Beginner problems in java

Question

I have been stuck for about an hour and after googling and doing research I could't get my code to run. It shows not a single error and when I press run it just opens debug and does nothing. I am using eclipse. I am trying to create a list of 10 objects and to give them random numbers.

class test {
public static void main(String[] args){
 int a [] = new int[9];{
 for (int i = 0; i < a.length; i++)
 a[i] = a[(int)(Math.random()*70+15)];
 for (int elem : a){
 System.out.println(elem);
 }; 
}}}

• Sam I am says Reinstate Monica: indenting, and not having random curly-brace blocks make code easier to read

Answer 1

If you are actually launching the application, it should fail with an exception at the following line:

 a[i] = a[(int)(Math.random()*70+15)];

Here, a[] consists of nine elements, so its highest index is 8. However, Math.random()*70+15 is guaranteed to generate numbers that are greater than 8.

• Cruncher: Yeah I was about to say... If he didn't get an error there, then he rolled rather low
• user1794625: Sorry I just tried to give one elemnt value and it didnt work I changed it.
• NPE: @Cruncher: He couldn't have. The +15 makes this fail every single time.
• Cruncher: @NPE: yeah I realised after I posed
• Sam I am says Reinstate Monica: it might be worthwhile to edit the failing code excript to reflect the op's updated code. just so that nobody is confused.

Answer 2

I don't know any Java, but I would say :

for (int i = 0; i < a.length; i++)
a[2] = a[(int)(Math.random()*70+15)];

should be

for (int i = 0; i < a.length; i++)
a[i] = (int)(Math.random()*70+15);

Answer 3

I would suggest using the Random number generator instead. I would also suggest using better names than i or a for your programs.

import java.util.Random;
class test {
 public static void main(String[] args){
 Random object = new Random ();//declare for your object
 int a; //declare your integer type (I would suggest 
 // changing that to be more descriptive)
 for (int i = 1; i <=10; i++) 
 {
 a = object.nextInt(100); // change 100 to however large 
 // parameter you want
 System.out.println(a + " ");
 }
 } 
}

• lrnzcig: For java7 and above it is best to use ThreadLocalRandom. Take a look e.g. here