Given the following two classes: ``java public class Customer { ElectricAccount acct = new ElectricAccount(); public void useElectricity(double kWh) { acct.addKwh(kWh); } } public class…

Question

public class Customer {
 ElectricAccount acct = new ElectricAccount();

 public void useElectricity(double kWh) {
 acct.addKwh(kWh);
 }
}

public class ElectricAccount {
 private double kwh;
 private double rate = 0.07;
 private double bill;

 //line n1
}

Options

• A

  public void addKwh(double kWh) {
   this.kwh += kWh;
   this.bill = this.kwh*this.rate;
  }
  

• B

  public void addKwh(double kWh) {
   if (kWh > 0) {
   this.kwh += kWh;
   this.bill = this.kwh * this.rate;
   }
  }
  

• C

  private void addKwh(double kWh) {
   if (kWh > 0) {
   this.kwh += kWh;
   this.bill = this.kwh*this.rate;
   }
  }
  

• D

  public void addKwh(double kWh) {
   if (kWh > 0) {
   this.kwh += kWh;
   setBill(this.kwh);
   }
  }
  
  public void setBill(double kWh) {
   bill = kWh*rate;
  }
  

• A
  11% (3)
• B
  56% (15)
• C
  26% (7)
• D
  7% (2)

Explanation

Option B is correct because it satisfies all three constraints simultaneously: it is public so Customer can call it, the if (kWh > 0) guard prevents a customer from passing a negative value to reduce the bill, and bill is immediately recalculated as kwh * rate on every valid update.

Option A fails the tamper-proof requirement - without the positive check, a Customer could call addKwh(-100) to fraudulently reduce their bill.

Option C has the right logic but uses private access, which means Customer.useElectricity() can't call acct.addKwh(kWh) at all - it won't even compile.

Option D introduces a public setBill() method, which hands the Customer a direct backdoor to set bill to any arbitrary value, breaking the tamper-proof requirement.

Memory tip: Two locks, one key - the method needs to be public (so Customer can reach it) but the door inside must check kWh > 0 (so Customer can't abuse it). If anything else is public or the check is missing, one of those locks is broken.

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