Must explicitly convert the char[] to a String

package com.java.learn;

import java.util.Random;

public class RandomDemo {

String str = "0123456789";

public static void main(String[] args) {

    int length = 10;

    System.out.println("otp is" + getOtp(length));//Must explicitly convert the char[] to a String

}

public static char[] getOtp(int len) {
    // TODO Auto-generated method stub

    String str = "0123456789";

    Random random = new Random();
    char[] otp = new char[len];
    for (int i = 0; i < len; i++) {

        otp[i] = str.charAt(random.nextInt(str.length()));

    }
    return otp;

}

}

While printing the otp why it's giving me warning of that i need to convert char[] to String explicitly.But when i print it without "otp is" it runs successfully.