java - 在子类的函数中使用字符串在 Java 中获取 null

Question

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已关闭 10 年前。

我最近一直在学习 Java，它与大多数其他语言有很大不同。我和其他一些人一直站在那里摸不着头脑，不明白为什么有些东西不起作用，这是我所看到的一个问题。这。时间。这令人沮丧和恼怒，希望我能为其他人阐明这一点。

有没有问过自己这个问题？:

“我已经尝试让它工作好几个小时了，但我似乎看不到它。我试图将字符串“reducerFractions”传递给 FractionReducer 类，但不知何故，当它进入该类时，它变成了空？

我对 Java 很陌生，这只是我用它编写的第二个程序，我不太明白为什么它不起作用......

父类:

public class FractionCalc extends FractionUI
{
//member variables for this class initialized here, all are kept within this class
protected String reducerFractions;
{
/*Paramiters in here math stuff ect*/
finalNum = (int)fin_num;
finalDem = (int)fin_dem;
reducerFractions = finalNum + "/" + finalDem; 
//puts these together in a protected string

System.out.println(reducerFractions); //<- testing here shows its working as needed
reducer.setFraction(finalNum, finalDem);
//overloading the function shows it's working for the two ints as ints themselves
reducer.setFraction(reducerFractions);
int rNum = reducer.getResultsNum();
int rDenom = reducer.getResultsDenom();
String debug = rNum + " " + rDenom;
System.out.println(debug); 
//right here gives back the correct numbers using an int approach
}

子类:

public class FractionReducer extends FractionCalc
{
  public void setFraction(String a)
{
    System.out.println(reducerFractions);
    //Right here it's saying it's null, I don't understand why...
    String[] stringReducedFraction = reducerFractions.split("/");
    double numerator = Float.valueOf(stringReducedFraction[0]);
    double denominator = Float.valueOf(stringReducedFraction[1]);
    //split up the string here for other uses
 }
//Other stuff
}

终端输出

2/1 //The string prints out fine in the parent class
null//but null in the child?
Exception in thread "main" java.lang.NullPointerException
    at FractionReducer.setFraction(FractionReducer.java:25)
    at FractionCalc.FractionCalc(FractionCalc.java:73)
    at FractionUI.Run(FractionUI.java:47)
    at MainP2.main(MainP2.java:19)

这里有一个明显的(对大多数人来说)问题，但只有当您经常使用对象时才会明显。

Answer 1

据我所知，您将 reducerFractions 作为 String 参数 a 传递到 FractionReducer 中。因此，如果您将对 reducerFractions 的引用替换为对 a 的引用，它应该适合您。像这样:

     public void setFraction(String a)
{
    System.out.println(a);
    //Right here it's saying it's null, I don't understand why...
    String[] stringReducedFraction = a.split("/");
    double numerator = Float.valueOf(stringReducedFraction[0]);
    double denominator = Float.valueOf(stringReducedFraction[1]);
    //split up the string here for other uses
 }

使字符串 protected 且静态并不是真正的正确方法。如果您在 FractionReducer 中需要它，您应该将其传入。