Java Array Inside of Object

Question

I am trying to access an array that is part of an object.

I am getting the error "Exception in thread "main" java.lang.NullPointerException at OrderedStringList.add(OrderedStringList.java:21) at Main.main(Main.java:24)"

I have cut my program down to the bare bones, cutting out everything that might be interfering with the output.

public class Main {

public static void main(String[] args) {

    int x = 5;

    OrderedStringList myList = new OrderedStringList();

    myList.add(x);
    }
} //end class

This code references the class OrderedStringList.

public class OrderedStringList {

public int values[];

OrderedStringList(){
    int values[] = new int[5];
}

public void add(int y) {
    values[0] = y;
    System.out.print(values[0]);
}

I assume that the numbers 21 and 24 in the error are line numbers. Because I have some things commented out in my original code, the code I have posted would normally have some content in the middle of it. Line 24 in main is: myList.add(x);. Line 21 of OrderedStringList is: values[0] = y;.

I'm guessing that there is something really simple that I am missing. Anything is appreciated.

Thanks.

Main.java:24 and OrderedStringList.java:21 do indeed point to the class and line number you're having the error at. — nhgrif
– nhgrif, Commented Sep 27, 2013 at 0:41
In your constructor, int values[] = new int[5]; declares a local variable values! Note, your compiler should have warned you about this unused variable. — Ken Y-N
– Ken Y-N, Commented Sep 27, 2013 at 0:59

jedwards · Accepted Answer · 2013-09-27 00:40:50Z

5

Here

OrderedStringList(){
    int values[] = new int[5];
}

You shadow the class member values.

Change this to:

OrderedStringList(){
    values = new int[5];
}

answered Sep 27, 2013 at 0:40

jedwards

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1 Comment

UnlovedPanda Over a year ago

Thanks a lot! it's been several months since I worked with java and I guess I was rustier than I thought. Greatly Appreciated.

MadProgrammer · Accepted Answer · 2013-09-27 00:40:58Z

2

You've declared values twice....

public int values[];

OrderedStringList(){
    int values[] = new int[5];
}

This commonly known as shadowing.

Change the initialization of the array in the constructor to something like...

public int values[];

OrderedStringList(){
    value = new int[5];
}

Instead...

answered Sep 27, 2013 at 0:40

MadProgrammer

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Comments

Diego C Nascimento · Accepted Answer · 2013-09-27 03:16:17Z

2

This declares the values[] array just inside the scope of the method.

OrderedStringList(){
    int values[] = new int[5];
}

If want to refer to the Class scope use

OrderedStringList(){
    values = new int[5];
}

edited Sep 27, 2013 at 3:16

answered Sep 27, 2013 at 0:40

Diego C Nascimento

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1 Comment

jedwards Over a year ago

:-) Sorry, have just seen a lot of unbelievable "answers" tonight

musical_coder · Accepted Answer · 2013-09-27 00:41:19Z

1

With the line int values[] = new int[5];, you're declaring an entirely new int[] that exists only in the constructor. Change it to values = new int[5];.

answered Sep 27, 2013 at 0:41

musical_coder

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