Returning Integer Array List from another class returns null pointer exceptions when getting specifi-CodePudding

new to java and need some pointers

I am trying to create an array list in one class then return it and iterate through its values in a separate main class. However when i use the get method to return a specific value in the array it produce a null pointer exception. What am I doing Wrong ?

import java.util.*
public class ReadFile 
{
 
    private ArrayList<Integer> height;

    public ReadFile()
    {
        ArrayList<Integer> height = new ArrayList<Integer>();

        for(int x = 0; x <= 3; x  )
        {
            height.add(x);
        }
    }

    public ArrayList<Integer> getHeights()
    {
        return height;
    }
}

Main Class

import java.util.*
    
public class Jumper
{

    private ReadFile readFile;

    public Jumper()
    {
        readFile = new ReadFile();
    }

    public static void main(String[] arg)
    {
        Jumper game = new Jumper();
        System.out.println(game.readFile.getHeights().get(1));
    }
}

CodePudding user response：

You're declaring a field named height:

private ArrayList<Integer> height;

and then you're déclaring a local variable also named height:

public ReadFile()
{
    ArrayList<Integer> height = new ArrayList<Integer>();

everything you do with height in the constructor is done on the local variable, and nothing is done on the field.

CodePudding user response：

This line is the problem

ArrayList<Integer> height = new ArrayList<Integer>();

Instead of assigning a value to the height member field, you create a new height variable that only exists inside the constructor.

You should instead do height = new ArrayList<Integer>();, similar to how you do with readFile.

P.S: Having a local variable with the same name as a member field is called "variable shadowing". Depending on what IDE you are using, it might warn you about it, as it's not an uncommon mistake.

CodePudding user response：

Please change this line ArrayList<Integer> height = new ArrayList<Integer>(); to height = new ArrayList<Integer>(); inside the constructor of ReadFile. What happens is both the height references are different.

In this code

public ReadFile() {

    ArrayList<Integer> height = new ArrayList<Integer>();

    for(int x = 0; x <= 3; x  ) {
        height.add(x);
    }
}

The height is local to the Constructor ReadFile(), so only that is initialized and the private ArrayList<Integer> height; remains null and that's what causes the Exception.