Question

我觉得至少打印出每个子类的toString方法是有点正确的，但由于某种原因，它不会被打印出任何东西。任何帮助将不胜感激。

data.txt中：

1 meter
1 inch
1 foot
1 yard
401.336 meters
15839 inches
1319 feet
439 yards

码

public abstract class Length implements Comparable<Length> {

    public String toString() {
        return this.getClass() + ": " + getLength() + " " + getUnit();
    }



public static void main(String[] args){
        ArrayList<Length> lo = new ArrayList <Length>();
        Scanner in = null;
        try {
            in = new Scanner(new File("src/length/data.txt"));
        } catch (FileNotFoundException exception) {
            throw new RuntimeException("failed to open data.txt");
        }
        // need more code for other parts of this assignment
        while (in.hasNextDouble()) {
            double value = in.nextDouble();
            String unit = in.next();
            Length length = null;

            if(unit.equals("Meter")){
                length = new Meter(value);
                lo.add(length);             
            }

            else if(unit.equals("Foot")){

                length = new Foot(value);
                lo.add(length);
            }

            else if(unit.equals("Inch")){
                length= new Inch(value);
                lo.add(length);
            }

            else if(unit.equals("Yard")){
                length = new Yard(value);
                lo.add(length);
            }

        }
        Length[] myLengths = new Length[lo.size()]; //declares myLengths integer array 
        for(int i=0; i < lo.size(); i++){       
            myLengths[i]=lo.get(i);     //copies arrayList to lo to Length array myLengths

        }


    for(int i = 0; i < myLengths.length; i++) {
            System.out.println(myLengths[i].toString());  
        }
    }   
}

Answer 1

此代码中的问题是字符串比较。请使用unit.equalsIgnoreCase（“Inch”）代替unit.equals（“Inch”）等。更正代码：

public abstract class Length implements Comparable<Length> {

    public String toString() {
        return this.getClass() + ": " + getLength() + " " + getUnit();
    }

    public static void main(String[] args) {
        ArrayList<Length> lo = new ArrayList<Length>();
        Scanner in = null;
        try {
            in = new Scanner(new File("src/length/data.txt"));
        } catch (FileNotFoundException exception) {
            throw new RuntimeException("failed to open data.txt");
        }
        // need more code for other parts of this assignment
        while (in.hasNextDouble()) {
            double value = in.nextDouble();
            String unit = in.next();
            Length length = null;

            if (unit.equalsIgnoreCase("Meter")) {
                length = new Meter(value);
                lo.add(length);
            } else if (unit.equalsIgnoreCase("Foot")) {

                length = new Foot(value);
                lo.add(length);
            } else if (unit.equalsIgnoreCase("Inch")) {
                length = new Inch(value);
                lo.add(length);
            } else if (unit.equalsIgnoreCase("Yard")) {
                length = new Yard(value);
                lo.add(length);
            }

        }
        Length[] myLengths = new Length[lo.size()]; //declares myLengths integer array 
        for (int i = 0; i < lo.size(); i++) {
            myLengths[i] = lo.get(i);     //copies arrayList to lo to Length array myLengths

        }


        for (int i = 0; i < myLengths.length; i++) {
            System.out.println(myLengths[i].toString());
        }
    }
}

Answer 2

单位字符串比较中的代码存在两个问题。

其他答案提到，在代码中你正在使用“英寸”，而在文件中你正在使用“英寸”。可以使用equalsIgnoreCase()
即使您更改了1，您仍将面临阅读不完整文件的问题。原因是在文件中，您将遇到单数和复数单位名称。它可以通过像if (unit.equalsIgnoreCase("foot") || unit.equalsIgnoreCase("feet"))

为什么我的for循环对象没有打印出来？

2 个答案: