Trouble creating a simple array in Java

Question

I'm having a bit of trouble creating a simple array in Java where a random number is generated every time it loops. My code is as follows:

public class Q1 {
public static void main(String[] args) {
    Scanner listScan = new Scanner(System.in);
    System.out.println("Size of list to sort?");
        int j = listScan.nextInt();


    int listArray[] = new int[j];

    for (int i = 0; i <= j; i++){
        listArray[i] = (int)(Math.random() * 100 +1);
    }

    System.out.println(listArray);


}

}

but the code gives me this:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3
    at Lab3.Q1.main(Q1.java:18)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

Answer 1

int listArray[] = new int[j];  // Assume j =5. creates array of 5 elements.

    for (int i = 0; i <= j; i++){  // iterates from index 0 to 5 i.e, 6 elements . change it to i<j. it will work. 
        listArray[i] = (int)(Math.random() * 100 +1);
    }

Note : You get ArrayIndexOutOfBoundsException when you try to access an index which is greater than the array's length.

Answer 2

You're 1 index too high. Use

for (int i = 0; i < j; i++)

Answer 3

Take a look at this part of your code:

int listArray[] = new int[j];

for (int i = 0; i <= j; i++){
    listArray[i] = (int)(Math.random() * 100 +1);
}

It should be changed to this:

int listArray[] = new int[j];

for (int i = 0; i < j; i++){
    listArray[i] = (int)(Math.random() * 100 +1);
}

Notice how <= was changed to < :

for (int i = 0; i <= j; i++){

vs:

for (int i = 0; i < j; i++){

For pretty much all coding, including java , counting starts at 0 . So where you would normally say 5 , in coding you would say 4 .

Let's say the length of the array is 3 , meaning it contains 0, 1, and 2 . So, when doing this:

for (int i = 0; i <= j; i++){

j = the length of the array, so 3.

for (int i = 0; i <= 3; i++){

now, it will loop threw the numbers 0, 1, 2, and 3 . Yet, our array does not contain a 3, it contains only 0, 1, and 2 . So, if we were to do this:

for (int i = 0; i < j; i++){

j = 3

for (int i = 0; i < 3; i++){

Using this new code, it will loop threw 0, 1, and 2 , the same size of our array, 0, 1, and 2 .

Answer 4

Two problems:

1. You're for-loop is going 1 index position too far. You can't include the actual j value as an index position. The last index position is 1 less than the length. So do this:

    for (int i = 0; i < j; i++) {

2. You're not printing your array properly. You can't print an array like this: System.out.println(array); so you have two options:

    System.out.println( Arrays.toString(listArray) ); // or iterate through the array for (int i = 0; i < listArray.length; i++) { System.out.println(listArray[i]); }

Answer 5

The problem is, the array, in Java is an static structure, and it won´t change it size directly, as you add elements. You can use another types of objects, more suitable as ArrayList.