Question

我正在尝试制作一个程序来计算一系列BankAccount余额的平均值，同时过滤那些低于1000的那些。我们应该使用一个Measurable和Filter接口，所以我创建了一个方法来过滤BankAccounts数组并返回一个已过滤的数组，并使用该数组来计算平均值，这就是我所拥有的：

public class Data{
   public static double average(Object[] objects, Measurable meas){
      double sum = 0;
      for(int i = 0; i < objects.length; i++){
        sum = meas.measure(objects[i]) + sum;
      }
      if(objects.length > 0)
        return sum / objects.length;
      else 
        return 0;
   }

   public static Object[] filter(Object[] objects, Filter filter){
     Object[] newarray = new Object[objects.length];
     int cntr = 0;
     for(int i = 0; i < objects.length; i++){
        if(filter.accept(objects[i])){
            newarray[i] = objects[i];
            cntr++;
        }
    }
     Object[] result = new Object[cntr];
     System.arraycopy(newarray, 0, result, 0, cntr);
     return result;
}

接口及其实现：

public interface Measurable{
double measure(Object x);
}
public interface Filter{
boolean accept(Object x);
}

public class BankFilter implements Filter{
public boolean accept(Object x){
    BankAccount account = (BankAccount) x;
    return account.getBalance() > 1000;
}

public class BankMeasure implements Measurable{
public double measure(Object x){
    BankAccount account = (BankAccount) x;
    return account.getBalance();
}

和测试它的类：

public class Tester{
public static void main(String[] args){
    Measurable bankMeas = new BankMeasure();
    Filter filter = new BankFilter();
    BankAccount acc1 = new BankAccount(200);
    BankAccount acc2 = new BankAccount(200);
    BankAccount acc3 = new BankAccount(3000);
    BankAccount acc4 = new BankAccount(50);
    BankAccount acc5 = new BankAccount(4000);
    BankAccount[] ar = {acc1, acc2, acc3, acc4, acc5};
    Object[] filteredAcc = Data.filter(ar,filter);
    for(int i = 0; i < filteredAcc.length; i++){
        System.out.println(bankMeas.measure(filteredAcc[i]));
    }

    System.out.println(Data.average(filteredAcc, bankMeas));
    }
}

BankAccount类

public class BankAccount{
private double balance;
public BankAccount(double initialBalance){
    balance = initialBalance;
}
public double getBalance(){
    return balance;
}
}

NPE发生在BankMeasure.measure（BankMeasure.java:4）和Tester.main（Tester.java:13）我已经读过NullPointerException意味着我正在尝试访问或修改null对象，但我不明白为什么会出现这种情况，任何帮助都非常感谢！

为什么我会收到java.lang.NullPointerException错误？

0 个答案: