Adding to a random number

Question

I have used the Random class to generate a random number and the number shows up, but when I try to add that generated random number with 10 it just get concatenated i.e if the random number is 20 it will show as 2010

import java.util.Random;

public class MainClass{
    static int x = 50;
    static  int y = 10;

    public class InnerClass {    
        double myDouble = x;

        void show() {
            System.out.println(myDouble + 5);
        }
    }

    public static void main(String args[]){
        Random rand = new Random();
        MainClass obj1 = new MainClass();
        MainClass.InnerClass obj2 = obj1.new InnerClass();

        int int_random = rand.nextInt(x);

        System.out.println(int_random + " :" + " This is the Random Number and its plus 10 is "+ int_random + y);

        // obj2.show();
    }
}

+ is left associative in Java, meaning "... is " + int_random + y is the same as ("... is " + int_random) + y — Johannes Kuhn
– Johannes Kuhn, Commented Apr 20, 2020 at 8:40
I've removed the random tag, you'd have the same issue with printing fixed values the way you've done it. Randomness has nothing to do with the result. — pjs
– pjs, Commented Apr 20, 2020 at 15:39

Thomas · Accepted Answer · 2020-04-20 08:41:43Z

3

The + operator in Java can do two very different things: adding two numbers, and concatenating two strings. If you "add" a number to a string, the number is first converted into a string, and then the two strings are concatenated.

The other piece of the puzzle is that + is applied left to right (it's left-associative). So if you write a + b + c, that means (a + b) + c and not a + (b + c). So if either a is a string and b and c are integers, (a + b) will be a string, and c is converted to a string before being added to it.

The solution is to force the order of evaluation using parentheses: write a + (b + c) explicitly, or in your case:

    System.out.println(int_random + " :" + " This is the Random Number and its plus 10 is "+ (int_random + y));

answered Apr 20, 2020 at 8:41

Thomas

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3 Comments

Arvind Kumar Avinash Over a year ago

So if you write a + b + c, that means (a + b) + c - This is wrong. int x = 5, y = 10, z = 15; System.out.println("Hello" + x + y + z); will print Hello51015 . As per your explanation, it would have printed Hello1515.

Coderino Javarino Over a year ago

@ArvindKumarAvinash As per his explanation, you would add "Hello" and x first, not x and y like you imply with a result Hello1515. His explanation is correct

Arvind Kumar Avinash Over a year ago

I've got it. Thank you. As per his explanation, a is Hello in my expression and therefore it will be evaluated as ("Hello" + 5) + 10 and finally, "Hello510" + 15 which will be evaluated to Hello51015.

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