Java - Error en tiempo de ejecucion

import java.util.Scanner;

public class Array_Of_Int_1

{//START CLASS

	Scanner input = new Scanner (System.in); // READ FROM THE KEYBOARD

		//FIELS:

		public int array[];

		public int number;

		public int length;

		public String st ="";

		//CONSTRUCTOR:

		public Array_Of_Int_1 ()

		{

		}

		//POPULATE ARRAY METHOD:

		public int Populate_Array (int array[])

		{//START POPULATE ARRAY

            System.out.println("How much numbers do you want to add?");

            length = input.nextInt();

            array = new int[length];

            int value = 0;

            for(int ind = 0; ind < array.length;ind++)

            {//START FOR

            	System.out.println("What is the " +(ind+1)+ "º value?");

                value =input.nextInt();

                if(value >= 0)

                {//START IF

                	array[ind] = value;

                    number = ind + 1;

                    st+= number + ".- "+array[ind] + "\n";

                }//FINISH IF

                else

                {//START ELSE

                	break;

                }//FINISH ELSE

            }//FINISH FOR

            return number;

		}//FINISH POPULATE ARRAY

		//STRING METHOD:

		public String List ()

		{//START LIST

			return st;

		}//FINISH LIST

		//ADD INTEGERS METHOD:

		public int Add_Integers (int array[], int number)

		{//START ADD INTEGERS

            int sum = 0;

            for(int ind2 =0; ind2 < number; ind2++)

            {//START FOR

            	sum += array[ind2];

            }//FINISH FOR

            return sum;

		}//FINISH ADD INTEGERS

		//BIGGEST METHOD

		public int Biggest (int array[], int number)

		{//START BIGGEST

            int biggest =0;

            for(int ind3=0;ind3 < number;ind3++)

            {//START FOR

                    if(array[ind3] > biggest)

                    {//START IF

                            biggest = array[ind3];

                    }//FINISH IF

            }//FINISH FOR

            return biggest;

		}//FINISH BIGGEST

}//FINISH CLASS

/code]

CLASE PRINCIPAL:

[code]

import java.util.Scanner;

public class Use_Array_Of_int_1 {//START CLASS

	public static void main(String[] args) {//START MAIN

		Array_Of_Int_1 aoi = new Array_Of_Int_1 ();

		int array[] = new int[0];

		int number = aoi.Populate_Array(array);

		String st = aoi.List();

		int sum = aoi.Add_Integers(array, number);

		System.out.println("\n"+ number + " numbers have been added. \n");

		System.out.println("The numbers were: \n");

		System.out.println("-----------------------------------------------------------------");

		System.out.println(st);

		System.out.println("-----------------------------------------------------------------");

		System.out.println("\n The sum of the numbers was: \n");

		System.out.println(sum);

	}//FINISH MAIN

}//FINISH CLASS

import java.util.Scanner;

public class ArrayOfInt1 {

    public int array[];

    public int number;

    public String st = "";

    public void populateArray() {

        Scanner input = new Scanner(System.in);

        System.out.print("How much numbers do you want to add?\nquantity = ");

        int length = input.nextInt();

        array = new int[length];

        System.out.println();

        int value = 0;

        for (int ind = 0; ind < array.length; ++ind) {

            System.out.print("What is the " + (ind + 1) + "º value? ");

            value = input.nextInt();

            if (value >= 0) {

                array[ind] = value;

                ++number;

                st += number + ".- " + array[ind] + "\n";

            } else {

                break;

            }

        }

        input.close();

    }

    public int addIntegers() {

        int sum = 0;

        for (int ind = 0; ind < number; ++ind) {

            sum += array[ind];

        }

        return sum;

    }

    public int biggest() {

        int biggie = 0;

        for (int ind = 0; ind < number; ++ind) {

            if (array[ind] > biggie) {

                biggie = array[ind];

            }

        }

        return biggie;

    }

    public int getNumber() {

        return number;

    }

    @Override

    public String toString() {

        return st;

    }

}

public class UseArrayOfInt1 {

    public static void main(String[] args) {

        ArrayOfInt1 aoi = new ArrayOfInt1();

        aoi.populateArray();

        System.out.println(

            "\n" + aoi.getNumber() + " numbers have been added.\n\n"

            + "The numbers were:\n"

            + "--------------------------------------------------------------\n"

            + aoi

            + "--------------------------------------------------------------\n"

            + "\nThe sum of the numbers was: " + aoi.addIntegers() + "\n"

            + "\nThe biggest number was: " + aoi.biggest()

        );

    }

}