Why return happened resource leak but I do close Scanner(System.in)

Question

Java show the warning mean it may cause error. I don't want to ignore the warning of "Resource leak". How to solve resource leak, show in "switch-case" return even write keyin.close()? If I don't write keyin.close(), the "Resource leak" show in Scanner(System.in). It mean "Resource leak" must happen?? That is tricky.

Below is the program. Please help. Thanks a lot.

package JPack001_20210722;
import java.util.Scanner;

public class JClass008_20210722 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int n,sum;
        char addChoice;
        Scanner keyin=new Scanner(System.in);
        
        System.out.print("1+2+...+n=? Please input n: ");
        n=Integer.parseInt(keyin.nextLine());
        System.out.print("Which you need as odd sum(O), even sum(E) or Integer(I)?");
        System.out.print("Please choose: ");
        
        addChoice=keyin.nextLine().charAt(0); 
        /* charAt(index), is char at index. Get the character from the string. 
         * string Odd -> O is index(0), d is index(1), d is index(2)
         * charAt(0), if string is Odd, will get index(0)is 'O'
         * then set the O to addChoice to run switch(addChoice)
         */
        
        switch(addChoice) {
            case 'O':
                sum=odd(n); // call odd(n) function and input n(line 13)
                break;
            case 'E':
                sum=even(n); // call even(n) function and input n(line 13)
                break;
            case 'I':
                sum=totalSum(n); // call totalSum(n) function and input n(line 13)
                break;
            default:
                System.out.println("Your choice is wrong.");
                return; 
                /* return :if had value will return the value to function and transfer control to who's call
                 * return :if no value, just transfer control to JVM, break the program.
                 */
                
        }
        System.out.println("The Total Sum is "+sum);
        keyin.close();
        
    }
    
    public static int odd(int U) {
        int i,total=0;
        for(i=1;i<=U;i++)
            if(i%2==1)
                total=total+i;
        return total;
    }
    
    public static int even(int U) {
        int i,total=0;
        for(i=1;i<=U;i++)
            if(i%2==0)
                total=total+i;
        return total;
    }
    
    public static int totalSum(int U) {
        return odd(U)+even(U);
    }
    
}