ArrayList 不会添加我的随机数元素 [JAVA]

Question

this is my first question and if you knew me you'd know id have to be completely lost to be asking a question on here.

My arraylist is continuously empty and refuses to add elements no matter what I do, I am just trying to fill it with random numbers between 1 and 50, then print those numbers as a string. I've been trying to find a solution for 4-5 hours with no luck. I'm not receiving an error when I run my code, but the arraylist is always empty and won't add any elements. If anyone can see what I'm doing wrong I would be very grateful, thank you in advance.

This is my numbers class:

import java.util.ArrayList;
import java.util.Collections;
import java.util.Random;

public class Numbers {

private int size = 0;

ArrayList<Integer> array = new ArrayList<Integer>(size);

public Numbers() {

}

public Numbers(int n) {
    size = n;
}

public void generateNumbers() {
    Random random = new Random();
    for (int i = 0; i < size; i++) {

        array.add(random.nextInt(49) + 1);
    }

}

public void findCount(int find) {
    int count = Collections.frequency(array, find);
    System.out.println("Number " + find + " occurred " + count + " times in the array");
}

@Override
public String toString() {
    return array.toString();
}

public void isArrayCreated() {
    if (size == 00) {
        System.err.print("Array is not created... please create the array first");
        System.out.println();
    }
}

public void isEmpty() {
    if (array.isEmpty()) {

        System.err.print("Array is empty");
        System.out.println();
        System.out.println(size);

    }
}

}

and this is my test class:

import java.util.InputMismatchException;
import java.util.Scanner;

public class Lab2Test {

public static void main(String[] args) {
    int n = 0;
    boolean loop = true;
    Scanner input = new Scanner(System.in);

    while (loop == true) {
        int option = 0;
        System.out.println("1. Create array with new size");
        System.out.println("2. Generate random numbers and store it in the array");
        System.out.println("3. Search a number and display its number of occurrences");
        System.out.println("4. Display array");
        System.out.println("5. Quit");
        System.out.println("Enter your option: ");

        try {
            option = input.nextInt();
        } catch (InputMismatchException e) {
            System.out.print("******Input mismatch exception*****");
            System.out.println();
            System.out.println();
        }

        if (option == 1) {
            try {
                System.out.println("Enter required size: ");
                n = input.nextInt();
            } catch (InputMismatchException a) {
                System.out.print("******Input mismatch exception*****");
                System.out.println();
                System.out.println();
            }

        }
        Numbers numb = new Numbers(n);

        if (option == 2) {
            numb.generateNumbers();

        }

        if (option == 3) {
            int find = 0;
            System.out.println("Enter the number to be searched: ");
            try {
                find = input.nextInt();
            } catch (InputMismatchException e) {
                System.out.print("******Input mismatch exception*****");
                System.out.println();
                System.out.println();
            }

            numb.findCount(find);

        }

        if (option == 4) {
            // numb.isArrayCreated();
            numb.isEmpty();
            numb.toString();

        }

        if (option == 5) {
            System.out.println("Bye.... have a nice day!");
            loop = false;

        }

    }

}

}

Answer 1

The problem is that Numbers object is declared and instantiated inside the while loop. So in each iteration your Numbers is destroyed and created again. To solve it you should move Numbers outside the loop.

Try this

import java.util.InputMismatchException;
import java.util.Scanner;

public class Lab2Test {

public static void main(String[] args) {
    int n = 0;
    boolean loop = true;
    Scanner input = new Scanner(System.in);
    Numbers numb; // Here we have a reference to Numbers outside the loop, so it's not destroyed and recreated in each iteration.

    while (loop == true) {
        int option = 0;
        System.out.println("1. Create array with new size");
        System.out.println("2. Generate random numbers and store it in the array");
        System.out.println("3. Search a number and display its number of occurrences");
        System.out.println("4. Display array");
        System.out.println("5. Quit");
        System.out.println("Enter your option: ");

        try {
            option = input.nextInt();
        } catch (InputMismatchException e) {
            System.out.print("******Input mismatch exception*****");
            System.out.println();
            System.out.println();
        }

        if (option == 1) {
            try {
                System.out.println("Enter required size: ");
                n = input.nextInt();
                numb = new Numbers(n); // Here we create a new sized Numbers

            } catch (InputMismatchException a) {
                System.out.print("******Input mismatch exception*****");
                System.out.println();
                System.out.println();
            }

        }

        if (option == 2) {
            numb.generateNumbers();

        }

        if (option == 3) {
            int find = 0;
            System.out.println("Enter the number to be searched: ");
            try {
                find = input.nextInt();
            } catch (InputMismatchException e) {
                System.out.print("******Input mismatch exception*****");
                System.out.println();
                System.out.println();
            }

            numb.findCount(find);

        }

        if (option == 4) {
            // numb.isArrayCreated();
            numb.isEmpty();
            numb.toString();

        }

        if (option == 5) {
            System.out.println("Bye.... have a nice day!");
            loop = false;

        }

    }

}

}

PD: After these changes you could get NullpointerExceptions when you use other options before 1, so you need to fix it. Just provided a solution to your problem, but it needs more work.

Answer 2

As @Ezequiel said the problem is that every time you are creating a new object and assign it to numb .

I hope this can help you, now it also display the value of the array

import java.util.ArrayList;
import java.util.Collections;
import java.util.Random;
import java.util.InputMismatchException;
import java.util.Scanner;

class Numbers {

private int size;

ArrayList<Integer> array ;

public Numbers() {

}

public Numbers(int n) {
    size = n;
    array = new ArrayList<Integer>(size);
}

public void generateNumbers() {
    Random random = new Random();
    for (int i = 0; i < size; i++) {
        array.add(random.nextInt(49) + 1);
    }

}

public void findCount(int find) {
    int count = Collections.frequency(array, find);
    System.out.println("Number " + find + " occurred " + count + " times in the array");
}

@Override
public String toString() {
 return array.toString();
}

public void isArrayCreated() {
    if (size == 00) {
        System.err.print("Array is not created... please create the array first");
        System.out.println();
    }
}

public void isEmpty() {
    if (array.isEmpty()) {

        System.err.print("Array is empty");
        System.out.println();
        System.out.println(size);

    }
}

}




public class Main {

public static void main(String[] args) {
    int n = 0;
    boolean loop = true;
    Scanner input = new Scanner(System.in);
Numbers numb = new Numbers(n);
    while (loop == true) {
        int option = 0;
        System.out.println("1. Create array with new size");
        System.out.println("2. Generate random numbers and store it in the array");
        System.out.println("3. Search a number and display its number of occurrences");
        System.out.println("4. Display array");
        System.out.println("5. Quit");
        System.out.println("Enter your option: ");

        try {
            option = input.nextInt();
        } catch (InputMismatchException e) {
            System.out.print("******Input mismatch exception*****");
            System.out.println();
            System.out.println();
        }

        if (option == 1) {
            try {
                System.out.println("Enter required size: ");
                n = input.nextInt();
                numb = new Numbers(n);
            } catch (InputMismatchException a) {
                System.out.print("******Input mismatch exception*****");
                System.out.println();
                System.out.println();
            }

        }
        

        if (option == 2) {
            numb.generateNumbers();

        }

        if (option == 3) {
            int find = 0;
            System.out.println("Enter the number to be searched: ");
            try {
                find = input.nextInt();
            } catch (InputMismatchException e) {
                System.out.print("******Input mismatch exception*****");
                System.out.println();
                System.out.println();
            }

            numb.findCount(find);
            numb.toString();

        }

        if (option == 4) {
            // numb.isArrayCreated();
            numb.isEmpty();
           System.out.println( numb.toString());

        }

        if (option == 5) {
            System.out.println("Bye.... have a nice day!");
            loop = false;

        }

    }

}

}

Answer 3

The problem lies with your Test class, in the line Numbers numb = new Numbers(n); . Everytime your loop is run, the value of numb from the previous iteration gets destroyed and a new object numb is created again. You should rather declare your numb variable outside the loop as

import java.util.InputMismatchException;
import java.util.Scanner;

public class Lab2Test {

public static void main(String[] args) {
    int n = 0;
    boolean loop = true;
    Numbers numb = new Numbers(0);
    Scanner input = new Scanner(System.in);

    while (loop == true){
        int option = 0;
        System.out.println("1. Create array with new size");
        System.out.println("2. Generate random numbers and store it in the array");
        System.out.println("3. Search a number and display its number of occurrences");
        System.out.println("4. Display array");
        System.out.println("5. Quit");
        System.out.println("Enter your option: ");

        try {
            option = input.nextInt();
        } catch (InputMismatchException e) {
            System.out.print("******Input mismatch exception*****");
            System.out.println();
            System.out.println();
        }

        if (option == 1) {
            try {
                System.out.println("Enter required size: ");
                n = input.nextInt();
            } catch (InputMismatchException a) {
                System.out.print("******Input mismatch exception*****");
                System.out.println();
                System.out.println();
            }

        }
        Numbers numb = new Numbers(n);

        if (option == 2) {
            numb.generateNumbers();

        }

        if (option == 3) {
            int find = 0;
            System.out.println("Enter the number to be searched: ");
            try {
                find = input.nextInt();
            } catch (InputMismatchException e) {
                System.out.print("******Input mismatch exception*****");
                System.out.println();
                System.out.println();
            }

            numb.findCount(find);

        }

        if (option == 4) {
            // numb.isArrayCreated();
            numb.isEmpty();
            numb.toString();

        }

        if (option == 5) {
            System.out.println("Bye.... have a nice day!");
            loop = false;

        }

    }

}

}

Generally too, it is considered to be a nice practice to declare all your variables outside the loop. This also reduces your space complexity, and removes the possibility of such errors as encountered by you.

Also, in your functions, you can add a condition to check if (size == 0) . If it is the case, a proper message should be displayed and an appropriate return code returned. This way no NullPointerException will be generated.