如何修复数组OutOfBoundsException？

Question

I'm working on a project for school where we have to spin a cube a number of times and find the longest run of the numbers that are received from the cube. I am almost finished with the code, and it complies, but whenever I run it I get the same error:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
    at number_cube.number_cube.cubeToss(number_cube.java:20)
    at number_cube.number_cube.main(number_cube.java:10)

Can anybody help me with this?

Here is my code:

public class number_cube { 
    public static int ans;
    public static void main(String[]args){
        Scanner scan = new Scanner(System.in);
        System.out.println("How many times would you like to toss the cube?");
        ans = scan.nextInt();
        cubeToss();
        getLongestRun();
        System.out.println();
    }

    public static int arr[] = new int[ans];

    public static int[] cubeToss(){
        for(int i = 0; i < ans; i++){
            int randnum = (int) (1 + Math.random() * (6-1));
            arr[i] = randnum;
        }
        return arr;
    }

    public static void getLongestRun(){
        int longest = 0;
        int length = 1;
        for(int i = 1; i < ans; i++)
            if(arr[i] == arr[i-1]){
                length++;
            } 

            else{
                length = 1;
            }

        if(length > longest){
            longest = length;
        }

        System.out.println("The longest run is " + longest + ".");
    }
}

Answer 1

You used the variable ans as the length of your arr array, but when the array was created, ans didn't have a value (Java initialized it to 0), so the arr array has length 0. The main method isn't called until the class is initialized; all static variables are initialized before the main method is called.

Don't create the array until you have a valid length:

ans = scan.nextInt();
arr = new int[ans];  // Add this line.
cubeToss();

Answer 2

You need to initialize the array after you got the value for ans. Currently you are initialize the Array with ans = 0 at the startup for the program (where ans is 0).

A correct solution may be:

 public static void main(String[]args){
        Scanner scan = new Scanner(System.in);
        System.out.println("How many times would you like to toss the cube?");
        arr[] = new int[scan.nextInt()];

Answer 3

public static int arr[] = new int[ans];

From the jls-12.4: when The JVM is still trying to execute the method main of class number_cube , the invocation is permitted only if the class has been initialized. However, Initialization of a class consists of executing its static initializers and the initializers for static fields (class variables) declared in the class.

That is why, static fields declared in class context always gets instantiated first. For your class which are: int[] arr and int ans . static variable ans initialized to the value 0 which is default value of integer. Hence, Instantiating the static array arr has the length 0 and being empty. When you are trying to access it in the cubeToss() function using a loop, an AIBE 0 is being thrown as the array has length 0 (or, the array is empty).

So, after reading the ans in the main method in the main method using ans = scan.nextInt(); , try creating the array in the main method: arr = new int[ans]

Answer 4

public static int[] arr;

然后仅在从System.in获取ans之后初始化