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);
            };              
}}}

indenting, and not having random curly-brace blocks make code easier to read — Sam I am says Reinstate Monica, Mar 19 '13 at 16:18

NPE · Accepted Answer · 2013-03-19T22:11:15.760

6

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.

edited Mar 19 '13 at 22:11

answered Mar 19 '13 at 16:13

NPE

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Yeah I was about to say... If he didn't get an error there, then he rolled rather low – Cruncher Mar 19 '13 at 16:13
Sorry I just tried to give one elemnt value and it didnt work I changed it. – user1794625 Mar 19 '13 at 16:14
@Cruncher: He couldn't have. The `+15` makes this fail every single time. – NPE Mar 19 '13 at 16:14
@NPE: yeah I realised after I posed – Cruncher Mar 19 '13 at 16:15
it might be worthwhile to edit the failing code excript to reflect the op's updated code. just so that nobody is confused. – Sam I am says Reinstate Monica Mar 19 '13 at 16:22

Laurent S. · Answer 2 · 2013-03-19T16:20:42.907

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);

edited Mar 19 '13 at 16:20

answered Mar 19 '13 at 16:14

Laurent S.

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score 1 · Answer 3 · edited Dec 17 '16 at 21:54

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 + " ");
        }

    }              
}

For java7 and above it is best to use `ThreadLocalRandom`. Take a look e.g. [here](http://stackoverflow.com/a/363692/3519000) — lrnzcig, Dec 17 '16 at 20:05

Beginner problems in java

3 Answers3