Question

我对如何使用和显示数组列表有疑问。在我的情况下，我认为我正在使用数组列表，因为编译器没有抱怨。但是当我显示它时，会出现一个看起来像这样的错误：

run:
Average Monthly Electricity Bill:                             463.26
Exception in thread "main" java.util.IllegalFormatConversionException: f !=  java.util.ArrayList
Average Monthly Electricity Price Per Kilowatt:     at java.util.Formatter$FormatSpecifier.failConversion(Formatter.java:4045)
at java.util.Formatter$FormatSpecifier.printFloat(Formatter.java:2761)
at java.util.Formatter$FormatSpecifier.print(Formatter.java:2708)
at java.util.Formatter.format(Formatter.java:2488)
at java.io.PrintStream.format(PrintStream.java:970)
at java.io.PrintStream.printf(PrintStream.java:871)
at CO2FromElectricityTester.main(CO2FromElectricityTester.java:72)
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)

我该如何解决这个问题？任何帮助将不胜感激。以下是我的代码：

public class CO2FromElectricityTester {

public static void main(String args[]){

    // declare & initialize variables
    double months = 3.0;
    double emissionFactor = 1.37;
    int kilowattHoursSept = 109;
    int kilowattHoursOct = 87;
    int kilowattHoursNov = 93;
    double monthlyCostSept = 551.51;
    double monthlyCostOct = 392.84;
    double monthlyCostNov = 445.42;
    double avgKilowattHours = (kilowattHoursSept + kilowattHoursOct + 
            kilowattHoursNov) / 3;
    double avgMonthlyCost = (monthlyCostSept + monthlyCostOct + 
            monthlyCostNov) / 3;

    // create object
    CO2FromElectricity CO2 = new CO2FromElectricity();

    // // // declare & initialize variables for methods

    // // create array lists
    ArrayList<Double> monthlyPriceForElec = new ArrayList<Double>();
    ArrayList<Double> monthlyElectricBill = new ArrayList<Double>();

    // average monthly price for electricity
    double avgPricePerKilowatt = CO2.calcPricePerKilowatt(avgKilowattHours, 
            avgMonthlyCost);

    // average monthly electric bill
    double avgCO2Emission = CO2.calcCO2Emission(emissionFactor, months, 
            avgMonthlyCost, avgPricePerKilowatt);

    // initialize array lists
    monthlyPriceForElec.add(avgPricePerKilowatt);
    monthlyElectricBill.add(avgCO2Emission);

    // display results
    System.out.printf("%1s%34.2f%n", "Average Monthly Electricity Bill: ", 
            avgMonthlyCost);
    System.out.printf("%1s%20.2f%n", "Average Monthly Electricity Price Per "
            + "Kilowatt: ", monthlyPriceForElec);
    System.out.printf("%1s%10.2f%n", "CO2 Emissions from Electricity Usage "
            + "in a 3 Month Period: ", monthlyElectricBill);
}

}

如果有帮助，下面是我声明方法的其他文件：

public class CO2FromElectricity {

// default constructor
CO2FromElectricity(){

}

// method for calculating price per kilowatt
public double calcPricePerKilowatt(double kilowattHours, double monthlyCost){
    return monthlyCost / kilowattHours;
}

// method for calculating CO2 emission
public double calcCO2Emission(double emissionFactor, double months, 
        double avgMonthlyCost, double avgPricePerKilowatt){
    return (avgMonthlyCost / avgPricePerKilowatt) * emissionFactor * months;
}

}

Answer 1

错误很明显。您正在尝试格式化指定浮点格式的ArrayList。你的问题有点不清楚，但不是

System.out.printf("%1s%20.2f%n", "Average Monthly Electricity Price Per "
        + "Kilowatt: ", monthlyPriceForElec); // you want the result

我想你想要

System.out.printf("%1s%20.2f%n", "Average Monthly Electricity Price Per "
        + "Kilowatt: ", avgPricePerKilowatt); // like this

然后在下一行，您需要avgCO2Emission而不是monthlyElectricBill。

如果您想打印整个列表，则需要分别迭代列表。

Answer 2

协助，不是答案的一部分。使用接口而不是具体类。不要这样做：

 ArrayList<Double> monthlyPriceForElec = new ArrayList<Double>();

请改为：

List<Double> monthlyPriceForElec = new ArrayList<Double>();

列表（包括ArrayList）是数据的集合。通过循环遍历列表并显示每个元素来显示列表的内容。

List<Double> hooty = new ArrayList<Double>();
... put stuff in hooty.
for (Double blammy : hooty)
{
    System.out.printf("blammy not hooty: %20.2f", blammy);
}

Answer 3

System.out.printf没有对Collections的特殊支持，它会将它视为常规对象（不是Iterable）。如果要格式化集合中的项目，则需要循环遍历它们并在每个项目上调用System.out.printf（）。

Answer 4

在你做打印语句的地方，你放了“System.out.printf”。我认为您应该放置“System.out.println”，否则编译器只会有一个巨大的脑放屁。我认为这是错误，但我不确定。尝试将打印语句的“f”部分更改为“ln”，看看是否有效。

Java显示和使用数组列表

4 个答案: