错误：变量可能尚未初始化 - 如何初始化ArrayList？

Posted 2021-05-06

我已经审查了所有以前的问题，并且标题与此类似，但我无法找到解决方案。所有错误都表明我没有初始化ArrayList ..我是不是用= new ArrayList<Double>初始化ArrayList？

import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Scanner;
import java.io.PrintWriter;

public class main
{
    public static void main (String[] args) throws FileNotFoundException
    {
        ArrayList<Double> RPM, bCoeffs, filteredRPM = new ArrayList<Double>();
        Scanner RPMFile = new Scanner(new File("RotationSpeed.txt"));

        while(RPMFile.hasNextLine()){
            String line = RPMFile.nextLine();

            Scanner scanner = new Scanner(line);
            scanner.useDelimiter(",");
            while(scanner.hasNextDouble()){
                RPM.add(scanner.nextDouble());
            }
            scanner.close();
        }
        RPMFile.close();

        int windowSize = 10;
        int filterItterations = 1;

        for (int i = 0; i < windowSize; i++){
                double temp = 1/windowSize;
                bCoeffs.add(temp);
        }

        for (int k = 1; k <= filterItterations; k++){
            if (k == 1){
                for (int n = windowSize; n < RPM.size(); n++){
                    int m = 0;
                    double tempYSum = 0;
                    for (int j = 0; j < windowSize; j++){
                        double tempY = (bCoeffs.get(j))*(RPM.get(n-m));
                        tempYSum += tempY;
                        m++;
                    }
                    filteredRPM.add(tempYSum);
                }
            }else{
                int i = 1;
                for (int n = windowSize; n < filteredRPM.size(); n++){
                    int m = 0;
                    double tempYSum = 0;
                    for (int j = 0; j < windowSize; j++){
                        double tempY = (bCoeffs.get(j))*(filteredRPM.get(n-m));
                        tempYSum += tempY;
                        m++;
                    }
                    filteredRPM.set(i, tempYSum);
                    i++;
                }
            }
        }
    }
}

我收到的错误如下：

main.java:20: error: variable RPM might not have been initialized
                RPM.add(scanner.nextDouble());
                ^
main.java:31: error: variable bCoeffs might not have been initialized
                                bCoeffs.add(temp);
                                ^
main.java:36: error: variable RPM might not have been initialized
                                for (int n = windowSize; n < RPM.size(); n++){
                                                             ^
main.java:40: error: variable bCoeffs might not have been initialized
                                                double tempY = (bCoeffs.get(j))*(RPM.get(n-m));
                                                                ^
main.java:52: error: variable bCoeffs might not have been initialized
                                                double tempY = (bCoeffs.get(j))*(filteredRPM.get(n-m));
                                                                ^
5 errors

答案

只需分配变量：您当前正在声明3个变量，但只为最后一个赋值。

ArrayList<Double> RPM = new ArrayList<>(), bCoeffs = new ArrayList<>(), filteredRPM = new ArrayList<>();

注意，有些人不鼓励在同一行上声明许多变量，例如： it is forbidden by Google's style guide：如果你每行做一个，它会更容易阅读：

ArrayList<Double> RPM = new ArrayList<>();
ArrayList<Double> bCoeffs = new ArrayList<>();
ArrayList<Double> filteredRPM = new ArrayList<>();

另一答案

虽然您的第一行可能看起来像初始化所有三个ArrayLists，但它基本上执行以下操作：

ArrayList<Double> RPM;
ArrayList<Double> bCoeffs;
ArrayList<Double> filteredRPM = new ArrayList<Double>();

如果它按照您可能想要的方式工作，但您会创建许多需要澄清的潜在问题。例如：现在所有三个变量都指向相同的内存（换句话说是同一个ArrayList）？或者它们都指向不同的ArrayList。

有一个原因是不鼓励在一行中初始化几个变量，因为它非常混淆。

你这样做的正确方法是：

ArrayList<Double> RPM = new ArrayList<>();
ArrayList<Double> bCoeffs = new ArrayList<>();
ArrayList<Double> filteredRPM = new ArrayList<>();