Question

我很困惑为什么这个数组（cumulativeWeights）被认为是null。这是我的（简化）代码：

public class WeightedDieTester
{
    public static void main(String[] args)
    {
        WeightedDie d = new WeightedDie();
    }
}

public class Die
{
    public Die()
    {
        this.roll();
    }
}

public class WeightedDie extends Die {
    double[] cumulativeWeights = {.1, .2, .3, .4, .5, 1};

    public WeightedDie() {
    }
    public int roll() {

        for (int i = 0; i < cumulativeWeights.length; i++) { //this is where the nullpointerException is occuring
            if (randNum < cumulativeWeights[i]) {
                return i + 1;
            }
        }
        return 6;
    }
}

所以是的，cumulativeWeights已初始化，不应为null。有谁知道我为什么会得到nullpointerexception？

Answer 1

Java按此顺序执行

Parent static block
Child static block
Parent initialization  block
Parent Constructor
Child initialization block
Child Constructor

因此，当您从Die调用roll时，数组尚未初始化。

Answer 2

在超类的构造函数内部，类的字段未初始化。你应该从这一点开始调用roll。从最远的超类到实例的类初始化实例（并调用构造函数）。

Answer 3

我花了很多时间自己搞清楚，

首先，您在代码中已经犯了一些基本错误

您不应该从构造函数中调用可覆盖的方法。在这里，您调用roll()函数，该函数在子类中被覆盖。
```
public Die()
{
    this.roll(); // shouldn't be done.
}
```

2.您没有收到编译时错误，因为您已在子类中声明了变量cumulativeWeights。由于WeightedDie的对象尚未完成，因此尚未初始化这些变量的值。

3.由于值未初始化为cumulativeWeights，因此您将获得空指针异常。（但它已经声明，所以编译器让你编译没有错误）

4.查看以下格式化代码的输出。下次从构造函数中调用可覆盖的方法时要小心

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;
class Die
{
    public double[] cumulativeWeights = {.1, .2, .3, .4, .5, 1};
    public Die()
    {
        System.out.println("constructing die");
        this.roll();
    }
    public int roll(){
        return 1;
    }
}
class WeightedDie extends Die {
     int randNum = 33; // value 0 is used because randNum isn't even initialized
    // even in your code!! cumulativeWeights is just declared, so no compile time error, you are using it even before the constructor of the child class is called!!!!
    public WeightedDie() {
        System.out.println("constructing weighteddie");
    }
    @Override
    public int roll() {
        System.out.println("using weighteddie and randNum = "+randNum);
        for (int i = 0; i < cumulativeWeights.length; i++) { //this is where the nullpointerException is occuring
            if (randNum < cumulativeWeights[i]) {
                return i + 1;
            }
        }
        return 6;
    }
}
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main(String[] args)
    {
        WeightedDie d = new WeightedDie();
    }
}

输出：

constructing die
using weighteddie and randNum = 0
constructing weighteddie

我希望你明白我的观点。干杯！

Answer 4

如果cumulativeWeights不是特定于对象的数据，则将其设为静态。

为什么这个数组在初始化时是null？

4 个答案: