i have exceed number error in nextInt() of util.Random in Java 1.8.74 (Intellij Idea 2016.1.1)

Question

I tried to get a random number between 3 and 6 (inclusive):

public class A {              

 Random ran = new Random();                            
 public func1(){            
  x = 3 + ran.nextInt(7); // need Random number x from 3 to 6   
  y = 3 + ran.nextInt(7)             
 }
}    

but it often gives results 8 and 9! How it can be ?

I used

 x=(int) (3 + Math.random()*6);  

its give 8 sometimes too...
where is an Error???

Answer 1

3 + ran.nextInt(7); // need Random number x from 3 to 6   

ran.nextInt() will give you numbers between 0 (inclusive) and 7(exclusive). If you add 3, you'll end up with range 3 - 9.

Answer 2

This is a good example of why you need write your own code and understand what you are writing.

   x = 3 + ran.nextInt(7);

If you read the javadoc for Random.nextInt(int) it says:

Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator's sequence.

So nextInt(7) is going to return a value between 0 (inclusive) and 7 (exclusive); i.e. 0 or 1 or 2 or 3 or 4 or 5 or 6.

That means that

   x = 3 + ran.nextInt(7);

gives you 3 or 4 or 5 or 6 or 7 or 8 or 9. That is what you are seeing.

The solution? How to generate a number in the range 3, 4, 5, 6?

Work it out for your self¹. The exercise will do you good!

---

Lessons:

1. Read the javadocs.
2. Don't just copy code fragments. Copying code is not proper programming.

---

^{1 - SPOILER: x = 3 + ran.nextInt(4);}