How can I make my push() method work properly within private ArrayLists and other constraints below?

Question

I am working on coding something as an assignment we are working with stacks and queues right now.

This is the first part of what I have in the Stack.java file:

import java.util.*;

class Stack
{
 private ArrayList<Integer> array;
/*
this should implement a Stack that holds Integers. It should have a constructor and the methods push(Integer), pop()and toString().
*/
    public void push(int value)
    {
        // adds the value to the end of the array
        array.add(value);
    }

A small part of runner class is:

 class Main {
 public static void main(String[] args) {

   Stack myStack = new Stack();
   myStack.push(1);
   myStack.push(2);
   myStack.push(3);
   myStack.push(4);
   myStack.push(5);
   myStack.push(6);

The error message I get is

"Exception in thread "main" java.lang.NullPointerException
    at Stack.push(Stack.java:12)
    at Main.main(Main.java:5)"

What is wrong?

When did you initialize the `array`? Try `private List array = new ArrayList<>();` — KevinO, Dec 17 '18 at 02:32

score 0 · Answer 1 · answered Dec 17 '18 at 05:49

First，like what GBlodgett said,you must initialize your ArrayList

// please user interface
private List<Integer> array = new ArrayList(8);

Second,ArrayList is not safe for multiple threads,if you just want to use List,you can use CopyOnWriteArrayList instead,but it perfomance is not good enough for stack.

How can I make my push() method work properly within private ArrayLists and other constraints **below**?

1 Answers1

How can I make my push() method work properly within private ArrayLists and other constraints below?