java - 调用构造函数时出现空指针异常

Question

每当我尝试调用任何构造函数时，我都会收到空指针异常

public class Poly{

private static class Pair{

int coeff;
int exponent;

}

int count=1;

private Pair[] poly =new Pair[count];

public Poly(){

    poly = new Pair[count];
    poly[count-1].coeff=0;
    poly[count-1].exponent=0;
    count++;
}
public Poly(int coeff1, int exp){

    poly = new Pair[count];
    //poly.add(new Poly(coeff1, exp));
    poly[count-1].coeff=coeff1;
    poly[count-1].exponent=exp;
    count++;
}

private Poly (int exp) {

       poly = new Pair[exp+1];
       poly[0].exponent = exp;
    }





public String toString(){

    String str ="";
    for(int i=0; i<=count; i++){
        str+= poly[i].coeff +"x^" +poly[i].exponent;
    }
    return str;
}

public static void main(String[] args) {

        Poly p =new Poly(5, 3);
        System.out.println(p.toString());
    }


}

Answer 1

这段代码:

poly = new Pair[count];

从不调用构造函数，因此下一行

poly[count-1].coeff=0;

...失败并出现 NPE。

第一行所做的只是创建一个 null 引用数组，您尚未创建任何 Pair 对象。要实际创建 Pair 对象，您必须这样做:

poly = new Pair[count];                    // Not calling the constructor
for (int i = 0; i < poly.length; ++i) {
    poly[i] = new Pair(/*...args...*/);    // Calling the constructor
}
poly[count-1].coeff=0;