
// 13b^2x^3z
Monomial m1 = new Monomial(13);
m1.setDegree('b', 2);
m1.setDegree('x', 3);
m1.setDegree('z', 1);
// 15
Monomial m2 = new Monomial(15);
Polynomial p1 = new Polynomial(new Monomial[] { m1, m2 });
}
}
public class Monomial
{
private int coefficient;
private int[] var;
public Monomial(int coefficient)
{
this.coefficient = coefficient;
this.var = new int[26];
}
public int getCoefficient()
{
return coefficient;
}
public void setCoefficient(int coefficient)
{
this.coefficient = coefficient;
}
public int getDegree(char variable)
{
return var[variable - 'a'];
}
public void setDegree(char variable, int degree)
{
this.var[variable - 'a'] = degree;
}
public Monomial getCopy()
{
Monomial monom = new Monomial(this.coefficient);
for (char ch = 'a'; ch <= 'z'; ch++)
{
if (this.getDegree(ch) > 0)
monom.setDegree(ch, this.getDegree(ch));
}
return monom;
}
public String toString()
{
String monom = "";
monom += Integer.toString(this.getCoefficient());
for (char ch = 'a'; ch <= 'z'; ch++)
{
if (this.getDegree(ch) > 0)
{
monom += ch;
if (this.getDegree(ch) > 1)
{
monom += "^";
monom += Integer.toString(this.getDegree(ch));
}
}
}
return monom;
}
}
public class Polynomial
{
private Monomial[] monomials;
public Polynomial(Monomial[] monomials)
{
this.monomials = new Monomial[monomials.length];
for (int i = 0; i < monomials.length; i++)
{
this.monomials[i] = monomials[i].getCopy();
}
}
public String toString()
{
String str = "";
for (int i = 0; i < this.getMonomialCount(); i++)
{
str += this.getMonomial(i).toString();
str += "+";
}
return str;
}
}
我构建了m1和m2，我尝试构建p1。
有时我会得到这个：[13b^2x^3z, 15]，有时我会得到这个：[null, 30]
为什么？
感谢。
=============================================== ========================= 编辑：这是我的完整代码，当我运行它时，我在控制台中得到它
m1 = 13b^2x^3z
m2 = 15
m3 = -4b^2x^3z
m4 = 10b^3x^4z^2
13b^2x^3z does not have the same degrees as 15
13b^2x^3z has the same degrees as -4b^2x^3z
Exception in thread "main" java.lang.NullPointerException
at Polynomial.toString(Polynomial.java:98)
at java.lang.String.valueOf(Unknown Source)
at java.lang.StringBuilder.append(Unknown Source)
at TestPolynomials.main(TestPolynomials.java:44)
public class TestPolynomials
{
private static String tempstr = ""; // For printing results;
public static void main(String[] args)
{
// 13b^2x^3z
Monomial m1 = new Monomial(13);
m1.setDegree('b', 2);
m1.setDegree('x', 3);
m1.setDegree('z', 1);
System.out.println("m1 = " + m1);
// 15
Monomial m2 = new Monomial(15);
System.out.println("m2 = " + m2);
// -4b^2x^3z
Monomial m3 = new Monomial(-4);
m3.setDegree('b', 2);
m3.setDegree('x', 3);
m3.setDegree('z', 1);
System.out.println("m3 = " + m3);
Monomial m4 = new Monomial(10);
m4.setDegree('b', 3);
m4.setDegree('x', 4);
m4.setDegree('z', 2);
System.out.println("m4 = " + m4);
if (m1.hasSameDegrees(m2))
tempstr = " has";
else
tempstr = " does not have";
System.out.println(m1 + tempstr + " the same degrees as " + m2);
if (m1.hasSameDegrees(m3))
tempstr = " has";
else
tempstr = " does not have";
System.out.println(m1 + tempstr + " the same degrees as " + m3);
Polynomial p1 = new Polynomial(new Monomial[] { m1, m2 });
Polynomial p2 = new Polynomial(new Monomial[] { m3 });
Polynomial p3 = p1.add(p2);
System.out.println("p1 = " + p1); // should be 13b^2x^3z+15
System.out.println("p2 = " + p2); // should be -4b^2x^3z
System.out.println("p3 = " + p3); // should be 13b^2x^3z+15-4b^2x^3z up to reordering the monomials
System.out.println("-p3 = " + p3.inverse()); // should be -13b^2x^3z-15+4b^2x^3z up to reordering the monomials
System.out.println("p4 = p3 + p2");
Polynomial p4 = p3.add(p2);
int[] assignment = { 0, 1, 6, 4, 3, 0, 0, 2, 3, 5, 2, 6, 3, 8, 7, 0, 0, 4, 2, 6, 0, 4, 1, 5, 1, 9 };
System.out.println("p4 value = " + p4.computeValue(assignment) + " --- before normalization");
p4.normalize();
System.out.println("p4 = " + p4);
System.out.println("p4 value = " + p4.computeValue(assignment) + " --- after normalization");
Polynomial p5 = new Polynomial(new Monomial[] { m1, m2, m3, m4, m1, m2, m3, m4, m4 });
System.out.println("p5 items are m1,m2,m3,m4,m1,m2,m3,m4,m4");
System.out.println("p5 = " + p5);
Monomial m5 = new Monomial(0);
m5.setDegree('z', 1);
Polynomial p6 = new Polynomial(new Monomial[] { m5 });
System.out.println("p6 = " + p6);
Polynomial p7 = new Polynomial(new Monomial[] {});
System.out.println("p7 = " + p7);
}
public class Monomial
{
private int coefficient;
private int[] var;
public Monomial(int coefficient)
{
this.coefficient = coefficient;
this.var = new int[26];
}
public int getCoefficient()
{
return coefficient;
}
public void setCoefficient(int coefficient)
{
this.coefficient = coefficient;
}
public int getDegree(char variable)
{
return var[variable - 'a'];
}
public void setDegree(char variable, int degree)
{
this.var[variable - 'a'] = degree;
}
public boolean hasSameDegrees(Monomial other)
{
for (char ch = 'a'; ch <= 'z'; ch++)
{
if (!(this.getDegree(ch) == other.getDegree(ch)))
return false;
}
return true;
}
public Monomial getCopy()
{
Monomial monom = new Monomial(this.coefficient);
for (char ch = 'a'; ch <= 'z'; ch++)
{
if (this.getDegree(ch) > 0)
monom.setDegree(ch, this.getDegree(ch));
}
return monom;
}
public String toString()
{
String monom = "";
monom += Integer.toString(this.getCoefficient());
for (char ch = 'a'; ch <= 'z'; ch++)
{
if (this.getDegree(ch) > 0)
{
monom += ch;
if (this.getDegree(ch) > 1)
{
monom += "^";
monom += Integer.toString(this.getDegree(ch));
}
}
}
return monom;
}
}
import java.lang.Math;
public class Polynomial
{
private Monomial[] monomials;
public Polynomial(Monomial[] monomials)
{
this.monomials = new Monomial[monomials.length];
for (int i = 0; i < monomials.length; i++)
{
this.monomials[i] = monomials[i].getCopy();
}
}
public int getMonomialCount()
{
return this.monomials.length;
}
public Monomial getMonomial(int index)
{
if (index <= this.getMonomialCount())
return this.monomials[index];
return null;
}
public Polynomial inverse()
{
Monomial[] inverse = new Monomial[this.getMonomialCount()];
for (int i = 0; i < inverse.length; i++)
{
inverse[i] = this.monomials[i].getCopy();
inverse[i].setCoefficient(inverse[i].getCoefficient() * -1);
}
return new Polynomial(inverse);
}
public Polynomial add(Polynomial other)
{
Monomial[] add = new Monomial[this.getMonomialCount() + other.getMonomialCount()];
for (int i = 0; i < other.getMonomialCount(); i++)
{
add[i] = other.getMonomial(i).getCopy();
}
for (int i = other.getMonomialCount(); i < add.length; i++)
{
add[i] = this.monomials[i - other.getMonomialCount()].getCopy();
}
return new Polynomial(add);
}
public int computeValue(int[] assignment)
{
int result = 1, sum = 0;
for (int i = 0; i < this.getMonomialCount(); i++)
{
for (char ch = 'a'; ch <= 'z'; ch++)
{
if (this.getMonomial(i).getDegree(ch) > 0)
result *= (int) Math.pow((double) assignment[i], (double) this.getMonomial(i).getDegree(ch));
}
result *= this.getMonomial(i).getCoefficient();
sum += result;
result = 1;
}
return sum;
}
public void normalize()
{
Monomial[] normalize = new Monomial[this.getMonomialCount()];
for (int i = 0; i < this.getMonomialCount(); i++)
{
for (int j = 1; j < this.getMonomialCount(); j++)
{
if (this.getMonomial(i).hasSameDegrees(this.getMonomial(j)))
{
normalize[i] = new Monomial(this.getMonomial(i).getCoefficient() + this.getMonomial(j).getCoefficient());
for (char ch = 'a'; ch <= 'z'; ch++)
{
if (this.getMonomial(i).getDegree(ch) > 0)
normalize[i].setDegree(ch, this.getMonomial(i).getDegree(ch));
}
this.monomials[i] = null;
}
}
}
this.monomials = normalize;
}
public String toString()
{
normalize();
String str = "";
for (int i = 0; i < this.getMonomialCount(); i++)
{
str += this.getMonomial(i).toString();
str += "+";
}
return str;
}
}
为什么？