在 Java 中对数组列表进行排序时无法转换为类错误

Question

I'm trying to add a number to an object array list and I keep getting an 'Cannot be cast' error.

I'm fairly new to Java so please go easy on me

My code is below, any help would be appreciated. If anyone has any questions please contact me

public class runningClass {

    public class Sortbymake implements Comparator<runningClass> 
    { 
        @Override
        public int compare(runningClass a, runningClass b) 
        { 
            return b.numb.compareTo(a.numb);
        }
    }

    private String numb;

    public runningClass(String nNumb)
    {
        this.numb = nNumb;
    }

    public String getNumber(){
        return numb;
    }

    public void oof()
    {
        Scanner scan = new Scanner(System.in);
        while(ptr <= 5)
        {
            System.out.println("What number do you want to add");
            System.out.println("Ptr = "+ptr);
            this.numb = scan.next();
            numberList.add(new runningClass(this.numb));
            ptr++;
        }
        Collections.sort(numberList, null);
    }

    public void searchList()
    {
        int Sptr = 0;
        Scanner scan = new Scanner(System.in);
        System.out.println("What is the number your searching for?");
        String userSearch = scan.next();
        for(int u = 0; u < numberList.size();u++)
        {
            //loop is created, will check if any index contains the entered text
            if(numberList.get(Sptr).getNumber().contains(userSearch))
            {
                System.out.println("The "+userSearch+" is located at index "+u);
                break;
            }
            else
            {
                Sptr++;
            }
        }
    }

}

And here's the error I'm getting, please contact me if you want me to post the full error

Exception in thread "main" java.lang.ClassCastException: class arraysearch.runningClass cannot be cast to class java.lang.Comparable (arraysearch.runningClass is in unnamed module of loader 'app'; java.lang.Comparable is in module java.base of loader 'bootstrap')

Answer 1

Your class RunningClass is not implementing Comparable and Collections.sort is expecting a Comparable since you are not specifying any comparator instance.

Why do you define a new class Sortbymake that implementes Comparator and never use it anywhere?

I think you don't understand the difference between Comparator and Comparable (from source ):

• Comparable : Comparable is an interface defining a strategy of comparing an object with other objects of the same type. This is called the class's “natural ordering”;
• Comparator : The Comparator interface defines a compare(arg1, arg2) method with two arguments which represent compared objects and works similarly to the Comparable.compareTo() method.

Now, you have two solutions (and the first one feels the right one to me):

Use the Comparable interface in this case on your RunningClass and call the Collections.sort(numberList) method. This will make use of the implementation of the compareTo method enclosed in the RunningClass class.

Use the Sortbymake class but specify the comparator when calling the sort method: Collections.sort(numberList, new Sortbymake())