java - 我的程序中出现 NullPointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 5 年前。

我正在尝试运行以下程序。然而，NullPointerException正在出现。到目前为止，我已经调试了我的代码的每一步。看来是我实现numA.add()后出现空指针异常的在主要。

Main.java

public class Main {

    public static void main(String[]args){

        NumberSystem numA = new NumberSystem(new Number(20),new Number(10));
        numA.add();
        System.out.println(numA.getResult().toString());
    }
}

NumberSystem.java

public class NumberSystem {

    private Number n1,n2,result;

    public NumberSystem(){
        n1 = new Number(0);
        n2 = new Number(0);
        result = new Number(0);
    }

    public NumberSystem(Number n1,Number n2){       
        n1 = new Number();
        n2 = new Number();
        result = new Number(0);
    }   

    public NumberSystem(NumberSystem n){
        n1 = new Number(n.n1);
        n2 = new Number(n.n2);
        result = new Number(n.result);
    }

    public Number getN1(){
        return n1 = new Number(n1.getNum());
    }

    public Number getN2(){
        return n2 = new Number(n2.getNum());
    }

    public Number getResult(){
        return result = new Number(result.getNum());
    }

    public void setN1(int n1Value){
        if (n1Value != 0) n1 = new Number(n1Value);
    }

    public void setN2(int n2Value){
        if (n2Value != 0) n2 = new Number(n2Value);
    }

    public void add(){
        result = new Number(n1.getNum() + n2.getNum());
    }

    public String toString(){
        return "" + n1.getNum();
    }

}

数字.java

public class Number {

    private int num;

    public Number() {num = 0;}
    public Number(int numValue){
        if (numValue != 0) num = numValue;
    }

    public Number(Number anotherNum){
        num = anotherNum.getNum();
    }

    public int getNum(){
        return num;
    }

    public void setNum(int numValue){
        if (numValue != 0) num = numValue;
    }   

    public String toString(){
        return new String("" + num);
    }

}

Answer 1

您需要在 NumberSystem 的构造函数中为类成员变量赋值。目前，您仅将这些值分配给传入的具有相同名称的本地值。

public NumberSystem(Number n1,Number n2){       
    n1 = new Number();  // Number assigned to local variable n1
    n2 = new Number();  // ditto for n2
    ...

这会导致 NPE，然后您尝试在此行上分配结果:

result = new Number(n1.getNum() + n2.getNum());

您可以使用:

this.n1 = new Number();
this.n2 = new Number();

或者使用传入的值(!?):

this.n1 = n1;
this.n2 = n2;