In the below code , i use RunTimeException to control when scanner finishes reading data. Reasons for this being

1. I cannot use any other library than java.lang
2. I cannot use Scanner either , We have been provided a StdIn class which has a static scanner.next() method only for reading strings. My stdin class in the code below is an attempt to post a SSCCE .

Note : All the code is inside the main() because that is how the api demands it.

Can someone review my code , and possibly tell if there's some other way i can do this better ?

import java.io.BufferedInputStream;
import java.util.Scanner;

public class Subset {
    
    public static void main(String[] args) {
        int k = Integer.parseInt(args[0]);
        String[] arr = new String[k];
        int i = 0;
        String str;     
        while((str=stdin.readString())!=null && i < arr.length){
            arr[i++] = str;
            // System.out.println(str);
        }
        try{
            for(; (str=stdin.readString())!=null; i++){
                int r = (int)(Math.random()*(i + 1));
                if(r < k){
                    arr[r] = str;
                    //System.out.println(str);
                }
            }
        }catch(RuntimeException e){ // do nothing }     
        for(String s : arr){
            System.out.print(s + " ");
        }
        System.out.println("\n");
    }
}
class stdin{
    private stdin(){}
    private static Scanner sc = new Scanner(new BufferedInputStream(System.in));
    public static String readString(){
        return sc.next();
    }
}

Command line input : echo A B C D E F G H I J | java Subset 3
Output A F C