Question

我正在尝试制作手机资费计算器。但是我得到了nullpointerexceptions。无法弄清楚他们来自哪里。帮助赞赏!!

注意我有另一个接受用户输入到列表中的类。但那是按预期工作的

  import java.util.ArrayList;
  import java.util.List;

public class TariffCalc {

public void calculator(List<Integer> tariff){

    int whattariff = 0;
    int mins = 0;
    int texts = 0;
    int surchargeMin = 0;
    int surchargeText = 0;
    int totalsurcharge= 0;

    for( Integer s: tariff ){
        whattariff = tariff.get(0);
        mins = tariff.get(1);
        texts = tariff.get(2);
    }

 if ( whattariff == 1 && mins <= 200 && texts <= 150 ){
     System.out.println("Tariff 1: Within Allowance: £20 per month: Mins used: " + mins + " Texts used: " + texts );
 }
 else if( whattariff == 1 && mins > 200 ){
     surchargeMin = mins - 200;
     surchargeMin = (int) (surchargeMin * 0.1);
 }

 if ( whattariff == 1 && texts > 150 ){
     surchargeText = texts - 150;
     surchargeText = (int) (surchargeText * 0.05);
 }



 if ( whattariff == 2 && mins <= 400 && texts <= 350 ){
     System.out.println("Tariff 2: Within Allowance: £35 per month");
 }
 else if ( whattariff == 2 && mins > 400 ){
     surchargeMin = mins - 400;
     surchargeMin = (int) (surchargeMin * 0.1);
 }

 if ( whattariff == 2 && texts > 350 ){
     surchargeText = texts - 350;
     surchargeText = (int) (surchargeText * 0.05);
 }

 totalsurcharge = surchargeMin + surchargeText;

 if( whattariff == 1)
 System.out.println("Tariff 1: Allowance 200mins + 150 text. Used: " + mins + " + " + texts + ": Total Cost is £" + (20 + totalsurcharge) );
 else if (whattariff == 2){
     System.out.println("Tariff 2: Allowance 400mins + 350 text. Used: " + mins + " + " + texts + ": Total Cost is £" + (35 + totalsurcharge) );
 }

}
public static void main(String[] args) {


    TariffCalc c = new TariffCalc();
    c.calculator(null);

}

}

Answer 1

您在此行中传递null个对象：

c.calculator(null);

并且您尝试迭代此行

for( Integer s: tariff )

这使您的代码抛出异常。

你应该这样做：

List<Integer> tariffs = new ArrayList<Integer>();
tariffs.add(1);
tariffs.add(2);
tariffs.add(3);
c.calculator(tariffs);

给了我输出：

Tariff 1: Within Allowance: £20 per month: Mins used: 2 Texts used: 3
Tariff 1: Allowance 200mins + 150 text. Used: 2 + 3: Total Cost is £20

Answer 2

您正在将null传递给calculator()方法并尝试在该方法中迭代它。

c.calculator(null); // passing null
....
public void calculator(List<Integer> tariff) { // tariff is null as you passed null from main method
...
for (Integer s : tariff) { // Trying to iterator over null, hence, NPE
    whattariff = tariff.get(0);
    mins = tariff.get(1);
    texts = tariff.get(2);
}

您可能需要向calculate方法发送一个Integers列表。像这样的东西

// In main method
List<Integer> tariff = new ArrayList<>(); // new integer list for example
tariff.add(1); // add 1
tariff.add(2); // add 2
c.calculator(tariff); // pass the list to calculate method

电话关税计算器

2 个答案: